Proposed Minimum and Guidance Levels for Crews Lake in Pasco … · 2020-05-17 · Proposed...

Proposed Minimum and Guidance Levels for Crews Lake in Pasco County, Florida

January 18, 2016 Draft

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Proposed Minimum and Guidance Levels for Crews Lake in Pasco County, Florida

January 18, 2016 Draft

Douglas A. Leeper

Yonas Ghile Springs and Environmental Flows Section Natural Systems and Restoration Bureau

Tamera McBride

Resource Evaluation Section Water Resources Bureau

Southwest Florida Water Management District 2379 Broad Street

Brooksville, Florida 34604-6899 The Southwest Florida Water Management District (District) does not discriminate upon the basis of any individual’s disability status. This non-discriminatory policy involves every aspect of the District’s functions, including one’s access to, participation, employment, or treatment in its programs or activities. Anyone requiring accommodation as provided for in the American with Disabilities Act should contact (352) 796-7211 or 1-800-423-1476, extension 4215; TDD ONLY 1-800-231-6103; FAX (352) 754-6749. Cover: Aerial photograph (looking south) of Crews Lake in 2004 showing the fishing pier along the western lakeshore in Crews Lake Wilderness Park (Southwest Florida Water Management District files).

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Table of Contents

Page Executive Summary ........................................................................................................ 5

Acknowledgements ......................................................................................................... 7

Introduction ..................................................................................................................... 8

Establishment of Minimum and Guidance Levels for Crews Lake ................................... 8

Minimum Flows and Levels Program Overview .............................................................. 8

Legal Directives ........................................................................................................... 8

Development of Minimum Lake Levels .......................................................................... 10

Programmatic Description and Major Assumptions ................................................... 10 Consideration of Changes and Structural Alterations and Environmental Values .. 11

Established Levels ..................................................................................................... 13

Lake Setting and Description ..................................................................................... 14

Location ..................................................................................................................... 14

Physiography and Hydrogeology ............................................................................... 17

Bathymetry and Basin/Watershed Description and History ....................................... 17

Hydrology .................................................................................................................. 36 Climate and Rainfall ............................................................................................... 36 Water Level (Lake Stage) Record .......................................................................... 38 Water Use in the Lake Area and Evaluation of Withdrawal Impacts ...................... 39

Historical Management Levels and Currently Adopted Guidance Levels ...................... 43

Methods, Results and Discussion ................................................................................. 45

Summary Data Used for Minimum and Guidance Level Development ...................... 45

Bathymetry .................................................................................................................... 46

Classification of Lake Stage Data and Exceedance Percentiles ................................... 48

Normal Pool, Control Point Elevation and Determination of Structural Alteration Status ...................................................................................................................................... 50

Proposed Guidance Levels ........................................................................................... 52

Lake Classification ........................................................................................................ 52

Significant Change Standards and Other Information for Consideration ....................... 53

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Proposed Minimum Levels ............................................................................................ 55

Consideration of Environmental Values ........................................................................ 59

Minimum Levels Status Assessment ............................................................................. 60

Documents Cited and Reviewed for Development of Minimum and Guidance Levels for Crews Lake ................................................................................................................... 62

Appendix A ………………………………………………………………………………..…..A1

Appendix B ………………………………………………………………………………..…..B1

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Executive Summary This report describes the development of proposed Minimum and Guidance Levels by the Southwest Florida Water Management District for Crews Lake in Pasco County, Florida. Minimum levels are the levels at which further water withdrawals would be significantly harmful to the water resources of the area (Section 373.042(1)(b), Florida Statutes; F.S.). Minimum levels adopted by the District for lakes, wetlands and aquifers, and minimum flows adopted for rivers, springs and estuaries are used to support water resource planning and permitting activities. Guidance levels are adopted for lakes and used as advisory guidelines for construction of lakeshore development, water dependent structures, and operation of water management structures. The proposed levels, which are expressed as elevations in feet above the National Geodetic Vertical Datum of 1929 (and corresponding elevations in feet above the North American Vertical Datum of 1988), are listed in Table ES-1 along with descriptions for the levels included in District rules (Rule 40D-8.624, F.A.C). The proposed Minimum Levels were developed using current District methods for establishing minimum levels for Category 1 Lakes, which are lakes that are contiguous with at least 0.5 acres of cypress-dominated wetlands. The proposed levels were also developed with consideration of and are protective of all relevant environmental values identified for consideration in the Water Resource Implementation Rule when establishing minimum flows and levels (see Rule 62-40.473, F.A.C.). The proposed Minimum Levels will be used to support ongoing assessment of the status of minimum flows and levels water bodies and the need for additional recovery in the northern Tampa Bay Water Use Caution Area, a region of the District where strategies are being implemented to support recovery to MFLs thresholds. If adopted by the District Governing Board, the proposed Minimum and Guidance Levels identified in this report will replace currently adopted levels for Crews Lake. Based on available water level records, the currently proposed minimum levels for Crews Lake are being met, so development of a recovery strategy is not required. In the event that adopted levels for the lake are not met, the Comprehensive Environmental Resources Recovery Plan for the Northern Tampa Bay Water Use Caution Area and the Hillsborough River Recover Strategy (Rule 40D-80.073, F.A.C.) will apply for recovery of minimum levels for the lake. The District plans to continue regular monitoring of water levels in Crews Lake and will also routinely evaluate the status of the lake’s water levels with respect to adopted minimum levels for the lake that are included in Chapter 40D-8, F.A.C.

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Table ES-1. Proposed Minimum and Guidance Levels for Crews Lake and level descriptions. Minimum and

Guidance Levels

Elevation (feet above NGVD29a)

Elevation (feet above NAVD88b)

Level Descriptions

High Guidance Level 55.3

54.4

Advisory guideline for construction of lake shore development, water dependent structures, and operation of water management structures. The High Guidance Level is the elevation that a lake's water levels are expected to equal or exceed ten percent of the time on a long-term basis.

High Minimum Lake Level 52.4

51.5

Elevation that a lake's water levels are required to equal or exceed ten percent of the time on a long-term basis.

Minimum Lake Level 51.0

50.1

Elevation that the lake's water levels are required to equal or exceed fifty percent of the time on a long-term basis.

Low Guidance Level 48.9

48.0

Advisory guideline for water dependent structures, information for lakeshore residents and operation of water management structures. The Low Guidance Level is the elevation that a lake's water levels are expected to equal or exceed ninety percent of the time on a long-term basis.

a National Geodetic Vertical Datum of 1929 b National American Vertical Datum of 1988

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Acknowledgements The authors would like to thank several of our Southwest Florida Water Management District colleagues for their contributions to the work summarized in this report. We thank Richard Gant for his assistance with field-data collection and review of previous draft project reports, Mark Fulkerson for providing information on high water levels at the lake, and Don Ellison, Jason Patterson and Dave Arnold for hydrologic modeling support. We also thank our former District colleague, Lisa Henningsen for review of previous draft reports for the project and hydrologic modelings support.

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Introduction Establishment of Minimum and Guidance Levels for Crews Lake This report describes the development of proposed minimum and guidance levels by the Southwest Florida Water Management District (District or SWFWMD) for Crews Lake in Pasco County, Florida. The proposed levels were developed using peer-reviewed District methods for establishing minimum and guidance levels for lakes. The proposed levels were also developed with consideration of and are protective of all relevant environmental values identified for consideration in the Water Resource Implementation Rule when establishing minimum flows and levels (see Rule 62-40.473, Florida Administrative Code, hereafter F.A.C.). If adopted into District rules by the District Governing Board, the proposed minimum and guidance levels identified in this report will replace guidance levels currently adopted for Crews Lake. Minimum Flows and Levels Program Overview

Legal Directives State law (Section 373.042, Florida Statutes; hereafter F.S.) directs the Department of Environmental Protection or the water management districts to establish minimum flows and levels for lakes, wetlands, rivers and aquifers. As currently defined by statute, the minimum flow for a given watercourse "shall be the limit at which further withdrawals would be significantly harmful to the water resources or ecology of the area", and the minimum level of an aquifer or surface water body is "the level of groundwater in the aquifer and the level of surface water at which further withdrawals would be significantly harmful to the water resources of the area." Minimum flows and levels (MFLs) are established and used by the Southwest Florida Water Management District for water resource planning, as one of the criteria used for evaluating water use permit applications, and for the design, construction and use of surface water management systems. Established MFLs are key components of resource protection, recovery and regulatory compliance, as Section 373.0421(2) F.S., requires the development of a recovery or prevention strategy for water bodies “[i]f the existing flow or level in a water body is below, or is projected to fall within 20 years below, the applicable minimum flow or level established pursuant to S. 373.042.” Section 373.0421(2)(a), F.S, requires that recovery or prevention strategies be developed to: "(a) [a]chieve recovery to the established minimum flow or level as soon as practicable; or (b) [p]revent the existing flow or level from falling below the established minimum flow or level." Periodic reevaluation and, as necessary, revision of established MFLs are required by Section 373.0421(3), F.S.

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Minimum flows and levels are to be established based upon the best information available, and when appropriate, may be calculated to reflect seasonal variations (Section 373.042(1), F.S.). Also, establishment of MFLs is to involve consideration of, and at the governing board or department’s discretion, may provide for the protection of nonconsumptive uses (Section 373.042(1), F.S.). Consideration must also be given to "…changes and structural alterations to watersheds, surface waters and aquifers, and the effects such changes or alterations have had, and the constraints such changes or alterations have placed, on the hydrology of the affected watershed, surface water, or aquifer…", with the requirement that these considerations shall not allow significant harm caused by withdrawals (Section 373.0421(1)(a), F.S.). Sections 373.042 and 373.0421 provide additional information regarding the prioritization and scheduling of MFLs, the independent scientific review of scientific or technical data, methodologies, models and scientific and technical assumptions employed in each model used to establish a minimum flow or level, and exclusions that may be considered when identifying the need for MFLs establishment. The Florida Water Resource Implementation Rule, specifically Rule 62-40.473, F.A.C., provides additional guidance for MFLs establishment, requiring that "…consideration shall be given to natural seasonal fluctuations in water flows or levels, non-consumptive uses, and environmental values associated with coastal, estuarine, riverine, spring, aquatic and wetlands ecology, including: a) Recreation in and on the water; b) Fish and wildlife habitats and the passage of fish; c) estuarine resources; d) Transfer of detrital material; e) Maintenance of freshwater storage and supply; f) Aesthetic and scenic attributes; g) Filtration and absorption of nutrients and other pollutants; h) Sediment loads; i) Water quality; and j) Navigation." Rule 62-40.473, F.A.C., also indicates that "[m]inimum flows and levels should be expressed as multiple flows or levels defining a minimum hydrologic regime, to the extent practical and necessary to establish the limit beyond which further withdrawals would be significantly harmful to the water resources or the ecology of the area as provided in Section 373.042(1), F.S." It further notes that, “…a minimum flow or level need not be expressed as multiple flows or levels if other resource protection tools, such as reservations implemented to protect fish and wildlife or public health and safety, that provide equivalent or greater protection of the hydrologic regime of the water body, are developed and adopted in coordination with the minimum flow or level.” The rule also includes provision addressing: protection of MFLs during the construction and operation of water resource projects; the issuance of permits pursuant to Section 373.086 and Parts II and IV of Chapter 373, F.S.; water shortage declarations; development of recovery or prevention strategies, development and updates to a minimum flow and level priority list and schedule, and peer review for MFLs establishment.

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Development of Minimum Lake Levels

Programmatic Description and Major Assumptions Since the enactment of the Florida Water Resources Act of 1972 (Chapter 373, F.S.), in which the legislative directive to establish MFLs originated, and following subsequent modifications to this directive and adoption of relevant requirements in the Water Resource Implementation Rule, the District has actively pursued the adoption, i.e., establishment of MFLs for priority water bodies. The District implements established MFLs primarily through its water supply planning, water use permitting and environmental resource permitting programs, and through the funding of water resource and water supply development projects that are part of a recovery or prevention strategy. The District’s MFLs program addresses all relevant requirements expressed in the Florida Water Resources Act and the Water Resource Implementation Rule. A substantial portion of the District’s organizational resources has been dedicated to its MFLs Program, which logistically addresses six major tasks: 1) development and reassessment of methods for establishing MFLs; 2) adoption of MFLs for priority water bodies (including the prioritization of water bodies and facilitation of public and independent scientific review of proposed MFLs and methods used for their development); 3) monitoring and MFLs status assessments, i.e., compliance evaluations; 4) development and implementation of recovery strategies; 5) MFLs status assessment reporting; and 6) ongoing support for minimum flow and level regulatory concerns and prevention strategies. Many of these tasks are discussed or addressed in this proposed minimum levels report for Crews Lake; additional information on all tasks associated with the District’s MFLs Program is summarized by Hancock et al. (2010). The MFLs Program is implemented based on three fundamental assumptions. First, it is assumed that many water resource values and associated features are dependent upon and affected by long-term hydrology and/or changes in long-term hydrology. Second, it is assumed that relationships between some of these variables can be quantified and used to develop significant harm thresholds or criteria that are useful for establishing MFLs. Third, the approach assumes that alternative hydrologic regimes may exist that differ from non-withdrawal impacted conditions but are sufficient to protect water resources and the ecology of these resources from significant harm. Support for these assumptions is provided by a large body of published scientific work addressing relationships between hydrology, ecology and human-use values associated with water resources (e.g., see reviews and syntheses by Postel and Richer 2003, Wantzen et al. 2008, Poff et al. 2010, Poff and Zimmerman 2010). This information has been used by the District and other water management districts within the state to identify significant harm thresholds or criteria supporting development of MFLs for hundreds of water bodies, as summarized in the numerous publications associated with these efforts (e.g., SFWMD 2000, 2006, Flannery et al. 2002, SRWMD 2004, 2005, Neubauer et al. 2008, Mace 2009).

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With regard to the assumption associated with alternative hydrologic regimes, consider a historic condition for an unaltered river or lake system with no local groundwater or surface water withdrawal impacts. A new hydrologic regime for the system would be associated with each increase in water use, from small withdrawals that have no measurable effect on the historic regime to large withdrawals that could substantially alter the regime. A threshold hydrologic regime may exist that is lower or less than the historic regime, but which protects the water resources and ecology of the system from significant harm. This threshold regime could conceptually allow for water withdrawals, while protecting the water resources and ecology of the area. Thus, MFLs may represent minimum acceptable rather than historic or potentially optimal hydrologic conditions.

Consideration of Changes and Structural Alterations and Environmental Values

When establishing MFLs, the District considers “…changes and structural alterations to watersheds, surface waters and aquifers, and the effects such changes or alterations have had, and the constraints such changes or alterations have placed, on the hydrology of an affected watershed, surface water, or aquifer…” in accordance with Section 373.0421(1)(a), F.S. Also, as required by statute, when considering the changes, alterations and their associated effects and constraints, the District does not establish MFLs that would allow significant harm caused by withdrawals. These considerations are based on review and analysis of best available information, such as water level records, environmental and construction permit information, water control structure and drainage alteration histories, and observation of current site conditions. When establishing, reviewing or implementing MFLs, considerations of changes and structural alterations may be used to: adjust measured flow or water level historical records to account for existing

changes/alterations; model or simulate flow or water level records that reflect long-term conditions that

would be expected based on existing changes/alterations and in the absence of measurable withdrawal impacts;

develop or identify significant harm standards, thresholds and other criteria; aid in the characterization or classification of lake types or classes based on the

changes/alterations; support status assessments for water bodies with proposed or established MFLs

(i.e., determine whether the flow and/or water level are below, or are projected to fall below the applicable minimum flow or level); and

support development of lake guidance levels (described in the following paragraph).

The District has developed specific methodologies for establishing MFLs for lakes, wetlands, rivers, estuaries and aquifers, subjected the methodologies to independent, scientific peer-review, and incorporated the methods for some system types, including lakes, into its Water Level and Rates of Flow Rule (Chapter 40D-8, F.A.C.). This rule also provides for the establishment of Guidance Levels for lakes, which serve as

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advisory information for the District, lakeshore residents and local governments, or to aid in the management or control of adjustable water level structures. Information regarding the development of adopted methods for establishing minimum and guidance lake levels is included in SWFWMD (1999a, b) and Leeper et al. (2001). Additional information relevant to developing lake levels is presented by Schultz et al. (2005), Carr and Rochow (2004), Caffrey et al. (2006, 2007), Carr et al. (2006), Hoyer et al. (2006), Leeper (2006), Hancock (2006, 2007) and Emery et al. (2009). Independent scientific peer-review findings regarding lake level methods are summarized by Bedient et al. (1999), Dierberg and Wagner (2001) and Wagner and Dierberg (2006). For lakes, methods have been developed for establishing Minimum Levels for systems with fringing cypress-dominated wetlands greater than 0.5 acre in size, and for those without fringing cypress wetlands. Lakes with fringing cypress wetlands where water levels currently rise to an elevation expected to fully maintain the integrity of the wetlands are classified as Category 1 Lakes. Lakes with fringing cypress wetlands that have been structurally altered such that lake water levels do not rise to levels expected to fully maintain the integrity of the wetlands are classified as Category 2 Lakes. Lakes with less than 0.5 acre of fringing cypress wetlands are classified as Category 3 Lakes. Categorical significant change standards and other available information are developed to identify criteria that are sensitive to long-term changes in hydrology and can be used for establishing minimum levels. For all lake categories, the most sensitive, appropriate criterion or criteria is/are used to develop recommend minimum levels. For Category 1 or 2 Lakes, a significant change standard, referred to as the Cypress Standard, is developed. For Category 3 Lakes, six significant change standards, including a Basin Connectivity Standard, a Recreation/Ski Standard, an Aesthetics Standard, a Species Richness Standard, a Lake Mixing Standard and a Dock-Use Standard are typically developed. Other available information, including potential changes in the coverage of herbaceous wetland and submersed aquatic plants is also considered when establishing minimum levels for Category 3 Lakes. The standards and other available information are associated with the environmental values identified for consideration in Rule 62-40.473, F.A.C., when establishing minimum flows or levels (Table 1). Descriptions of the specific standards and other information are provided in subsequent sections of this report.

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Table 1. Environmental values identified in the state Water Resource Implementation Rule for consideration when establishing MFLs, and associated significant change standards and other information used by the District for consideration of the environmental values.

Environmental Value Associated Significant Change Standards and Other Information for Consideration

Recreation in and on the water Basin Connectivity Standard, Recreation/Ski Standard, Aesthetics Standard, Species Richness Standard, Dock-Use Standard, Herbaceous Wetland Information, Submersed Aquatic Macrophyte Information

Fish and wildlife habitats and the passage of fish

Cypress Standard, Wetland Offset, Basin Connectivity Standard, Species Richness Standard, Herbaceous Wetland Information, Submersed Aquatic Macrophyte Information

Estuarine resources NA1 Transfer of detrital material Cypress Standard, Wetland Offset, Basin

Connectivity Standard, Lake Mixing Standard, Herbaceous Wetland Information, Submersed Aquatic Macrophyte Information

Maintenance of freshwater storage and supply NA2 Aesthetic and scenic attributes Cypress Standard, Dock-Use Standard, Wetland

Offset, Aesthetics Standard, Species Richness Standard, Herbaceous Wetland Information, Submersed Aquatic Macrophyte Information

Filtration and absorption of nutrients and other pollutants

Cypress Standard Wetland Offset Lake Mixing Standard Herbaceous Wetland Information Submersed Aquatic Macrophyte Information

Sediment loads NA1 Water quality Cypress Standard, Wetland Offset, Lake Mixing

Standard, Dock-Use Standard, Herbaceous Wetland Information, Submersed Aquatic Macrophyte Information

Navigation Basin Connectivity Standard, Submersed Aquatic Macrophyte Information

NA1 = Not applicable for consideration for most priority lakes NA2 = Environmental value is addressed generally by development of minimum levels base on appropriate significant change standards and other information and use of minimum levels in District permitting programs

Established Levels

Two Minimum Levels and two Guidance Levels are typically established for lakes. The levels, which are expressed as elevations in feet above the National Geodetic Vertical Datum of 1929 (NGVD 29), may include the following (refer to Rule 40D-8.624, F.A.C.).

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A High Guidance Level that is provided as an advisory guideline for construction of lake shore development, water dependent structures, and operation of water management structures. The High Guidance Level is the elevation that a lake's water levels are expected to equal or exceed ten percent of the time on a long-term basis.

A High Minimum Lake Level that is the elevation that a lake's water levels are

required to equal or exceed ten percent of the time on a long-term basis.

A Minimum Lake Level that is the elevation that the lake's water levels are required to equal or exceed fifty percent of the time on a long-term basis.

A Low Guidance Level that is provided as an advisory guideline for water

dependent structures, information for lakeshore residents and operation of water management structures. The Low Guidance Level is the elevation that a lake's water levels are expected to equal or exceed ninety percent of the time on a long-term basis.

The District is in the process of converting from use of the NGVD 29 datum to use of the North American Vertical Datum of 1988 (NAVD88). While the NGVD 29 datum is used for most elevation values included within this report, in some circumstances notations are made for elevation data that was collected or reported relative to mean sea level or relative to NAVD88 and converted to elevations relative to NGVD 29. All datum conversions were derived using the Corpscon 6.0 software distributed by the United States Army Corps of Engineers.

Lake Setting and Description

Location Crews Lake is located in north-central Pasco County, Florida within the Tampa Bay Planning Region of the Southwest Florida Water Management District (Figure 1). The lake extends into portions of Sections 10, 15, 16, 20, 21 and 29, Township 24 South, Range 18 East, and is approximately centered around 28°22’59’’ latitude and -82°30’58” longitude (Figure 2). Public access to Crews Lake is available through the Crews Lake Wilderness Park, located along the western lakeshore. A public boat ramp, fishing pier and observation tower are available for use in this 113-acre Pasco County park. Pasco County also owns and maintains the Jumping Gully Preserve, a 598 acre Conservation Area adjacent to the central-eastern portion of the lake, although established public uses are currently not supported at the preserve.

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Figure 1. Location of Crews Lake, the Pithlachascotee River and other regional water bodies, highways and major roads within and near Pasco County, Florida.

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Figure 2. Location of Crews Lake Wilderness Park and Jumping Gully Preserve adjacent to Crews Lake, with numeric section, township (south) and range (east) information labeled for gridded Public Land Survey sections A portion of the Cross Bar Ranch Wellfield is also shown.

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Physiography and Hydrogeology White (1970) classified the region of central or mid-peninsular Florida containing Crews Lake as the Northern Gulf Coastal Lowlands (see Figure 4 in McBride 2016, included as Appendix A to this report). Brooks (1981) categorized the area surrounding the lake as the Land O Lakes subdivision (see Figure 5 in Appendix A) of the Tampa Plain division of the Ocala Uplift District, and described the region as a plain with numerous small lakes imbedded in moderately thick silty sand deposits lying above the Tampa Limestone formation. As part of the Florida Department of Environmental Protection’s Lake Bioassessment/ Regionalization Initiative, the area has been identified as Weeki Wachee Hills (Griffith et al. 1997). Lakes in the region are mostly clear-water systems, with circumneutral pH, and moderately low alkalinity, nutrients and chlorophyll a levels. A thin, but mostly continuous clay layer underlies areas adjacent to Crews Lake and the southwestern part of the drainage basin (Trommer 1987). The clay layer thickens toward the east near the Brooksville Ridge. Trommer reports that driller’s logs indicate the clay layer is breached by relict sinks in many places. More breaches appear in the northern and eastern areas of the basin than other areas. Consequently, in some areas, the surficial deposits may contain water only during wet periods, or may be locally perched where confining layers retard recharge (Trommer 1987). McBride (2016) notes that there are several sinkholes and a history of sinkhole and subsidence occurrence in and around Crews Lake.

Bathymetry and Basin/Watershed Description and History Crews Lake is listed as a 693-acre lake in "Gazetteer of Florida Lakes" (Florida Board of Conservation 1969, Shafer et al. 1986). A topographic map of the lake basin generated in support of minimum levels development (Figure 3) indicates that the lake would extend over 1,188 acres when the water level is at the elevation of 56 feet above NGVD 29 included on the 1954 U.S. Geological Survey 1:24,000 Port Richey NE, Fla. and Fivay, Fla. quadrangle 7.5 minute topographic maps and the 1988 photorevised versions of the Port Richey NE and Fivay (Fivay Junction), Fla. maps. Based on review the 2011 Florida Land Use, Cover and Forms Classification System (FLUCCS) layer maintained by the District Mapping and GIS Section, most of the land in the vicinity of Crews Lake is classified as upland forests, agriculture, urban and built up, and wetlands (data not shown). The upland areas in the immediate lake basin include extensive areas of native vegetation, including the Crews Lake Wilderness Park, and altered areas that are used for livestock grazing, production of feed-grasses low-density residential development. The immediate lake basin includes extensive, forested and non-forested lacustrine and palustrine wetland areas (Figure 4). These areas are populated by wetland and aquatic plant species, including spatterdock (Nuphar luteum), fragrant water lily (Nymphaea odorata), arrowhead (Sagittaria sp.), pickerelweed (Pontederia cordata), cypress (Taxodium sp.) and a number of grasses and sedges.

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Figure 3. One-foot ground elevation contours (feet above NGVD 29) bounded by the 57-foot contour within the Crews Lake basin.

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Figure 4. Lake and wetland areas in the Crews Lake vicinity based on 2011 Florida Land Use, Cover and Forms Classification System Classification data (upper panel) and National Wetland Inventory information (lower panel).

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The lake lies within the Crews Lake Outlet drainage basin in the Upper Coastal Areas watershed (United States Geological Survey Hydrologic Unit Classification System), and drains an area of 138 square miles (Florida Board of Conservation 1969, Foose 1981). A recent, alternative basin delineation by Ardaman and Associates, Inc. (2015) includes the lake in the Pithlachascotee River/Bear Creek Watershed (see Figure 2 in Appendix A). Surface water inputs to the lake include precipitation on the lake surface or immediate basin area, runoff from adjacent upland areas, and inflows from the Masaryktown Canal and Jumping Gully (Figure 5). The Masaryktown Canal, a District-owned flood management system, was constructed in the mid-1960s to drain areas south and east of Masaryktown into the north end of Crews Lake. Hutchinson (1985) reports that flow in the canal is "known to be zero during most of the year.” Jumping Gully conveys water into the lake from Unnamed Lake Number 22 (aka Loyce Lake) and other upstream water bodies, including Unnamed Lake Number 10, Pasco Lake, and various wetlands and ponds on the Cross Bar Ranch Wellfield. Jumping Gully discharge has been measured by the United States Geological Survey at a site named Jumping Gully at Loyce, FL (USGS Number 02310240, aka District Site Identification or SID number 20524) near U.S. Highway 41, from the mid-1960s through January 1988, and since January 1998 to September 2010. Prior to 1984, flows were typically less than 100 cubic feet per second, but reached as high as 890 cubic feet per second on September 19, 1964. An earthen berm located about three quarters of a mile north of the point where Jumping Gully enters the lake bisects the Crews Lake basin into roughly equal-sized northern and southern sub-basins (Figures 5 and 6). A secondary berm partitions the most southeastern portion of the northern sub-basin from the rest of the sub-basin. Conveyance across the berm that separates the northern and southern sub-basins is provided by a 60-inch diameter corrugated metal pipe when water levels on either side of the pipe exceeds 51.8 feet above NGVD 29 (Figure 6). Dry-season drainage into a sinkhole (Figure 7) located north of the berm may reduce the inundated area of the northern sub-basin and may induce northward flow through the berm-culvert from the southern basin (Hutchinson 1985). As summarized in Appendix A, Trommer (1987) provides information on additional sinkholes in the lake vicinity that may affect lake levels. Historical photographs indicate that the berm has been in place since the 1940s, and also show that much of the northern sub-basin has frequently been dry (see Figures 8 through 22). Crews Lake is the headwaters of the Pithlachascotee River, which extends about twenty-five miles from the southern sub-basin of the lake to the Gulf of Mexico (Figure 5, see also Figure 1). Discharge from the lake to the river may occur when the lake surface exceeds 54.1 feet above NGVD 29, the elevation at a high spot in the riverine wetlands south of the lake (Ardaman & Associates, Inc. 2007).

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Figure 5. Aerial photograph of the Crews Lake area in 2006, showing major surface water bodies and other features in the lake vicinity, the current District water-level gauge site, lake inlets and the lake outlet, sink hole and sites where hydrologic indicators were measured (photographic image source: Woolpert, Inc. 2006).

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Secondary berm Northern Culvert Sub-basin Southern Sub-Basin Berm Figure 6. Photographs of the earthen berm that separates the northern and southern sub-basins of Crews Lake and the culvert that provides for surface water conveyance past the berm. The upper photograph, from 1971 (District files), also shows a portion of a secondary berm in the northern sub-basin that partitions a portion of the sub-basin from the rest of the sub-basin. The lower photograph shows flow between the northern and southern sub-basins in 1982 (District files).

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Figure 7. Sinkhole in the northern sub-basin of Crews Lake in 1985 (District files).

Figure 8. Aerial photograph of Crews Lake in January 1941 (United States Department of Agriculture 1941b).

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Figure 9. Aerial photograph of the northern portion of Crews Lake in March 1952 (United States Department of Agriculture 1952b).

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Figure 10. Aerial photograph of the southern portion of Crews Lake in March 1952 (United States Department of Agriculture 1952c).

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Figure 11. Aerial photograph of northern portion of Crews Lake in March 1957 (United States Department of Agriculture 1957c).

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Figure 12. Aerial photograph of the southern portion of Crews Lake in March 1957 (United States Department of Agriculture 1957b).

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Figure 13. Aerial photograph (looking northeast) of Crews Lake in 1967 (District files).

Figure 14. Aerial photograph (looking east) of the northern portion of Crews Lake in 1969 (District files).

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¯ Figure 15. Aerial photograph of Crews Lake in the 1973 (image source: Woolpert 2005a).

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¯ Figure 16. Aerial infrared photograph of Crews Lake in 1984 (image source: United States Geological Survey 2004a).

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¯ Figure 17. Aerial infrared photograph of Crews Lake in 1994 (image source: Southwest Florida Water Management District, date unknown).

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Figure 18. Aerial photograph (looking northwest) of the Crews Lake outlet in 1996 (District files). This region marks the headwaters of the Pithlachascotee River.

Figure 19. Aerial photograph (looking southwest) of Crews Lake and the southern terminus of the Masaryktown Canal in 1998 (District files).

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¯ Figure 20. Aerial infrared photograph of Crews Lake in 1999 (image source: Southwest Florida Water Management District 2002a).

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Figure 21. Aerial photograph (looking south) of Crews Lake in 2000 (District files). The fishing pier and observation tower in Crews Lake Wilderness Park are shown along the west shore of the lake basin in the right portion of the photograph.

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¯ Figure 22. Aerial photograph of Crews Lake in 2005 (image source: Woolpert, inc. 2005b).

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Hydrology

Climate and Rainfall

The climate of west-central Florida, where Crews Lake occurs, may be characterized as humid southern temperate to subtropical, with frost and freezing temperatures occurring at least once a year. Local weather patterns are strongly influenced by the Gulf of Mexico, which moderates winter and summer temperatures. Daily temperatures in the Tampa Bay watershed, just to the south of Crews Lake average about 70° Fahrenheit on an annual basis, with mean summer temperatures in the low 80s and mean winter temperatures are in the upper 50s (Wolfe et al.1990, as cited in SWFWMD 2002b). Based on data available from long-term rainfall gaging stations in Pasco and Hernando counties (see Appendix A), annual rainfall in the vicinity of Crews Lake ranged from 32.4 to 81.0 inches and averaged 55.5 inches for the 85-year period from 1930 through 2014 (Figure 23). On an annual basis, rainfall for this period was typically highest during the months of June through September, likely as a result of the significant rainfall events that may be associated with convective and tropical storms that occur during these wet-season months. Evapotranspiration for the area has been reported at approximately 39 inches per year (Hutchinson 1984) and annual evaporation rates of 47 to 59 inches are reported for shallow, central Florida lakes (e.g., see Henderson 1983, Schiffer 1998, Swancar et al. 2000, Metz and Sacks 2003). Cherry et al. (1970) note that evaporation in the region is highest in May and June, prior to and during the early phase of the summer wet season. No statistically significant linear trend is evident for the 85-year rainfall record, based on ordinary least squares regression analysis. Shorter-term trends are, however, apparent in the record, especially when annual values are aggregated as moving-average values (e.g., see Figure 23). A plot of annual departure from the long-term average annual rainfall in the Crews Lake area provides another means for identifying periods of above or below average area rainfall. Since 1930, the three years with the highest departure above the mean occurred in the earlier part of the record, in 1945, 1953, and 1975. The three years with the lowest departure below the mean occurred in the recent record, in 1999, 2000, and 2006. The longest sustained period of rainfall departure below the long-term mean lasted 9 years, from 2005 to 2013. Prior to that period, there were three years above the annual mean (2002-2004) and three years below the annual mean (1999-2001), two of which were the first and second largest departure below the mean for the period of record. Annual rainfall was below the annual mean for 21 of the 35 years since the wellfield began operation in 1980.

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Figure 23. Annual rainfall in the Crews Lake area from 1930 through 2014 (see Appendix A for description of the rainfall data).

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Figure 24. Annual departure from the mean annual rainfall of 55.4 inches in the vicinity of Crews Lake from 1930 through 2014 (data source: same as for Figure 23).

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Water Level (Lake Stage) Record

Water levels in Crews Lake are currently monitored at a District-maintained site named Crews Lake (SID number 20506) located adjacent to the fishing pier in Crews Lake Wilderness Park (see Figures 5). Period of record (POR) lake stage data, i.e., surface-water elevations relative to NGVD 29 for this and other monitoring sites within Crews Lake were obtained from the District's Water Management Information System (WMIS). Records available for SID numbers 734367, 782429, 20506, and 777811 (see Figure 20 in Appendix A) were used to develop a long-term lake stage record for the period from March 20, 1964 through August 31, 2015 (Figure 25) for characterizing water level fluctuations in the basin and for hydrologic modeling purposes. As described in Appendix A, records for another monitoring site within the lake basin, SID 734366, were not used for the long-term stage record because the site was located in a sinkhole at the northern end of the lake.

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Figure 25. Measured water surface elevations for Crews Lake from March 20, 1964 through August 31, 2015 based on records from five sites (SIDs = Site identification numbers). Period of record P10, P50 and P90 elevations, i.e., water levels equaled or exceeded ten, fifty and ninety percent of the time, respectively, for the long-term lake stage record are 53.7, 51.7 and 48.6 feet above NGVD 29. The highest surface water elevation for the lake included in the long-term record, 56.60 feet above NGVD 29, occurred on September 24 and October 3, 1964. Based on review of federal survey notes of the

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General Land Office, Wharton (1984b) estimates that Crews Lake was staged at approximately 55 feet above mean sea level in April 1847. Notes from this nineteenth century survey indicate that the lake level was "very high" at the time of the survey (Wharton 1984a). The low of record, 42.63 feet above NGVD 29, was recorded on April 16, 2001.

Water Use in the Lake Area and Evaluation of Withdrawal Impacts

Surface water withdrawals from Crews Lake may have occurred historically, and there may be small withdrawals from the lake that fall below District permitting thresholds, but there are currently no permitted surface withdrawals at the lake. There are, however, numerous permitted groundwater withdrawals in lake vicinity (Figure 26).

Figure 26. Permitted water use permit (WUP) withdrawal sites within one to six miles of Crews Lake. Central System Facility wellfields (Public Supply Wellfields) near the lake are also shown.

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Some of these withdrawals are part of eleven public water supply wellfields collectively referred to as the Central System Facilities. The Central System Facilities are operated by the regional water supplier Tampa Bay Water and include wellfields in Pasco, northeastern Pinellas and northern Hillsborough counties (Figure 27). In the early 1930’s the first facility wellfield, the Cosme-Odessa Wellfield, which is located southwest of Crews Lake in Hillsborough County, began operation. In the late 1950s and during subsequent decades, additional wellfields that comprise the current Central System Facilities became operational. The Cross Bar Ranch Wellfield, which is located 1.5 miles east of Crews Lake and is the closest Central System Facility to the lake (see Figures 2, 26 and 27), began in 1980. The peak monthly withdrawal quantity from the wellfield was approximately 41 mgd in 1993 (see Figure 16 in Appendix A). Groundwater withdrawals at Cross Bar Ranch Wellfield have declined significantly since they peaked in the early 1990s and have recently been at a 12-month moving average quantity of 12 to 15 mgd. These recent withdrawal quantities are similar to the quantities withdrawn when the wellfield began operation in 1980.

As summarized in the Tampa Bay Planning Region portion of the District Water Management Plan (SWFWD 2011), investigations of interactions between water use, other factors and the water resources of the northern Tampa Bay area have been completed by the District and many others during the past half century. Much of this work, in particular the information compiled for the District’s water resource assessment project for the area (e.g., see SWFWMD 1996b), contributed to the 1989 establishment and 2007 expansion of the Northern Tampa Bay Water Use Caution Area (NTBWUCA), which includes Pinellas County, a northern portion of Hillsborough County and Pasco County, where Crews Lake is located (see Figure 27). Water Use Caution Areas are areas where “…regional action is necessary to address cumulative water withdrawals that are causing or may cause adverse impacts to the water and related land resources or the public interest…” (Rule 40D-2.801, F.A.C.). In an effort to address and better manage regional resource concerns, the District issued a consolidated water use permit to Tampa Bay Water in December 1998 for withdrawals at the Central System Facilities, entered with Tampa Bay Water and its member governments into what was referred to as the Partnership Agreement, and adopted MFLs for a number of lakes, wetlands and aquifers in the Northern Tampa Bay Region. The Partnership Agreement included a phased reduction in annual average groundwater pumping from 158 mgd to 90 mgd at the Central System Facilities by 2008. In accordance with the agreement, the District developed a recovery strategy for the northern Tampa Bay area and adopted a regulatory portion of the strategy into District rules (Chapter 40D-80, F.A.C.) that became effective in 2000 and were in place through 2010, when the Partnership Agreement expired. Implementation of the original Northern Tampa Bay area recovery strategy contributed to increasing water levels and flows and improving the condition of many wetlands, lakes, streams, springs and aquifer levels, but the need for additional recovery of some systems remained. To address this need, the District adopted a second phase of the area recovery strategy in 2010. This second recovery phase is referred to as the Comprehensive Environmental Resources Recovery Plan for the Northern Tampa Bay

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Water Use Caution Area Recovery and Prevention Strategy, or simply the “Comprehensive Plan.” The Comprehensive Plan addresses recovery of MFLs water bodies and avoidance and mitigation of unacceptable adverse impacts to wetlands, lakes streams springs and aquifer levels associated with Central System Facilities and other area facilities, which are collectively referred to in rule as the “90 MGD Facilities” (Rule 40D-80.873, F.A.C.). Adoption of the second phase of the area recovery plan was followed in January 2011 by renewal of the consolidated permit addressing withdrawals from the Central System Facilities by Tampa Bay Water through January 2021. Continued implementation of the Comprehensive Plan has resulted in a dramatic reduction in total groundwater withdrawals from Tampa Bay Water’s wellfield network. To compensate for the required reductions in groundwater withdrawals at the Central System Facilities, increased reliance has been placed on surface water withdrawals and a sea-water desalination facility for water supply. In keeping with the intent of the Comprehensive Plan, Tampa Bay Water now obtains surface water supplies from the Tampa Bypass Canal, the Hillsborough and Alafia Rivers, and maintains and operates a 25 mgd capacity seawater desalination plant on the eastern shore of Tampa Bay. Withdrawal-related changes to Upper Floridan aquifer levels in the vicinity of Crews Lake were evaluated (Patterson, personal communication, 2015) using the Integrated Northern Tampa Bay Model developed by Geurink and Basso (2013). A scenario where Central System Facility withdrawals were maintained at the mandated average annual rate of 90 mgd was evaluated. Simulation results indicated average predicted drawdown in the Upper Floridan aquifer near Crews Lake ranged from 0.5 feet at the southern end of the lake to 1.3 feet near the center of the lake and to 1.8 feet at the northern end of the lake. Drawdown in the surficial aquifer at Crews Lake was not simulated, because the northern edge of the surficial aquifer in the model domain lies south of Crews Lake. However, statistical modeling of Crews Lake water levels and area rainfall as described in the Classification of Lake Stage Data and Development of Exceedance Percentiles section of this report and in Appendix A to this report indicates that water level fluctuations in the lake are closely associated with rainfall variation, confirming the earlier District finding that impacts from groundwater withdrawals are minimal for most of the period of record at Crews Lake.

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Figure 27. Location of Tampa Bay Water’s Central System Facilities wellfields, Northern Tampa Bay Water Use Caution Area, Crews Lake and other area water bodies.

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Historical Management Levels and Currently Adopted Guidance Levels The Southwest Florida Water Management District has a long history of water resource protection through the establishment of lake management levels. With the development of the Lake Levels Program in the mid-1970s, the District began establishing management levels based on hydrologic, biological, physical and cultural aspects of lake ecosystems. By 1996, management levels for nearly 400 lakes had been established. In November 1984, the District adopted management levels, including minimum and flood levels that are currently referred to as Guidance Levels, for Crews Lake and incorporated (i.e., adopted) the levels into Chapter 40D-8, F.A.C. (Table 2). As part of the work leading to the adoption of the management levels, a Maximum Desirable Level of 54.50 feet above NGVD 29 was also developed but was not adopted by rule. Based on changes to sections of the Florida Statutes that address minimum flows and levels in 1996 and 1997, and the development of new approaches for establishing MFLs, District Water Levels and Rates of Flow rules were modified in 2000. The modifications included incorporation of rule language addressing MFLs development and the renaming of previously established levels as Guidance Levels. Subsequent revisions to District rules incorporated additional rule language associated with developing minimum lake levels, and the Ten Year Flood Guidance Level for Crews Lake and other lakes were removed from Chapter 40D-8, F.A.C. in 2007, when the Governing Board determined that flood-stage elevations should not be included in the District’s Water Levels and Rates of Flow rules. The intent of this latter action was not to discontinue development of regional and site-specific flood stage information, but rather to promote organizational efficiency by eliminating unnecessary rules. Flood stage levels for lakes will continue to be developed under the District's Watershed Management Program, but ten-year flood recurrence levels will not be incorporated into Chapter 40D-8, F.A.C. Historical and more recent flood-stage information for Crews Lake is available in numerous published reports (e.g., SWFWMD 1978, 1997,Turner et al. 1979, Ghioto & Asociates 1997, Ardaman and Associates, Inc. 2007, 2015). The currently adopted Guidance Levels and the Maximum Desirable Level for Crews lake were developed using methods that differ from the current District approach for establishing Minimum and Guidance Levels. The adopted levels do not, therefore, necessarily correspond with levels developed using current methods. Upon adoption by the District Governing Board, Minimum and Guidance Levels established using current methodologies will replace the existing Guidance Levels. Annually since 1989, a list of stressed lakes has been developed to support the District's water-use permitting program. As described in the District's Consumptive Use of Water Rule (Chapter 40D-2, F.A.C.), "a stressed condition for a lake is defined to be chronic fluctuation below the normal range of lake level fluctuations". For lakes with adopted Guidance Levels, chronic fluctuation below the Low Level is considered a

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stressed condition. For lakes without adopted levels, the evaluation of stressed condition is conducted on a case-by-case basis. Crews Lake is classified as a stressed lake since 1992. Table 2. Currently and previously adopted management/guidance levels for Crews Lake.

Management Levels (as originally adopted)

Guidance Levelsa

Elevation (feet above NGVD 29)

Ten (10) Year Flood Warning Level Ten Year Flood Guidance Level 57.00b

Minimum Flood Level High Level 55.00

Minimum Low Management Level Low Level 52.00

Minimum Extreme Low Management Level Extreme Low Level 50.00 a Adopted management levels were renamed as Guidance Levels in District rules in 2000. a Removed from District rules in 2007.

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Methods, Results and Discussion Summary Data Used for Minimum and Guidance Level Development Proposed Minimum and Guidance Levels were developed for Crews Lake using the methodology for Category 1 lakes described in Chapter 40D-8, F.A.C. Proposed levels and additional information are listed in Table 3, along with lake surface areas for each elevation. Detailed descriptions of the development and use of these data are summarized in subsequent sections of this report. Table 3. Proposed Minimum and Guidance Levels, lake stage exceedance percentiles, Normal Pool, Control Point elevation, significant change standards and associated surface areas for Crews Lake.

Elevation In feet NGVD 29

Lake Area (acres)

Lake Stage Exceedance Percentiles Historic P10 55.3 1102.2 Historic P50 52.0 579.3

Historic P90 48.9 239.5

Period of Record P10 53.7 855.9

Period of Record P50 51.7 540.9

Period of Record P90 48.6 211.4 Normal Pool and Control Point Normal Pool 52.8 710.5

Control Point 54.1 916.6 Significant Change Standards Cypress Standard 51.0 465.3

Basin Connectivity Standard* NA NA

Recreation/Ski Standard* NA NA

Species Richness Standard* 51.3 494.2

Wetland Offset* 51.2 483.8

Aesthetic Standard* 48.9 239.5

Dock-Use Standard* NA NA

Lake Mixing Standard* 45.7 62.8 Proposed Minimum and Guidance Levels High Guidance Level 55.3 1102.2

High Minimum Lake Level 52.4 638.4

Minimum Lake Level 51.0 465.3

Low Guidance Level 48.9 239.5 NA = not available or not applicable * = developed or evaluated for comparative purposes only

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Bathymetry Relationships between lake stage, inundated area and volume can be used to evaluate expected fluctuations in lake size that may occur in response to climate, other natural factors, and anthropogenic impacts such as structural alterations or water withdrawals. Long term reductions in lake stage and size can be detrimental to many of the environmental values identified in the Water Resource Implementation Rule for consideration when establishing MFLs. Stage-area-volume relationships are therefore useful for developing significant change standards and other information identified in District rules for consideration when developing minimum lake levels. Stage-area-volume relationships were determined for Crews Lake by building and processing a digital elevation model (DEM) of the lake basin and surrounding watershed. To develop the DEM, Light Detection and Ranging Data (LiDAR) obtained from the SWFWMD Mapping and GIS Section was processed with QCoherent LP360 for ArcGIS and merged with bathymetric data collected using a combination of GPS and a sonar-based depth finder system by DC Johnson Associates Surveying and Mapping (2006) and Southeastern Surveying (2015). The overall process involves merging the terrain morphology of the lake drainage basin with the lake basin morphology to develop topographic contours of the lake basin (refer to Figure 3) and to create a triangulated irregular network (TIN). The TIN was used to calculate the stage areas and volumes using a Python script file to iteratively run the Surface Volume tool in the Functional Surface toolset of the ESRI® 3D Analyst toolbox at one-tenth of a foot elevation change increments, starting at the largest size of the lake at its peak or flood stage, and working downward to a base elevation associated with the deepest pools in the lake basin. selected stage-area-volume results are presented in Figure 28).

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Figure 28. Crews Lake surface area, volume, mean depth, maximum depth, herbaceous wetland area and dynamic ratio (basin slope) as a function of lake stage.

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Classification of Lake Stage Data and Exceedance Percentiles For the purpose of minimum levels determination, lake stage data are categorized as "Historic" for periods when there were no measurable impacts due to water withdrawals, and impacts due to structural alterations were similar to existing conditions. In the context of minimum levels development, "structural alterations" means man's physical alteration of the control point, or highest stable point along the outlet conveyance system of a lake, to the degree that water level fluctuations are affected. Lake stage data are categorized as "Current" for periods when there were measurable, stable impacts due to water withdrawals, and impacts due to structural alterations were stable. Based on water-use estimates and analysis of lake stage and regional ground water fluctuations, hydrologic data for Crews Lake collected through and after December 1980 were considered to be Historic and Current data, respectively. Based on the relatively short "historic" period of record, measured Historic data were considered insufficient for calculating Historic lake-stage exceedance percentiles (i.e., the Historic P10, P50 and P90 elevations) for the lake. Historic lake-stage exceedance percentiles were, instead, developed using a modeled 68.7-year Historic record of daily lake surface elevations. The 68.7-year period was considered sufficient for incorporating the range of lake-stage fluctuations that would be expected based on long-term climatic cycles that have been shown to be associated with regional hydrologic variability (Enfield et al. 2001, Basso and Schultz 2003, Kelly 2004). Modeled daily lake stage values for the Historic data set were estimated using a linear fitting procedure known as the line of organic correlation (LOC). The LOC is a linear fitting procedure that minimizes errors in both the x and y directions and defines the best-fit straight line as the line that minimizes the sum of the areas of right triangles formed by horizontal and vertical lines extending from observations to the fitted line (see Helsel and Hirsch 1992). The procedure was used to describe the relationship between available lake stage data for Crews Lake from the period that predated withdrawal impacts (i.e., from March 20, 1964 through December 31,1980) and regional rainfall, as measured at nearby long-term rainfall gauging stations in Pasco and Hernando counties (see Appendix A). Rainfall values used for the analysis consisted of weighted twenty-four month cumulative totals that were derived using a linear-decay series to weight monthly rainfall values for the 24-month periods. The LOC equation developed using lake and rainfall data from March 1964 through December 1980 was used to estimate water surface elevations for Crews Lake for the 68.7-year period from January 1, 1946 through August 31, 2015 (Figure 29). The modeled record included periods when estimated water surface elevations were higher than the highest value of 56.6 feet above NGVD 29 that has been measured at the lake gaging stations. Model predicted values higher than that elevation were assigned a value of 56.6 feet above NGVD 29 for further analyses, including development of lake stage exceedance percentiles, because physical limitations that constrain model peaks were not parameterized in the model. This data truncation did not affect estimated long-

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term lake stage exceedance percentiles used to develop minimum and guidance levels because the truncation elevation exceeded the estimated percentile elevations. For instances where modeled water levels were truncated, elevations are expected to have been at or above the highest observed stage of 56.6 feet above NGVD 29. The Historic P10 elevation, the elevation the lake water surface equaled or exceeded ten percent of the time during the 69.7-year historic period, was 55.3 feet above NGVD 29. The Historic P50, the elevation the lake water surface equaled or exceeded fifty percent of the time during the historic period, was 52.0 feet above NGVD 29. The Historic P90, the elevation the lake water surface equaled or exceeded ninety percent of the time during the historic period, was 48.9 feet above NGVD 29. It should be noted that these percentiles and the modeled record from which they were derived are not recommended for other water management analyses such as floodplain mapping.

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Model peaks were truncated at the maximum observed level. The only intended purpose of this model is to estimate long-term percentiles (not daily highs) to help establish minimum and guidance levels. Physical limitations that constrain model peaks are not parameterized in the model. Truncation in this case does not affect estimated long-term percentiles, since the truncation elevation exceeded the estimated percentiles. Where peaks are truncated, elevations are expected to have been at or above the highest observed stage.

Figure 29. Measured (blue) and modeled (orange) Historic water surface elevations and Historic lake-stage exceedance percentiles for Crews Lake, for the period from January 1946 through August 2015. Modeled water levels higher than the highest observed water level of 56.6 feet above NGVD 29 were assigned an elevation of 56.6 feet above NGVD 29. Historic exceedance percentiles include the Historic P10, Historic P50 and Historic P90, which are water surface elevations equaled or exceed ten, fifty and 90 percent of the time, respectively.

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Normal Pool, Control Point Elevation and Determination of Structural Alteration Status The Normal Pool elevation, a reference elevation used for development of minimum lake and wetland levels, is established based on the distribution of Hydrologic Indicators of sustained inundation. Hydrologic indicators of Normal Pool include biological and physical features that become established as a result of recent or long-term water levels. For development of Minimum Lake Levels, the Normal pool elevation is considered an approximation of the Historic P10. Based on elevation of buttress inflection points measured on 31 cypress (Taxodium sp.) trees along the southwestern lakeshore, the Normal Pool elevation was established at 52.8 feet above NGVD 29 (Table 4, see Figures 5). The Normal Pool elevation is similar to the safe upland line of 52.0 feet above NGVD 29 identified for the lake by the Florida Department of Environmental Protection, but is 2.7 feet lower than median ground elevations measured at the base of saw palmetto (Serenoa repens) shrubs and live oak (Quercus virginiana) trees (Table 5), two common indicators of the upland-wetland transition zone, that were observed along the lake shoreline. For development of Minimum and Guidance levels, lakes are classified as open or closed basin lakes. Open basin lakes are systems that are connected to, or are part of an ordered surface water conveyance system, i.e., they have outlets or inlets for conveyance of surface water. Closed basin lakes are those that are not part of an ordered conveyance system. Crews Lake was classified as an open basin lake because it receives inflow from the Masaryktown Canal and Jumping Gully, and is considered the headwaters of the Pithlachascotee River. The Control Point elevation is the elevation of the highest stable point along the outlet profile of a surface water conveyance system (e.g., a weir, canal or culvert) that is the principal control of water level fluctuations in the lake. A Control Point may be established at the invert or crest elevation associated with a water control structure at a lake outlet, or at a high, stable point in a lake-outlet canal, ditch or wetland area. The invert or crest elevation is the lowest point on the portion of a water control structure that provides for conveyance of water across or through the structure. Based on review of available aerial photography with contours maps of the region (SWFWMD 1974a-f, 1978, 1982a,b), LiDAR data (EarthData International, Inc. 2004a) and survey work completed for development of floodplain analyses for the Pithlachascotee River watershed (Ghiotto & Associates 1997), the control point elevation for Crews Lake was established at 54.1 feet above NGVD 29, at a high spot south of the lake in the wetlands associated with the Pithlachascotee River (Ardaman & Associates, Inc. 2007). Structural alteration status is determined to support development of the High Guidance Level. In addition to identification of outlet conveyance system modifications, comparison of the Control Point elevation with the Normal Pool is typically used to determine if a lake has been structurally altered. If the Control Point elevation is below the Normal Pool, the lake is classified as a structurally altered system. If the Control

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Point elevation is above the Normal Pool or the lake has no outlet, then the lake is not considered to be structurally altered. Although the Crews Lake basin has been modified from it's presumed natural state (see Figures 5 through 19), based on the relative elevations of the Normal Pool and Control Point elevations, the lake was determined not to be Structurally Altered. Table 4. Summary statistics for hydrologic indicator measurements (buttress inflection points of Taxodium sp. trees) used for establishing the Normal Pool Elevation for Crews Lake. Elevations were measured by District staff in February 2004 and July 2006.

Statistic Statistic Value (N) or Elevation (feet above NGVD 29)

N 31

Median 52.8

Mean (SD) 53.0 (0.7)

Minimum 51.7

Maximum 55.1

Table 5. Summary statistics for additional hydrologic indicator measurements (elevation at the base of Serenoa repens shrubs or Quercus virginiana trees) for Crews Lake. Elevations were measured by District staff in February 2004 and July 2006.

Indicator Statistic Statistic Value (N) or Elevation (feet above NGVD

29) Serenoa repens N 52 Median 55.5 Mean (SD) 55.4 (0.6) Minimum 53.0 Maximum 56.2 Quercus virginiana N 19 Median 55.4 Mean (SD) 55.1 (0.8) Minimum 53.4 Maximum 56.3

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Proposed Guidance Levels The High Guidance Level is provided as an advisory guideline for construction of lakeshore development, water dependent structures, and operation of water management structures. The High Guidance Level is the expected Historic P10 of the lake, and is established using historic data if it is available, or is estimated using the Current P10, the Control Point and the Normal Pool elevation. Based on the availability of the composite Historic data set developed for Crews Lake, the High Guidance Level was established at the Historic P10 elevation, 55.3 feet above NGVD 29. The Low Guidance Level is provided as an advisory guideline for water dependent structures, and as information for lakeshore residents and operation of water management structures. The Low Guidance Level is the elevation that a lake's water levels are expected to equal or exceed ninety percent of the time on a long-term basis, and is established using Historic or Current data and, in some cases, reference lake water regime statistics. Reference lake water regime statistics are used when adequate historic or current data are not available. These statistics represent differences between P10, P50 and P90 lake stage elevations for typical, regional lakes that exhibit little or no impacts associated with water withdrawals. Reference lake water regime statistics include the RLWR50, RLWR90 and RLWR5090, which are, respectively, median differences between P10 and P50, P50 and P90, and P10 and P90 lake stage percentiles for a set of reference lakes. Based on the availability of the composite Historic water level record for Crews Lake, the proposed Low Guidance Level was established at the Historic P90 elevation, 48.9 feet above NGVD 29. Lake Classification Lakes are classified as Category 1, 2 or 3 for the purpose of Minimum Levels development. Systems with fringing cypress wetlands greater than 0.5 acres in size where water levels regularly rise to an elevation expected to fully maintain the integrity of the wetlands (i.e., the Historic P50 is not more than 1.8 feet below the Normal Pool elevation) are classified as Category 1 Lakes. Lakes with fringing cypress wetlands greater than 0.5 acres in size that have been structurally altered such that the Historic P50 is more than 1.8 feet below the Normal Pool elevation are classified as Category 2 Lakes. Lakes without fringing cypress wetlands or with less than 0.5 acres of fringing cypress wetlands are classified as Category 3 Lakes. Based on the occurrence of lake-fringing cypress wetlands of 0.5 acre or more in size within the lake basin (see Figures 4 and 18), and because the Historic P50 (52.0 feet above NGVD 29) is less than 1.8 feet below the Normal Pool elevation of 52.8 feet above NGVD 29), Crews Lake was classified as a Category 1 lake.

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Significant Change Standards and Other Information for Consideration Lake-specific significant change standards and other available information are developed for establishing Minimum Levels. The standards are used to identify thresholds for preventing significant harm to environmental values associated with lake ecosystems (see Table 1), in accordance with guidance provided in the Florida Water Resources Implementation Rule (Rule 62-40.473, F.A.C.). Other information taken into consideration for Minimum Levels development includes potential changes in the coverage of herbaceous wetland and submersed aquatic plants. For Category 1 or 2 Lakes, a significant change standard is established 1.8 feet below the Normal Pool elevation. This standard identifies a desired median lake stage that if achieved, may be expected to preserve the ecological integrity of lake-fringing wetlands. Although not identified by name in the District's Minimum Flows and Levels rule, the elevation 1.8 feet below Normal Pool is typically referred to as the Cypress Standard in District documents pertaining to Minimum Levels development. For Crews Lake, the Cypress Standard was established at 51.0 feet above NGVD 29. The standard elevation was equaled or exceeded sixty-seven percent of the time during the Historic period; i.e., the standard elevation corresponds to the Historic P67 based on the composite Historic water level record. For Category 3 lakes, six significant change standards, including a Basin Connectivity Standard, a Recreation/Ski Standard, a Species Richness Standard, an Aesthetics Standard, a Dock-Use Standard, and a Lake Mixing Standard are developed. These standards identify desired median lake stages that if achieved, are intended to preserve various natural system and human-use environmental values. Although Crews Lake is a Category 1 Lake, Category 3 Lake standards were developed for comparative purposes, but were not used to establish proposed Minimum Levels. The Basin Connectivity Standard is developed to protect surface water connections between lake basins or among sub-basins within lake basins to allow for movement of aquatic biota, such as fish, and support recreational use of the lake. The standard is based on the elevation of lake sediments at a critical high spot between lake basins or lake sub-basins, identification of water depths sufficient for movement of biota and/or watercraft across the critical high spot, and use of Historic lake stage data or region-specific reference lake water regime statistics. Based on the elevation that ensures connectivity between the northern and southern sub-basins of the lake (51.8 feet above NGVD 29 (the invert elevation of the culvert in the earthen berm that bisects the lake basin), the Basin Connectivity Standard could be established at 55.9 feet above NGVD 29, by adding a one-foot water depth in the area of connectivity to allow for movement of biota between the sub-basins, and the difference between the Historic P50 and Historic P90 elevations (3.1 feet). However, because the standard elevation was equaled or exceeded only seven percent of the time during the Historic period and was not considered appropriate for development of Minimum Levels for Crews Lake.

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The Recreation/Ski Standard is developed to identify the lowest elevation within the lake basin that will contain an area suitable for safe water skiing. The standard is based on the lowest elevation (the Ski Elevation) within the basin that can contain a 5-foot deep ski corridor delineated as a circular area with a radius of 418 feet, or a rectangular ski area 200 feet in width and 2,000 feet in length, and use of Historic lake stage data or region-specific reference lake water regime statistics. For Crews Lake, the Recreation-Ski Standard was established at 55.2 feet above NGVD 29, based on the sum of the Ski Elevation (52.1 ft above NGVD 29) and the 3.1-foot difference between the Historic P50 and Historic P90. Based on the Historic, composite water level record, the standard elevation was equaled or exceeded eleven percent of the time during the Historic period; i.e., the standard elevation corresponds to the Historic P11. Because the standard exceeds the Historic P50, it was not, however, considered appropriate for Minimum Levels development. Also, based on the distribution of aquatic and wetland plants throughout most of the lake basin, it seems unlikely that Crews Lake is utilized for recreational skiing. The Species Richness Standard is developed to prevent a decline in the number of bird species that may be expected to occur at or utilize a lake. Based on an empirical relationship between lake surface area and the number of birds expected to occur at a lake (Hoyer et al. 2066, Emery et al. 2009), the standard is established at the lowest elevation associated with less than a fifteen percent reduction in lake surface area relative to the lake area at the Historic P50 elevation. For Crews Lake, the Species Richness Standard was established at 51.3 feet above NGVD 29. The Species Richness Standard was equaled or exceeded sixty percent of the time during the Historic period; i.e., the standard elevation corresponds to the Historic P60. The Aesthetics Standard is developed to protect aesthetic values associated with the inundation of lake basins. The standard is intended to limit potential change in aesthetic values associated with the median lake stage from diminishing beyond the values associated with the lake when it is staged at the Low Guidance Level. The Aesthetic Standard is established at the Low Guidance Level, which for Crews Lake occurs at an elevation of 48.9 feet above NGVD 29. Because the Low Guidance Level was established at the Historic P90 elevation, water levels equaled or exceeded the Aesthetics Standard ninety percent of the time during the Historic period. The Dock-Use Standard is developed to provide for sufficient water depth at the end of existing docks to permit mooring of boats and prevent adverse impacts to bottom-dwelling plants and animals caused by boat operation. The standard is based on the elevation of lake sediments at the end of existing docks, a two-foot water depth for boat mooring, and use of Historic lake stage data or region-specific reference lake water regime statistics. Because there are no docks currently located within the basin, a Dock-Use Standard was not developed for Crews Lake. The Lake Mixing Standard is developed to prevent significant changes in patterns of wind-driven mixing of the lake water column and sediment resuspension. The standard is established at the highest elevation at or below the Historic P50 elevation where the

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dynamic ratio (see Bachmann et al. 2000) shifts from a value of <0.8 to a value >0.8, or from a value >0.8 to a value of <0.8. The Lake Mixing Standard for Crews lake was established at 45.7 feet above NGVD 29. Review of the stage-area information indicates that the lake size would be 31 acre at elevation corresponding to the Lake Mixing Standard. The Lake Mixing Standard was equaled or exceeded 100 percent of the time during the Historic period defined by the Historic composite data record, i.e., the standard elevation corresponds to the Historic P100. Herbaceous Wetland Information is taken into consideration to determine the elevation at which changes in lake stage would result in substantial changes in potential wetland area within the lake basin (i.e., basin area with a water depth of four or less feet). Similarly, changes in lake stage associated with changes in lake area available for colonization by rooted submersed or floating-leaved macrophytes are also typically evaluated, based on water transparency values. Review of herbaceous wetland area in relation to change in lake stage did not indicate that use of any of the identified Category 3 Lake significant change standards would be inappropriate (Figure 28). Changes in the area available for aquatic plant colonization associated with changes in lake stage could not be evaluated for Crews Lake due to the lack of sufficient water transparency measurements. Because herbaceous wetlands are common within the Crews Lake basin (e.g., see Figure 4), it was determined that an additional measure of wetland change should be considered for minimum levels development. Based on a review of the development of minimum level methods for cypress-dominated wetlands, it was determined that up to an 0.8 foot decrease in the Historic P50 elevation would not likely be associated with significant changes in the herbaceous wetlands occurring within lake basins (Hancock 2006). A Wetland Offset elevation of 51.2 feet above NGVD 29 was therefore established for Crews Lake by subtracting 0.8 feet from the Historic P50 elevation. The standard elevation was equaled or exceeded sixty-two percent of the time during the Historic period defined by the Historic composite data record, i.e., the standard elevation corresponds to the Historic P62. Proposed Minimum Levels Minimum Lake Levels, including the Minimum Lake Level and the High Minimum Lake Level, are developed using specific lake-category significant change standards and other available information or unique factors, including: potential changes in the coverage of herbaceous wetland vegetation and aquatic macrophytes; elevations associated with residential dwellings, roads or other structures; frequent submergence of dock platforms; faunal surveys; aerial photographs; typical uses of lakes (e.g., recreation, aesthetics, navigation, irrigation); surrounding land-uses; socio-economic effects; and public health, safety and welfare matters. Minimum levels development is also contingent upon lake classification, i.e., whether a lake is classified as a Category 1, 2 or 3 lake.

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The Minimum Lake Level is the elevation that a lake's water levels are required to equal or exceed fifty percent of the time on a long-term basis. For Category 1 lakes, the Minimum Level is established 1.8 feet below the Normal Pool elevation, i.e., at the Cypress Standard elevation. The proposed Minimum Lake Level for Crews Lake was therefore established at 51.0 feet above NGVD 29. The High Minimum Lake Level is the elevation that a lake's water levels are required to equal or exceed ten percent of the time on a long-term basis. For Category 1 lakes, the High Minimum Lake Level is established 0.4 feet below the Normal Pool elevation. The proposed High Minimum Lake Level for Crews Lake was therefore established at 52.4 feet above NGVD 29. The proposed Minimum and Guidance levels for Crews Lake relative to NGVD 29 are listed in Table 6 and plotted in Figure 30 along with measured water surface elevations through October 2015. Because many federal, state, and local agencies, such as the U.S. Army Corps of Engineers, the Federal Emergency Management Agency, U.S. Geological Survey, and the District are in the process of migrating from the NGVD29 to the NAVD88 vertical control standard, proposed Minimum and Guidance Levels for Crews Lake relative to NAVD88 are also included in Table 6. The NAVD88 elevations were estimated using a datum conversion of 0.86 feet derived with Corpscon 6.0 software distributed by the United States Army Corps of Engineers. Table 6. Proposed Minimum and Guidance Levels for Crews Lake relative to the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88).

Minimum and Guidance Levels Elevation (feet above NGVD29)

Elevation (feet above NAVD88)

High Guidance Level 55.3 54.4

High Minimum Lake Level 52.4 51.5

Minimum Lake Level 51.0 50.1

Low Guidance Level 48.9 48.0

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Figure 30. Measured water surface elevations for Crews Lake through October 2015 based on records from five sites (SIDs – Site Identificatin numbers) and proposed levels including the High Guidance Level (HGL), High Minimum Lake Level (HMLL), Minimum Lake Level (MLL) and Low Guidance Level (LGL). The approximate locations of the lake margin when water levels equal the proposed minimum levels are shown in Figure 31. Staging of the lake at the proposed Minimum Levels would not be expected to flood any man-made features within the immediate lake basin. Based on field survey data (Southwest Florida Water Management District 2006a, unpublished District data), the proposed High Minimum Lake Level is approximately 7.1 feet below the lowest residential home floor slab within the immediate lake basin, and about 5.1 feet below the floor slab of the public restroom located adjacent to the lakeshore in Crews Lake Wilderness Park (Table 7). Water depth at the bottom of the public boat ramp in the park would be approximately 3.3 and 1.9 feet, respectively, when the lake is staged at the proposed High Minimum Lake Level and Minimum Lake Level (Table 7).

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Figure 31. Approximate location of the proposed High Guidance Level (HGL), High Minimum Lake Level (HMLL), Minimum Lake Level (MLL) and Low Guidance Level (LGL) for Crews Lake.

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Table 7. Elevations of selected man-made features occurring at relatively low elevations within the immediate Crews Lake basin.

Lake Basin Features Elevation (feet above NGVD 29)

Lowest floor slab – residential dwelling 59.50

Second lowest floor flab – residential dwelling 62.20 Concrete slab for picnic pavilions in Crews Lake Wilderness Park 57.62 & 58.45

Concrete slab for public restrooms in Crews Lake Wilderness Park 57.45

First floor of wooden observation tower in Crews Lake Wilderness Park 56.9

Platform of wooden boardwalk/fishing pier in Crews Lake Wilderness Park 54.9 – 55.4

Top/bottom of concrete boat ramp in Crews Lake Wilderness Park 54.2 / 49.1

Consideration of Environmental Values The proposed Minimum Levels for Crews Lake are protective of all relevant environmental values identified for consideration in the Water Resource Implementation Rule when establishing MFLs (see Rule 62-40.473, F.A.C.). When developing proposed minimum levels, the District evaluates categorical significant change standards and other available information to identify criteria that are sensitive to long-term changes in hydrology and represent significant harm thresholds. An elevation 1.8 feet below Normal Pool, i.e., the Cypress Standard was identified to support development of Minimum Levels for Crews Lake based on the occurrence of lake-fringing cypress wetlands of one-half an acre or greater in size and classification of the lake as a Category 1 Lake. The Cypress Standard and the comparable elevation 0.4 feet below Normal Pool used to develop proposed Minimum Levels for Crews Lake are associated with protection of several environmental values identified in the Water Resource Implementation Rule, including: fish and wildlife habitats and the passage of fish, transfer of detrital material, aesthetic and scenic attributes, filtration and absorption of nutrients and other pollutants, and water quality (refer to Table 1). Because the Cypress Standard is associated with an elevation that is higher than that associated with an Aesthetics Standard developed for the lake, the proposed Minimum Levels are also considered protective of the environmental value recreation in and on the water (refer to Tables 1 and 3). The proposed Minimum Levels are also considered protective of the environmental value, navigation, based on the assessment of water depths that would be expected at the public boat ramp at the lake when the lake surface is at the elevations associated with the proposed levels. In addition, the environmental value, maintenance of freshwater storage and supply is also expected to be protected by the proposed Minimum Levels based on inclusion of conditions in water use permits that

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stipulate that permitted withdrawals will not lead to violation of adopted minimum flows and levels. Two environmental values identified in the Water Resource Implementation Rule, were not considered relevant to development of proposed Minimum Levels for Crews Lake. Estuarine resources were not considered relevant because the lake is not directly connected to estuarine resources and water level fluctuations in the lake are not expected to affect the ecological structure and functions of any estuaries. Sediment loads were similarly not considered relevant for Minimum Levels development for the lake, because the transport of sediments as bedload or suspended load is a phenomenon typically associated with flowing water systems. Minimum Levels Status Assessment The goal of a Minimum Levels status assessment is to determine if lake levels are fluctuating in accordance with criteria associated with adopted or proposed levels, i.e., to determine whether or not the Minimum Levels are being met. In addition to use of a rainfall regression model and/or other types of models, the process typically includes comparison of long-term water levels with adopted or proposed levels, review of periodic groundwater modeling updates, and, if necessary, investigation of other factors that could help explain lake level fluctuations. Lake status was assessed (McBride et al. 2016, included as Appendix B to this report) by comparing the P50 and P10 of measured lake stage data for Crews Lake to the proposed Minimum Lake Level and the High Minimum Lake Level. The P50 and P10 statistics were calculated using data from March 1964 through October 2015 and from May 1979 (when pumping began at Cross Bar Ranch Wellfield) through October 2015. The proposed Minimum Levels for Crews Lake are considered to currently be met, because the P50 and P10 for both periods exceeded the proposed Minimum Levels. In addition, because the assessment included a period of groundwater withdrawals at the Cross Bar Ranch Wellfield that are greater than those anticipated for the twenty-year planning period that must be evaluated for minimum flows and levels per Section 373.0421, F.S., the proposed minimum levels are also expected to be achieved for the planning period. An alternate method for evaluating lake status that involved data aggregation was aslo investigated due to the variability in the frequency of observed data (Appendix B). Results from this evalution indicated that the P10 and P50 for the aggregated measured lake stage data from March 1964 through October 2015, repectively exceeded the proposed High Minimum Lake Level and Minimum Lake Level. The P10 for the aggregated data for May 1979 through October 2015 coincident with withdrawals at the Cross Bar Ranch Wellfield was also higher than the proposed High Minimum Lake Level. The P50 for May 1979 through October 2015 was, however, 0.3 feet lower than the proposed Minimum Lake Level. Based on expectations for P50 values derived for assessment periods shorter than 60 or more years, consideration of rainfall and wellfield

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withdrawal trends and other factors, the P50 value calculated for the aggregated data from the period of wellfield withrawals was not contraindicative of the status assessment findings based on use of the non-aggregated measured lake stage data. Crews Lake is currently in an area with a recovery and prevention strategy for minimum flows and levels. While recovery is not anticipated to be needed at this time, the Comprehensive Environmental Resources Recovery Plan for the Northern Tampa Bay Water Use Caution Area and the Hillsborough River Recovery Strategy (Rule 40D-80.073, F.A.C.) applies to the area. The District plans to continue regular monitoring of water levels in Crews Lake and will also routinely evaluate the status of the lake’s water levels with respect to adopted Minimum Levels for the lake that are included in Chapter 40D-8, F.A.C.

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Documents Cited and Reviewed for Development of Minimum and Guidance Levels for Crews Lake Ardaman & Associates, Inc. 2007. Recommended Ten Year Flood Guidance Level and other flood stage elevations for Crews Lake in Pasco County, Florida. Orlando, Florida. Orlando, Florida. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. Ardaman and Associates, Inc. 2015. Floodplain justification report for the Bear Creek and Pithlachascotee River watersheds within Pasco and Hernando County, Florida. Orlando, Florida. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. Bachmann, R. W., Hoyer, M. V., and Canfield, D. E., Jr. 2000. The potential for wave disturbance in shallow Florida lakes. Lake and Reservoir Management 16: 281-291. Basso, R. 2004. Draft technical memorandum to Doug Leeper, dated November 9, 2004. Subject: Hydrogeologic setting of lakes within the northern Tampa Bay region. Southwest Florida Water Management District. Brooksville, Florida. Basso, R. and Schultz, R. 2003. Long-term variation in rainfall and its effect on Peace River flow in west-central Florida. Southwest Florida Water Management District. Brooksville, Florida. Bedient, P., Brinson, M., Dierberg, F., Gorelick, S., Jenkins, K., Ross, D., Wagner, K., and Stephenson, D. 1999. Report of the Scientific Peer Review Panel on the data, theories, and methodologies supporting the Minimum Flows and Levels Rule for northern Tampa Bay Area, Florida. Prepared for the Southwest Florida Water Management District, the Environmental Confederation of Southwest Florida, Hillsborough County, and Tampa Bay Water. Published by the Southwest Florida Water Management District. Brooksville, Florida. Berryman & Henigar, Inc., SDI Environmental Services, Inc., Ormiston, B.G., HDR Engineering, Inc., Greeley and Hansen, Inc., Legette, Brashears, & Graham, Inc., and Reynolds, Smith, & Hills, Inc. 2001. Phase I mitigation plan, Volumes 1, 2 and 3. Prepared for Tampa Bay Water. Clearwater, Florida. Brenner, M. and Binford, M.W. 1988. Relationships between concentrations of sedimentary variables and trophic state in Florida lakes. Canadian Journal of Fisheries and Aquatic Sciences 45: 294-300. Brooks, H. K. 1981. Physiographic divisions of Florida: map and guide. Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Gainesville, Florida.

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Caffrey, A.J., Hoyer, M.V. and Canfield, D.E., Jr. 2006. Factors affecting the maximum depth of colonization by submersed aquatic macrophytes in Florida lakes. University of Florida Institute of Food and Agricultural Sciences Department of Fisheries and Aquatic Sciences. Gainesville, Florida. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. Caffrey, A.J., Hoyer, M.V. and Canfield, D.E., Jr. 2007. Factors affecting the maximum depth of colonization by submersed aquatic macrophytes in Florida lakes. Lake and Reservoir Management 23: 287-297 Carr, D.W. and Rochow, T.F. 2004. Technical memorandum to file dated April 19, 2004. Subject: comparison of six biological indicators of hydrology in isolated Taxodium acsendens domes. Southwest Florida Water Management District. Brooksville, Florida. Carr, D.W., Leeper, D.A., and Rochow, T.E. 2006. Comparison of six biological indicators of hydrology and the landward extent of hydric soils in west-central Florida, USA cypress domes. Wetlands 26: 1012-1019. Cherry, R.N., Stewart, J.W. and Mann, J.A. 1970. General hydrology of the middle Gulf area, Florida. Report of Investigation No. 56. State of Florida Department of Natural Resources, Bureau of Geology. Tallahassee, Florida. DC Johnson Associates Surveying and Mapping Inc. 2006. Crews Lake bathymetry using ab acoustic geophysical investigation. Dierberg, F. E. and Wagner, K. J. 2001. A review of “A multiple-parameter approach for establishing minimum levels for Category 3 Lakes of the Southwest Florida Water Management District” June 2001 draft by D. Leeper, M. Kelly, A. Munson, and R. Gant. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. EarthData International, Inc. 2004a. LiDAR topographic data collected to support of Federal Emergency Management Agency Map Modernization for Pasco County, Florida. Available from the Mapping and GIS Section of the Southwest Florida Water Management District. Brooksville, Florida. EarthData International. 2004b. 2004 digital orthophotographs natural color. Published by the United States Geological Survey. Reston, Virginia. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Emery, S., Martin, D., Sumpter, D., Bowman, R., Paul, R. 2009. Lake surface area and bird species richness: analysis for minimum flows and levels rule review. University of South Florida Institute for Environmental Studies. Tampa, Florida. Prepared for the Southwest Florida Water Management District. Brooksville, Florida.

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Enfield, D.B., Mestas-Nunez, A.M., and Trimble, P.J. 2001. The Atlantic multidecadal oscillation and its relation to rainfall and river flow in the continental U.S. Geophysical Research Letters 28: 2077-2080. Florida Board of Conservation. 1969. Florida lakes, part III: gazetteer. Division of Water Resources. Tallahassee, Florida. Florida Lakewatch. 2004. Florida Lakewatch water chemistry summaries 2004, Volume 2. Department of Fisheries and Aquatic Sciences, Institute of Food and Agricultural Sciences, University of Florida. Gainesville, Florida. Foose, D.W. 1981. Drainage areas of selected surface-water sites in Florida. Open-File Report 81-482. United States Department of the Interior, United States Geological Survey. Tallahassee, Florida. Gant, R. 2007. Memorandum to Ralph Kerr, John Parker, Michael Balser and Scott Laidlow dated January 23, 2006 Subject: 2007 stressed lakes. Southwest Florida Water Management District. Brooksville, Florida. Geurink, J.S. and Basso, R. 2013. Development, calibration, and evaluation of the Integrated Northern Tampa Bay Hydrologic Model. Prepared for Tampa Bay Water and Southwest Florida Water Management District. Clearwater and Brooksville, Florida. Hancock, M. 2006. Draft memorandum to file, dated April 24, 2006. Subject: a proposed interim method for determining minimum levels in isolated wetlands. Southwest Florida Water Management District. Brooksville, Florida. Ghioto and Associates. 1997. The Pithlachascotee River watershed flood plain analysis. Prepared for the Board of County Commissioners, Pasco County, Florida and the Southwest Florida Water Management District. Orlando, Florida. Griffith, G., Canfield, D., Jr., Horsburgh, C., Omernik, and J. Azevedo, S. 1997. Lake regions of Florida (map). United States Environmental Protection Agency, University of Florida Institute of Food and Agricultural Sciences, Florida Lakewatch, Florida Department of Environmental Protection, and the Florida Lake Management Society. Gainesville and Tallahassee, Florida. Hancock, M. 2006. Draft memorandum to file, dated April 24, 2006. Subject: a proposed interim method for determining minimum levels in isolated wetlands. Southwest Florida Water Management District. Brooksville, Florida. Hancock, M. 2007. Recent development in MFL establishment and assessment. Southwest Florida Water Management District, draft 2/22/2007. Brooksville, Florida. Helsel, D.R. and Hirsch, R.M. 1992. Statistical methods in water resources. Studies in Environmental Science 45. Elsevier. New York, New York.

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Henderson, S.E. 1983. Hydrology of Lake Padgett, Saxon Lake and adjacent area, Pasco County, Florida. U.S. Geological Survey Water-Resources Investigations Open-File Report 82-759. Tallahassee, Florida. Hoyer, M.V., Israel, G.D. and Canfield, D.E., Jr. 2006. Lake user’s perceptions regarding impacts of lake water level on lake aesthetics and recreational uses. University of Florida Institute of Food and Agricultural Sciences Department of Fisheries and Aquatic Sciences and Department of Agricultural Education and Communication. Gainesville, Florida. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. Hutchison, C.B. 1984. Hydrogeology of well-field areas near Tampa, Florida, phase 2 – development and documentation of a quasi-three-dimensional finite-difference model for simulation of steady-state groundwater flow. United States Geological Survey Open-File Report 84-4002. Hutchinson, C.B. 1985. Hydrogeology of the Cross Bar Ranch Well-Field area and projected impact of pumping, Pasco County, Florida. Water Resources Investigations Report 85-4001. United States Department of the Interior, United States Geological Survey. Tallahassee, Florida. Kelly, M. 2004. Florida river flow patterns and the Atlantic Multidecadal Oscillation. Southwest Florida Water Management District. Brooksville, Florida. Kolasa, K. 2015. Memorandum to Darrin Herbst, Claire Muirhead, April Breton and John Emery dated February 26, 2015. Subject: 2015 stressed lakes. Southwest Florida Water Management District. Brooksville, Florida. Leeper, D. 2006. Proposed methodological revisions regarding consideration of structural alterations for establishing Category 3 Lake minimum levels in the Southwest Florida Water Management District, April 21, 2006 peer-review draft. Southwest Florida Water Management District. Brooksville, Florida. Leeper, D., Kelly, M., Munson, A. and Gant, R. 2001. A multiple-parameter approach for establishing minimum levels for Category 3 Lakes of the Southwest Florida Water Management District, June 14, 2001 draft. Southwest Florida Water Management District. Brooksville, Florida. Malloy, R. L. 2005. E-mail to Doug Leeper, dated December 27, 2005. Subject: request for info on OHWL or SUL for selected lakes. Florida Department of Environmental Protection, Bureau of Surveying and Mapping. Tallahassee, Florida. McBride, T. 2016. Draft technical memorandum to Jerry L. Mallams, dated January 18, 2016. Crews Lake rainfall regression model and historic percentile estimations. Southwest Florida Water Management District. Brooksville, Florida.

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McBride,T. Leeper, D.A. and Ghile, Y. 2016. Draft technical memorandum to Jerry L. Mallams, dated January 18, 2016. Crews Lake initial minimum levels status assessment. Southwest Florida Water Management District. Brooksville, Florida. Metz, P.A. and Sachs, L.A. 2003. Comparison of the hydrogeology and water quality of a ground-water augmented lake with two non-augmented lakes in northwest Hillsborough County, Florida. United States Geological Survey Water-Resources Investigation Report 02-4032. Tallahassee, Florida. Pasco County Parks and Recreation Department. Date unknown. Crews Lake Park nature trail self guide. Land O' Lakes, Florida. Perry, R. 2000. Memorandum to John Parker, dated January 4, 2000. Subject: Jumping Gully discharge. Southwest Florida Water Management District. Brooksville, Florida. Romie, K. 2000. Water chemistry of lakes in the Southwest Florida Water Management District. Brooksville, Florida. Sacks, L.A. 2002. Estimating ground-water inflow to lakes in central Florida using the isotope mass-balance approach. Water-Resources Investigations Report 02-4192. United States Department of the Interior, United States Geological Survey. Tallahassee, Florida. Schiffer, D.M. 1998. Hydrology of central Florida lakes – a primer. United States Geological Survey Circular 1137. Washington, D.C. Prepared in cooperation with the Saint Johns River Water Management District and the South Florida Water Management District. Palatka and West Palm Beach, Florida. Schultz, R., Hancock, M., Hood, J., Carr, D. and Rochow, T. 2005. Technical memorandum to File, NTB II. Subject: use of biological indicators for the establishment of historic normal pool. Southwest Florida Water Management District. Brooksville, Florida. Shafer, M. D., Dickinson, R. E., Heaney, J. P., and Huber, W. C. 1986. Gazetteer of Florida lakes. Publication No. 96, Water Resources Research Center, University of Florida. Gainesville, Florida. Southestern Surveying Inc. 2015. Crews Lake topographic survey using Real Time Kinematic (RTS) GPS methods. Southwest Florida Water Management District. Date unknown. 1994 digital orthophotographs color infrared. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida.

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Southwest Florida Water Management District. 1974a. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 15-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1974b. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 16-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1974c. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 20-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1974d. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 21-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1974e. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 29-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1974f. Pithlachascotee River Basin, Pithlachascotee River aerial photography with contours, Sheet No. 30-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, Lansing, Michigan. Southwest Florida Water Management District. 1978a. Pithlachascotee River Basin, Gowers Corners aerial photography with contours, Sheet No. 29-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, St. Petersburg, Florida. Southwest Florida Water Management District. 1978b. Water management alternatives in the Squirrel Prairie area, Hernando Co. and Pasco Co., Florida. Brooksville, Florida. Southwest Florida Water Management District. 1982a. Coastal Rivers Basin, Wolf Prairie aerial photography with contours, Sheet No. 20-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, St. Petersburg, Florida. Southwest Florida Water Management District. 1982b. Coastal Rivers Basin, Wolf Prairie aerial photography with contours, Sheet No. 30-24-18. Brooksville, Florida. Prepared by Abraham Aerial Survey Corporation, St. Petersburg, Florida.

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Southwest Florida Water Management District. 1992. Flood-stage frequency relations for selected lakes in the Southwest Florida Water Management District. September 1991, updated November 1992. Brooksville, Florida. Southwest Florida Water Management District. 1996a. Lake Levels Program lake data sheets / 1977-1996: Coastal Rivers Basin – 15. Brooksville, Florida. Southwest Florida Water Management District. 1996b. Northern Tampa Bay Water Resources Assessment Project, volume 1, surface-water/ground-water interrelationships. Resource Evaluation Section. Brooksville, Florida. Southwest Florida Water Management District. 1997. Squirrel Prairie watershed management plan floodplain analysis. Brooksville, Florida. Southwest Florida Water Management District. 1999a. Establishment of minimum levels for Category 1 and Category 2 lakes, in Northern Tampa Bay minimum flows and levels white papers: white papers supporting the establishment of minimum flows and levels for isolated cypress wetlands, Category 1 and 2 lakes, seawater intrusion , environmental aquifer levels and Tampa Bypass canal, peer-review final draft, March 19, 1999. Brooksville, Florida. Southwest Florida Water Management District. 1999b. Establishment of minimum levels in palustrine cypress wetlands, in Northern Tampa Bay minimum flows and levels white papers: white papers supporting the establishment of minimum flows and levels for isolated cypress wetlands, Category 1 and 2 lakes, seawater intrusion , environmental aquifer levels and Tampa Bypass canal, peer-review final draft, March 19, 1999. Brooksville, Florida. Southwest Florida Water Management District. 2002a. 1999 digital orthophotographs color infrared. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Southwest Florida Water Management District. 2002b. Tampa Bay/Anclote River comprehensive watershed management plan. Brooksville, Florida. Southwest Florida Water Management District. 2002c. United States Geological Surveys 1:24,000 scale topographic map (DRG). Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Southwest Florida Water Management District. 2003a. 1:24,000 detailed roads. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Southwest Florida Water Management District. 2003b. 2002 satellite imagery, natural color. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida.

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Southwest Florida Water Management District. 2003c. Florida counties. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Southwest Florida Water Management District. 2003d. Generalized streams and rivers. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Southwest Florida Water Management District. 2006a. Surveyor's report: lake level data for the establishment of minimum flows and levels – Crews Lake – Paso County, Florida. Survey Section. Brooksville, Florida. Southwest Florida Water Management District. 2006b. Surveyor's report: lake level data for the establishment of minimum flows and levels – Pasco Lake – Paso County, Florida. Survey Section. Brooksville, Florida. Southwest Florida Water Management District. 2006c. Surveyor's report: lake level data for the establishment of minimum flows and levels – Unnamed Lake 22-Loyce Lake – Paso County, Florida. Survey Section. Brooksville, Florida. Southwest Florida Water Management District. 2011. 2010 Regional water supply plan – Tampa Bay Planning Region. Brooksville, Florida. Southwest Florida Water Management District, University of South Florida – Department of Marine Science (USF) and Pasco County. 1997. Water quality assessment of the Pithlachascotee River following remediation programs. Brooksville and Tampa, Florida. Swancar, A., Lee, T.M. and O’Hare, T.M. 2000. Hydrogeologic setting, water budget, and preliminary analysis of ground-water exchange at Lake Starr, a seepage lake in Polk County, Florida. United States Geological Survey Water-Resources Investigations Report 00-4030. Tallahassee, Florida. Trommer, J.T. 1987. Potential for pollution of the Upper Floridan Aquifer from five sinkholes and one internally drained basin in west-central Florida. U.S. Geological Survey Water-Resources Investigations Report 87-4013.United States Army Corps of Engineers. 1967. Four Rivers Basins, Florida multiple purpose project, part V, Gulf Coastal areas supplement 2, general and detail design memorandum, Masaryktown Canal area (C-534). U.S. Arm Engineer District, Jacksonville, Florida. Turner, J.F., Jr., Murphy, W.R., Jr. and Reeter, C.V. 1979. Flood profiles of the Pithlachascotee River, west-central Florida. U.S. Geological Survey Water-Resources Investigation 78-100. Prepared in cooperation with the Southwest Florida Water Management District. Tallahassee, Florida.

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United States Department of Agriculture. 1941a. Aerial photograph number CTT-6B-42, dated January 30, 1941. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1941b. Aerial photograph number CTT-6B-43, dated January 30, 1941. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1941c. Aerial photograph number CTT-9B-29, dated February 15, 1941. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1952a. Aerial photograph number CTT-7H-150, dated January 7, 1952. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1952b. Aerial photograph number DCR-8H-51, dated March 7, 1952. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1952c. Aerial photograph number DCR-8H-52, dated March 7, 1952. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1952d. Aerial photograph number DCR-8H-53, dated March 7, 1952. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1957a. Aerial photograph number CTT-1T-30, dated March 23, 1957. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Department of Agriculture. 1957b. Aerial photograph number CTT-1T-63, dated March 23, 1957. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida.

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United States Department of Agriculture. 1957c. Aerial photograph number CTT-1T-64, dated March 23, 1957. Washington, D.C. Available on-line at the Aerial Photography: Florida web site (www.uflib.ufl.edu/digital/collections/FLAP) maintained by the University of Florida. Gainesville, Florida. United States Geological Survey. 1954a. Ehren quadrangle, Florida – Pasco Co., 7.5 minute series (topographic) map; Ehren, Fla., N2815-W8222.5/7.5, 1954, AMS 4540 IV SW-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1954b. Fivay quadrangle, Florida – Pasco Co., 7.5 minute series (topographic) map; Fivay, Fla., N2815-W8230/7.5, 1954, AMS 4440 I SE-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1954c. Masaryktown quadrangle, Florida –7.5 minute series (topographic) map; Masaryktown, Fla., N2822-W8222.5/7.5, 1954, AMS 4540 IV NW-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1954d. Port Richey NE quadrangle, Florida –, 7.5 minute series (topographic) map; Port Richey NE, Fla., 28082-D5-TF-024, 1954, photorevised 1988, DMA 4440 I NE-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1988a. Ehren quadrangle, Florida – Pasco Co., 7.5 minute series (topographic) map; Ehren, Fla., 28082-C4-TF-024, 1954, photorevised 1988, DMA 4540 IV SW-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1988b. Fivay Junction quadrangle, Florida – Pasco Co., 7.5 minute series (topographic) map; Fivay Junction, Fla., 28082-C5-TF-024, 1954, photorevised 1988, DMA 4440 I SE-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1988c. Masaryktown quadrangle, Florida –7.5 minute series (topographic) map; Masaryktown, Fla., 28082-D4-TF-024, 1954, photorevised 1988, DMA 4540 IV NW-Series V847. United States Department of the Interior Geological Survey. Washington, D.C. United States Geological Survey. 1988d. Port Richey NE quadrangle, Florida –, 7.5 minute series (topographic) map; Port Richey NE, Fla., N2822.5-W8230/7.5, 1954. United States Department of the Interior Geological Survey. Washington, D.C.


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United States Geological Survey. 2004a. 1984 National high altitude photography (NHAP). Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. United States Geological Survey. 2004b. National hydrography dataset – water bodies and swamps. United States Department of the Interior Geological Survey. Washington, D.C. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Wagner, K. and Dierberg, F. 2006. A review of "Proposed methodological revisions regarding consideration of structural alterations for establishing Category 3 Lake minimum levels in the Southwest Florida Water Management District" by D. Leeper, 2006. Prepared for the Southwest Florida Water Management District. Brooksville, Florida. Wharton, B.R. 1984a. Memorandum to P.M. Dooris, dated September 17, 1984, regarding mid-nineteenth century lake levels for Unnamed Lake No. 10, Pasco County (Coastal Rivers Basin), Florida. Southwest Florida Water Management District. Brooksville, Florida. Wharton, B.R. 1984b. Memorandum to P.M. Dooris, dated September 18, 1984, regarding mid-nineteenth century lake levels for Crews Lake in northwestern Pasco County (Coastal Rivers Basin), Florida. Southwest Florida Water Management District. Brooksville, Florida. White, W. A. 1970. The geomorphology of the Florida peninsula. Geological Bulletin, No. 51. Bureau of Geology, Florida Department of Natural Resources. Tallahassee, Florida. Wolfe, S.H. and Drew, R.D. 1990. An ecological characterization of the Tampa Bay Watershed. Office of Research and Development, Fish and Wildlife Service, U.S. Department of the Interior. Washington, D.C. Woolpert, Inc. 2005a. 1970's black and white aerial photography. Englewood, Colorado. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Woolpert, Inc. 2005b. 2005 one foot natural color ortho photographs – west Pasco County. Winter Park, Florida. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida. Woolpert, Inc. 2006. 2006 one foot natural color ortho photographs – northern District. Orlando, Florida. Available from the Southwest Florida Water Management District Mapping and GIS Section. Brooksville, Florida.

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Appendix A

DRAFT Technical Memorandum

January 18, 2016

TO: Douglas A. Leeper, MFLs Program Lead, Natural Systems and Restoration Bureau

THROUGH: Jerry L. Mallams, P.G., Manager, Water Resources Bureau

FROM: Tamera McBride, P.G., Senior Hydrogeologist, Water Resources Bureau

Subject: Crews Lake Rainfall Regression Model and Historic Percentile Estimations

A. Introduction

A rainfall regression model was developed to assist the Southwest Florida Water Management District (District or SWFWMD) in the establishment of minimum and guidance levels for Crews Lake, located in northern Pasco County between the Suncoast Parkway and U.S. Highway 41 (Figure 1). This document discusses development of the model, hydrogeologic evaluations used to support model development, and derivation of lake stage percentiles used to help develop proposed levels for the lake.

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Figure 1. Location of Crews Lake in northern Pasco County, Florida.

B. Background and Setting

Lake and Watershed Characteristics

Crews Lake is located in northern Pasco County, approximately two miles south of the Pasco-Hernando county line (Figure 2). The lake is located in the Pithlachascotee River/Bear Creek Watershed (as delineated by Ardaman and Associates, Inc., 2015) and is headwaters to the Pithlachascotee River (Figure 2). It is about 1.2 square miles and less than 20 feet deep. An earthen dike bisects the lake into two nearly equal halves, and a culvert in the dike connects the two sections of the lake when the water level is above 51.8 feet, NGVD 29.

The northern portion of the lake is generally dry or extremely shallow and swampy except during the rainy season (Trommer, 1987). During wet periods, water from the southern portion of the lake generally flows northward through the culvert to provide water to the northern section of the lake. Cherry and others (1970) report that when water levels drop below the culvert, the northern portion of the lake recedes more rapidly than the southern portion. The difference in decline was attributed to the lake water being drained to the Floridan aquifer via Hernasco Sink, located in the northern portion of the lake (Figure 3). A large part of the Crews Lake drainage basin is internally drained due to the many sinkholes and depressions that collect local runoff and recharge the Floridan aquifer. Thus, drainage is highly localized and, under normal conditions, does not contribute much water to Crews Lake (Trommer, 1987).

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Figure 2. Watershed delineation and topography.

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Figure 3. Sinkhole locations in and near Crews Lake and other area hydrologic features (from Trommer, 1987).

Jumping Gully flows westward into the east side of the lake south of the earthen berm and provides intermittent inflow during wet periods. The Masaryktown Canal is connected to the northern portion of the lake. The 20-foot deep and 50-foot wide canal was constructed in the mid-1960s and provides flood drainage to the lake from the Masaryktown and Squirrel Prairie areas. The Masaryktown canal only flows into the lake from the canal following heavy rainfall, and water reverses flows north into the canal from Crews Lake during wet periods. Trommer (1987) notes that flow in the canal was observed flowing into a sinkhole in the canal bottom about one mile north of U.S. Highway 41 following heavy rains in the fall of 1979.

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Physiographic Setting

White (1970) classified the physiographic area where Crews Lake is located as the Gulf Coastal Lowlands physiographic region (Figure 4), which is a lowland area between the Brooksville Ridge and the Coastal Swamp (continuous areas of swamp adjacent to the coast). Brooks (1981) categorizes the Crews Lake area as the Land-O-Lakes area of the Tampa Plain. The Tampa Plain is described as a lowland area where karst features are related to the occurrence of the Tampa Limestone. The Land-O-Lakes area is described as a plain with elevations between 50 and 80 feet with many small lakes (Figure 5).

Figure 4. Physiographic Provinces (White, 1970) and topography.

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Figure 5. Physiographic Provinces (Brooks, 1981) and topography.

Gilboy and Moore (1982) identified two distinct areas in and near the Cross Bar Ranch Wellfield, which is located to the east of the lake (Figure 6). The northern province lacks wetlands and has large scrub oaks, while the southern province has many lakes, sinks, and cypress heads and is considered to be a stable wetlands area. The change largely coincides with a transition from a shallow water table semi-confined Upper Floridan aquifer over the southern half of Cross Bar Ranch wellfield to the mostly unconfined, deep water table setting over the northern half.

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Figure 6. Hydrologic Anomaly and Physiographic Provinces (from Hutchinson, 1985).

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Hydrogeologic Setting

A thin, but mostly continuous clay layer underlies areas adjacent to Crews Lake and the southwestern part of the drainage basin (Trommer, 1987). The clay layer thickens toward the east near the Brooksville Ridge. Trommer reports that driller’s logs indicate the clay layer is breached by relict sinks in many places. More breaches appear in the northern and eastern areas of the basin than other areas. Consequently, in some areas, the surficial deposits may contain water only during wet periods (Trommer, 1987).

There are several sinkholes and a history of sinkhole and subsidence occurrence in and around Crews Lake. Hernasco Sink (Figure 3) in Crews Lake is about 10 feet in diameter and was estimated to drain about half (6.5 million gallons per day (mgd)) of the inflow to the lake (Cherry and others, 1970). Other research estimated flow into the sinkhole to be between 13 and 16 mgd (U.S. Geological Survey, Sixth Advanced Ground-Water Seminar, unpublished field trip guidebook, 1970). Hernasco Sink is disconnected from the lake when the lake is dry and under the lake when water levels are high. The top of the Upper Floridan aquifer is about 20 to 30 feet above sea level (Figures 7 and 8), while the bottom of the sink was measured to be about 25 feet above sea level (Trommer, 1987). The sinkhole is open directly to the Floridan aquifer.

A second sinkhole is located about 800 feet west of Hernasco Sink. Trommer (1987) referred to it as Sinkhole A (Figure 3) and noted it as being about 150 feet in diameter. It has no external source of recharge except limited local runoff. Water level fluctuations corresponded, for the most part, to Hernasco Sink during Trommer’s study period. A third sinkhole, Sinkhole B (Figure 3), is located in Crews Lake about 800 feet southwest of Hernasco Sink and is about 250 feet in diameter. Water levels in Sinkhole B did not correspond to those in Hernasco Sink or Sinkhole A during Trommer’s study period. Water levels in Sinkhole B were reported to remain relatively constant during dry periods while levels continued to drop in the other two sinkholes. Water levels in Sinkhole B reacted rapidly to rainfall. Trommer concluded that Sinkhole B appears to be recharged from the surficial aquifer and probably has little or no direct connection to the Upper Floridan aquifer.

The existence of sinkholes and depressions in the vicinity of Crews Lake are evident in views of area topography and have also been noted in historic accounts. Figure 9 shows vertically enhanced topography near Crews Lake. Several sinkholes and circular depressions are evident on the west side of the lake. The locations of historical accounts of sinkholes and subsidence are shown in Figures 10 and 11, respectively.

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Figure 7. Hydrogeologic cross section of Cross Bar Ranch Wellfield (from Hutchinson, 1985).

Figure 8. Location of Hydrogeologic cross section in Figure 7 (from Hutchinson, 1985).

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Figure 9. Topography showing sinkholes and circular depression features near Crews Lake.

Figure 10. Sinkholes near Crews Lake (data accessed through SWFWMD GIS layer files and collected by the Florida Sinkhole Institute and the SWFWMD).

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Figure 11. Subsidence Incidence Reports (not confirmed sinkholes) near Crews Lake (data accessed through SWFWMD GIS layer files and collected by the Florida Geological Survey).

Head differences between the surficial aquifer and the Upper Floridan aquifer are generally small, and water level fluctuations in both aquifers mimic each other and the water level of Crews Lake (Figures 12 and 13). Figure 12 shows that water levels in the Cross Bar 1W Wolfe surficial and Upper Floridan aquifer wells located east of Crews Lake and Cross Bar 2W Crews Lake surficial and Upper Floridan wells located west of Crews Lake have fluctuated as much as 20 feet. Trommer (1987) indicates this correlation of surficial and Upper Floridan well levels suggests a hydraulic connection through a leaky confining bed. Groundwater flow in the Upper Floridan aquifer is generally from southeast to northwest in the vicinity of Crews Lake. Trommer determined that water level fluctuations in Hernasco Sink and Sinkhole A corresponded to water level fluctuations in an Upper Floridan aquifer well near Masaryktown.

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Figure 12. Water level elevations in Crews Lake and Cross Bar surficial and Floridan aquifer wells 1W and 2W (1979-2015).

Figure 13. Location of Cross Bar surficial and Floridan aquifer wells 1W and 2W.

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CROSS BAR 2W CREWS LAKE SURF AQ (CB-2W) (SID 20533)

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The approximate center of Crews Lake is about 1.5 miles west of the Cross Bar Ranch Wellfield western boundary (Figure 14). This wellfield is part of the eleven regional water supply wellfields collectively referred to as the Central System Facilities that are operated by Tampa Bay Water (TBW). Groundwater withdrawals began at the Cross Bar Ranch Wellfield in 1980 and incrementally increased to a peak 12-month moving average withdrawal quantity of approximately 31 mgd in 1992 (Figure 15). The peak monthly withdrawal quantity from the wellfield was approximately 41 mgd in 1993 (Figure 16). As can be seen in Figures 15 and 16, groundwater withdrawals have declined significantly since they peaked in the early 1990s and have recently been at a 12-month moving average quantity of 12 to 15 mgd. Current withdrawal quantities are similar to the quantities withdrawn when the wellfield began operation in 1980.

Figure 14. Crews Lake and the Cross Bar Ranch Wellfield.

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Figure 15. Cross Bar Ranch Wellfield 12-month moving average groundwater withdrawals (April 1980 – July 2015).

Figure 16. Cross Bar Ranch Wellfield monthly groundwater withdrawals (April 1980 - July 2015).

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Total estimated water use in the area inclusive of other uses such as agriculture and domestic supply wells is presented as sums for radial distances from the approximate center of the lake in Figures 17 through 19. The most current estimated and reported water use available when this report was written was for the period 1992 through 2012.

Changes to surficial and Upper Floridan aquifer levels in the vicinity of Crews Lake were evaluated using the Integrated Northern Tampa Bay Model developed by Geurink and Basso (2013). A scenario where TBW central wellfield system withdrawals were reduced to their mandated recovery quantity of 90 mgd from the 11 Central System Facilities wellfields was evaluated largely based on the 2008 distribution of TBW withdrawals. It was estimated that the average predicted drawdown in the Upper Floridan aquifer near Crews Lake ranges from 0.5 feet in the southern end of the lake to 1.3 feet near the center of the lake to 1.8 feet in the northern end of the lake. Drawdown in the surficial aquifer at Crews Lake was not simulated, because the northern edge of the surficial aquifer within the model domain was south of Crews Lake.

Figure 17. Location of withdrawals near Crews Lake.

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Figure 18. Reported and estimated water use within 1, 2 and 3 miles of Crews Lake (1992 – 2012).

Figure 19. Reported and estimated water use within 4, 5 and 6 miles of Crews Lake (1992 – 2012). Note that y-axis scale differs from that shown in Figure 18.

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Rainfall Regression Long-Term Historic Lake Percentile Estimation

The procedure to establish minimum and guidance levels for lakes considers long-term lake stage percentiles. In the absence of a long-term water-level data, a rainfall based regression model may be constructed and used to model lake stage fluctuations and create a long-term water level record. A first step in developing a rainfall regression model is the delineation of “Historic” and “Current” time periods. A Historic time period is a period when there are little to no groundwater withdrawal impacts on the lake, and the lake’s structural condition is similar or the same as the present day. In contrast, a Current time period is a recent long-term period during which withdrawals and structural alterations are stable. To identify Historic and Current time periods, an evaluation of hydrologic changes in the vicinity of the lake is conducted to determine if the water body has been significantly impacted by groundwater withdrawals. Examples of hydrological changes that are reviewed include drainage modifications, dredging, filling and modifications to the lake outlets.

Stage data from the Historic period are typically used to establish a statistical relationship (regression) with rainfall. The rainfall regression model is then used to extend the available stage record (i.e., develop a 60 year or longer record) for calculation of long-term P10, P50 (median), and P90 lake stage percentiles. The P10, P50 and P90 are, respectively, the water level elevations equaled or exceeded ten, fifty and ninety percent of the time on a long-term basis. The rainfall regression model can then be used to evaluate whether the lake has been fluctuating consistently with climate variability (primarily rainfall).

The rainfall regression method (Ellison, 2010) involves development of a Line of Organic Correlation (LOC) between lake stage and rainfall. The LOC is a linear fitting procedure that minimizes errors in both the x and y directions and defines the best-fit straight line as the line that minimizes the sum of the areas of right triangles formed by horizontal and vertical lines extending from observations to the fitted line (Helsel and Hirsch, 1992). The magnitude of the slope of the LOC line is calculated as the ratio of the standard deviations of the x and y variables and its sign, i.e., whether it is positive or negative, determined by the sign (+ or -) of the correlation coefficient (r). The LOC approach, rather than a simple linear regression approach is preferable for the rainfall-regression method since it produces a result that better retains the variance (and therefore retains the "character") of the original data.

Rainfall for the LOC model is correlated to lake water-level data using inverse linearly-weighted rainfall sums. The weighted-sums ascribe higher weight to more recent rainfall and progressively less weight to rainfall in the past. For the rainfall regression method, weighted sums varying from 6 months to 10 years are used to develop

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separate models, and the model with the highest coefficient of determination (r2) is chosen as the best-fit model.

Crews Lake Water-Level Data and Identification of Historic Data

Period of record (POR) lake stage data, i.e., surface-water elevations for Crews Lake relative to NGVD 29 obtained from the District's Water Management Information System (WMIS) database, Site Identification (SID) numbers 734366, 734367, 782429, 20506, and 777811 (Figures 20 and 21). Surface-water elevations have been recorded since March 1964 with the early data collected by the U.S. Geological Survey and the more recent data collected by the SWFWMD. A long-term composite stage record was developed, based on evaluation of available data (Figure 22) and was considered the long-term data for the lake. The resultant composite record was ultimately used to develop the rainfall LOC model.

The data for Crews Lake North near Loyce (SID 734366) were not included in the composite POR lake stage data set, because they were collected in a sinkhole at the far northern end of the lake (Figure 21) that separates from the lake when water levels in the lake decline. Although the gage location of the Crews Lake South near Loyce (USGS Pre-1979) gage (SID 734367) is upstream of the earthen dike that separates the two sections of the lake (Figure 21), most of the data were above the elevation of the bottom of the culvert that connects the two pools. Therefore, they were considered appropriate for characterization of lake water levels and use in rainfall regression modeling. It should be noted, however, that there were several dry gage readings noted in the U.S. Geological Survey’s field data collection notes for the early data. There was not a low-water gage in place during the early data collection period, which means lower levels occurred that were not measured. Historical aerial photographs show the lake has experienced extremely low levels, even prior to construction of the wellfield (Leeper, et al., 2016). However, those low levels were not included in the early data record, due to water levels that receded below the staff gage and absence of a low water gage. The District installed a low-water gage in 2007 in the southern pool, near the current main gage, so measurements for those lower levels appear in the data record.

The final, POR, composite data set used for characterization of lake water levels and model development is shown in Figure 22 and consists of records merged from several different gages. The period from 1964 through 1980 is considered historic, since pumping at the nearby Cross Bar Ranch Wellfield began in 1980 and started significantly increasing around 1985.

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Figure 20. Crews Lake water elevation data considered for the LOC model (1964 – 2015).

Figure 21. Location of Crews Lake water-level gages.

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Figure 22. Crews Lake water-level data (1964 – 2015).

Rainfall Data

Available rain-gage data were inventoried and sorted by distance from Crews Lake and by their POR to locate rain data from sites closest to the lake for compilation of a long-term rainfall record that could be used to develop a rainfall regression LOC model and predict long-term lake levels with the model. The rainfall data ultimately used for model development and application were selected based on distance of the gaging stations from the lake and the ability of the model to predict lake levels that reasonably matched measured lake levels. The main gages used to construct the rainfall time-series used in the rainfall regression LOC model were St. Leo NWS (SID 18901), Myrtle Lake (SID 19057), Weeki Wachee NWS (SID 20915), Gowers Corner Tower (SID 20189), Crews Lake East (SID 20537), and Cross Bar Ranch Wellfield (multiple gage average). The model time step is daily, which requires daily rainfall for use in the model. Daily data missing from the main rain gages were infilled with readings from the next closest gage with available data. Locations of the main gages and gages used to infill missing daily data are shown in Figure 23.

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Figure 23. Rain-gage locations for measurements used in the rainfall regression model for Crews Lake.

Crews Lake Rainfall Regression Model and Historic Percentiles

Multiple rainfall regression LOC models were developed using weighted sums varying from 6 months to 10 years. The models were optimized by selecting data from nearby rain gages that produced the fit between the model and observed data. After rainfall was selected, the model decay series with the highest coefficient of determination (r2) was chosen as the best-fit model. Models for the simulation period January 1, 1946 through August 31, 2015 were developed and calibrated to lake stage data and rainfall data collected from March 20, 1964 through December 31, 1980. The calibration period ended in 1980, when groundwater withdrawals at Cross Bar Ranch Wellfield began. A time series plot of actual (i.e., observed) and modeled water levels for the 1946-1980 calibration period is shown in Figure 24. The modeled record included periods when estimated water surface elevations were higher than the highest value of 56.6 feet above NGVD 29 that has been measured at the lake gaging stations. Model predicted values higher than that elevation occurred, but were assigned a value of 56.6 feet

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above NGVD 29 for further analyses. Physical limitations that constrain model peaks are not parameterized in the model.

A comparison of measured and modeled percentiles for the calibration period is presented in Table 1. For the calibration period, the model-derived P10 percentile was 0.1 foot lower than the measured P10; the model-derived P90 percentile was 0.4 foot lower than the measured P90; and the model-derived P50 percentile was 0.2 foot lower than the measured P50. The best-fit LOC model for predicting water levels in Crews Lake exhibited a coefficient of determination (r2) of 0.66 using a two-year rainfall decay series and may be simplified as:

𝑦�̂� = 𝑏0 + 𝑠𝑖𝑔𝑛[𝑟] ∗ 𝑏𝑖 ∗ 𝑥𝑖

where

𝑦�̂� = the estimate of lake stage expressed as an elevation in feet above NGVD 29

𝑏0 = the y intercept, in this case 38.81 feet above NGVD 29

𝑏𝑖 = the regression slope; in this case 0.2388

𝑠𝑖𝑔𝑛[𝑟] = the algebraic sign (+ or -) of the correlation coefficient; in this case “+”

𝑥𝑖 = the inversely, linearly-weighted two-year cumulative rainfall sum in inches

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Figure 24. Crews Lake LOC-modeled and actual (i.e., observed) water levels for the calibration period (1964 – 1980). Predicted water levels higher than the highest observed water level of 56.6 feet above NGVD 29 were assigned an elevation of 56.6 feet above NGVD 29.

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Table 1. Comparison of Crews Lake calibration period percentiles.

Calibration Percentiles

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P10 54.7 54.6

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* Percentiles listed include the water surface elevation equaled or exceeded ten (P10), fifty (P50) and ninety (P90) percent of the time

The resulting best-fit model is an estimated lake stage record intended to exclude the most significant groundwater withdrawals near the lake. The best-fit model was used to estimate water levels for the period from January 1, 1946 through August 31, 2015, and is shown in Figure 25. Model-predicted water levels matched actual, i.e., observed period of record data reasonably well when water levels were around the P50 or higher, indicating that the lake fluctuated as predicted during periods with average to high rainfall. The modeled record included periods when estimated water surface elevations were higher than the highest value of 56.6 feet above NGVD 29 that has been measured at the lake gaging stations. Model predicted values higher than that elevation occurred, but were assigned a value of 56.6 feet above NGVD 29 for further analyses, including development of lake stage exceedance percentiles (Figure 25). Physical limitations that constrain model peaks are not parameterized in the model. Truncation in this case does not affect estimated long-term percentiles used to help develop minimum and guidance levels, since the truncation elevation exceeded the estimated percentiles. Where peaks are truncated, elevations are expected to have been at or above the highest observed stage of 56.6 feet above NGVD 29. Model-predicted levels tended to be lower than observed water levels at lower lake stages, during dryer periods. The use of multiple rain gage combinations was evaluated, without complete success, in an attempt to improve the match between model-predicted and observed, lower lake stages. To assess whether groundwater withdrawals were the cause for the low observed lake stages that deviated from the LOC model-predicted estimates, predicted groundwater drawdown from the Integrated Northern Tampa Bay Model and reported and estimated groundwater use were analyzed.

Upper Floridan aquifer drawdown based on the average of Integrated Northern Tampa Bay model cells that include the lake south of the earthen dike is predicted to be less

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than 0.9 foot for the model scenario that simulates 90 mgd of groundwater withdrawals from the Central System Facilities. Groundwater withdrawals from Cross Bar wellfield in the 90 mgd scenario were 16.1 mgd. Actual 6-year average Cross Bar withdrawals from 2008-2013 were 14.2 mgd. Therefore, the model results for the 90 mgd scenario are showing slightly more drawdown than under current pumping conditions. Drawdown in the vicinity of Crews Lake is relatively small compared to other areas in the model domain. Groundwater use in the vicinity of the lake has decreased significantly over time, as shown in Figures 18 and 19. For example, groundwater use within four miles of the lake has decreased from a high in 1992 of nearly 30 mgd to less than 10 mgd more recently in 2012. Collectively, this information suggests that groundwater withdrawals are not the primary cause resulting in separation of the more recent observed low lake levels and the predicted lake levels.

Figure 25. Crews Lake LOC-model predicted and actual (i.e., observed) water levels for period from January 1, 1946 through August 31, 2015. Predicted water levels higher than the highest observed water level of 56.6 feet above NGVD 29 were assigned an elevation of 56.6 feet above NGVD 29.

One possible explanation for the differences between the recent low observed and the modeled water levels could be the existence of sinkholes within the lake basin. Figures

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9 through 11 show that the lake and area around it is vulnerable to sinkhole development. Furthermore, a sinkhole occurrence was documented in the southern portion of the lake (where the current staff gages are located) in 1986 as shown in Figure 10. Another possible explanation for the differences could be that low-lake levels occurred in the early part of the water level record, but were not measured. As noted in the earlier discussion of lake-level data, there were several dry gage readings noted in the U.S. Geological Survey’s field data collection notes for the early data. There was not a low-water gage in place during the early data collection period, which means lower levels occurred that were not measured. Historical aerial photographs show the lake has experienced extremely low levels, even prior to construction of the wellfield (Leeper et al., 2016). However, those low levels were not included in the early data record, since the gage was not located in the deepest area of the lake. This situation would tend to bias the LOC model to simulating lake stages higher than actual during similar rainfall conditions.

Dry gage days occurred during the early record of the calibration period, and were used to model and extend lake levels back to 1946 and forward in time to 2015. The lack of measured water-level data at low levels in the early record of the calibration period may have affected the predictive capability of the model when extreme low lake levels occurred in recent years.

The model was considered sufficient to develop a long-term, historic water level record to support development of proposed minimum and guidance levels, despite potential issues with predicting extreme high and low water levels. Model-predicted lake stage values truncated at a maximum elevation equal to the maximum observed elevation of 56.6 feet above NGVD 29 were used to calculate historic lake stage percentiles listed in Table 2. These percentiles were used to support development of proposed levels for Crews Lake. It should be noted that these percentiles and the modeled record from which they were derived are not recommended for other water management analyses such as floodplain mapping.

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Table 2. Crews Lake long-term Historic percentiles.

Crews Lake Long-term Historic Percentiles*

January 1, 1946 through August 31, 2015

(feet above NGVD 29)

Percentiles

P10 55.3

P50 52.0

P90 48.9

* Percentiles listed include the water surface elevation equaled or exceeded ten (P10), fifty (P50) and ninety (P90) percent of the time

Conclusions

Crews Lake is located in an area with a thin, but mostly continuous underground clay layer. Runoff is generally low, and transmissivity is high due to the presence of karst, resulting in a semi-confined Upper Floridan aquifer. There is a documented history of sinkhole occurrence in and around the lake as well as documented sinkholes in both the northern and southern sections of the lake. The Cross Bar Ranch Wellfield located just east of the lake began operating in 1980, with groundwater withdrawals peaking in the early 1990s. Reported and estimated water use in the area has significantly declined over time. Water use was nearly 30 mgd within four miles of the lake in 1992 and was reduced to less than 10 mgd in 2012. Direct withdrawals from the lake or occurrence of sinkholes could influence lake levels. There are no documented direct withdrawals from the lake; however, there are documented sinkhole occurrences that occurred both before and after wellfield withdrawals began in 1980. The lake is not classified as structurally altered.

Long-term water levels for Crews Lake were simulated using a rainfall regression technique. A best-fit LOC rainfall regression model was calibrated to water-level data from March 20, 1964 through December 31, 1980 using weighted two-year cumulative rainfall sums in inches. Observed data matched the model reasonably well for lake levels other than at extremely high and low levels. Groundwater withdrawals near the lake did not appear to be the driving cause of the mismatch between modeled and observed low lake levels, since water withdrawals in the vicinity of the lake declined significantly over the period that includes the mismatch. One major factor that could possibly cause the observed levels lower than the modeled levels is the role of

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sinkholes in the lake during dry periods. Another factor could be unmeasured levels in the early record when water levels were below the staff gage, as noted in the U.S. Geological Survey’s field data collection notes. Historical aerial photographs show the lake has experienced extremely low levels, even prior to construction of the wellfield. The absence of those low measured lake levels during the calibration period might have affected the predictive capability of the model to reproduce lower levels that matched observed levels. Since 2007, lower levels have been recorded, because a low-water gage was installed in the southern section of the lake. Those lower levels were documented in more recent years, due to installation of a low-water gage in 2007.

In conclusion, a water level record for Crews Lake developed using LOC-modeled data and observed lake stage data was used to develop a long-term, historic water level record. The historic record was considered sufficient for development of historic lake stage percentiles that could be used to support development of proposed minimum and guidance levels for Crews Lake. The record was not developed or recommended to support other water management activities such as floodplain mapping.

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References

Alt. D and H.K. Brooks. 1965. Age of the Floridan Marine Terraces: Journal of Geology, v. 73, no. 2, p. 406-411.

Ardaman and Associates, Inc. 2015. Floodplain Justification Report for the Bear Creek and Pithlachascotee River Watersheds within Pasco and Hernando County, Florida. Orlando, Florida. October 2015.

Brooks, H.K. 1981. Physiographic divisions of Florida: map and guide. Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Gainesville, Florida.

Cherry, R.N., J.W. Stewart, and J.A. Mann. 1970. General Hydrology of the Middle Gulf Area, Florida: Florida Bureau of Geology Report of Investigations.

Ellison, D.L. 2010. Rainfall Based Regression Lake Stage Model Used to Support Establishment of Lake Minimum levels and Evaluation of Compliance. Southwest Florida Water Management District. Brooksville, Florida.

Geurink, J.S. and R. Basso. 2013. Development, Calibration, and Evaluation of the Integrated Northern Tampa Bay Hydrologic Model. Prepared for Tampa Bay Water and Southwest Florida Water Management District. Clearwater and Brooksville, Florida. March 2013.

Gilboy, T. and D. Moore. 1982. Hydrogeologic Analysis Cross Bar Wellfield, Pasco County. Southwest Florida Water Management District. Brooksville, FL. February 1982.

Healy, H.G. 1975. Terraces and shorelines of Florida: Florida Bureau of Geology Map Series 71.

Helsel, D.R. and Hirsch, R.M. 1992. Statistical methods in water resources. Studies in Environmental Science 45. Elsevier. New York, New York.

Hutchinson, C.B. 1985. Hydrogeology of the Cross Bar Ranch Well-Field Area and Projected Impact of Pumping, Pasco County, Florida. U.S. Geological Survey Water-Resources Investigations Report 85-4001.

Leeper, D., Y. Ghile, and T. McBride. 2016. Proposed Minimum and Guidance Levels for Crews Lake in Pasco County, Florida.

Trommer, J.T. 1987. Potential for Pollution of the Upper Floridan Aquifer from Five Sinkholes and One Internally Drained Basin in West-Central Florida. U.S. Geological Survey Water-Resources Investigations Report 87-4013.

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White, W. A. 1970. The geomorphology of the Florida peninsula. Geological Bulletin, No. 51. Bureau of Geology, Florida Department of Natural Resources. Tallahassee, Florida.

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Appendix B Draft Technical Memorandum

January 18, 2016

TO: Jerry L. Mallams, P.G., Manager, Water Resources Bureau

FROM: Tamera McBride, P.G., Senior Hydrogeologist, Water Resources Bureau Douglas A. Leeper, MFLs Program Lead, Natural Systems and Restoration Bureau Yonas Ghile, Ph.D., Senior Environmental Scientist, Natural Systems and Restoration Bureau

Subject: Crews Lake Initial Minimum Levels Status Assessment

A. Introduction

The Southwest Florida Water Management District (District) is evaluating and proposing minimum levels for Crews Lake, in accordance with Section 373.042 and 373.0421, Florida Statutes (F.S). Documentation regarding development of the minimum levels is provided by Leeper, et al. (2016) and McBride (2016).

Section 373.0421, F.S. requires that a recovery or prevention strategy be developed for all water bodies that are found to be below their minimum flows or levels, or are projected to fall below the minimum flows or levels within 20 years. In the case of Crews Lake and other water bodies with established minimum flows or levels in the northern Tampa Bay area, an applicable regional recovery strategy, referred to as the “Comprehensive Plan”, has been developed and adopted into District rules (Rule 40D-80.073, F.A.C.). One of the goals of the Comprehensive Plan is to achieve recovery of minimum flow and level water bodies that are located in the area affected by the Consolidated Permit wellfields (i.e., the Central System Facilities) operated by Tampa Bay Water. This document provides information and analyses considered for evaluating the status of the proposed minimum levels for Crews Lake.

B. Background

Crews Lake is located in northern Pasco County, approximately two miles south of the Pasco-Hernando county line between the Suncoast Parkway and U.S. Highway 41. The lake is located in the Pithlachascotee River/Bear Creek Watershed (as delineated by Ardaman and Associates, Inc., 2015) and is headwaters to the Pithlachascotee River. It is nearly 1,200 acres in size and less than 20 feet deep. An earthen dike

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bisects the lake into two nearly equal halves, and a culvert in the dike connects the two sections of the lake when the water level exceeds 51.8 feet above NGVD 29.

The approximate center of Crews Lake is about 1.5 miles west of the Cross Bar Ranch Wellfield western boundary, one of the eleven regional water supply wellfields comprising the Central System Facilities operated by Tampa Bay Water (Figure 1). From 2002 to 2005, a cutback in the withdrawal rates at most Central System Facility wellfields occurred in response to the development of several alternative water supply sources. As a whole, the wellfields were reduced from approximately 158 million gallons per day (mgd) to 90 mgd, although the timing and amount of reduction at each wellfield was variable. These cutbacks are evident in the withdrawal rates reported for the Cross Bar Ranch Wellfield (Figures 2 and 3).

Figure 1. Location of Crews Lake and the Cross Bar Ranch Wellfield.

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Figure 2. Cross Bar Ranch Wellfield withdrawals in mgd (1980 – 2015).

Figure 3. 12-month moving average of Cross Bar Ranch Wellfield withdrawals in mgd (1980 – 2015).

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Total estimated water use in the area inclusive of other uses such as agriculture and domestic supply wells is presented as sums for radial distances from the approximate center of the lake in Figures 4 through 6. The most current estimated and metered water use available when this memorandum was drafted was for the period 1992 through 2012.

Changes to aquifer levels in the vicinity of Crews Lake were evaluated using the Integrated Northern Tampa Bay Model developed by Geurink and Basso (2013). A scenario where Tampa Bay Water Central System Facilities withdrawals were reduced to their mandated recovery quantity of 90 mgd from the 11 wellfields was evaluated. It was estimated that the average predicted drawdown in the Upper Floridan aquifer near Crews Lake ranges from 0.5 feet in the southern end of the lake to 1.3 feet near the center of the lake to 1.8 feet in the northern end of the lake. Drawdown in the surficial aquifer at Crews Lake was not simulated, because the northern edge of the surficial aquifer within the model domain was south of Crews Lake.

Figure 4. Location of withdrawals near Crews Lake.

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Figure 5. Metered and estimated water use within 1, 2 and 3 miles of Crews Lake (1992 – 2012).

Figure 6. Metered and estimated water use within 4, 5 and 6 miles of Crews Lake (1992 – 2012). Note that y-axis scale differs from that shown in Figure 5.

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C. Minimum Levels Proposed for Crews Lake

Minimum levels proposed for Crews Lake are presented in Table 1, as described by Leeper, et al. (2016). Minimum levels represent long-term conditions that, if achieved, are expected to protect water resources and the ecology of the area from significant harm that may result from water withdrawals. The Minimum Lake Level (MLL) is the elevation that a lake's water levels are expected to equal or exceed fifty percent of the time on a long-term basis. The High Minimum Lake Level (HMLL) is the elevation that a lake's water levels are expected to equal or exceed ten percent of the time on a long-term basis. The MLL therefore represents the expected 50th percentile (P50) of long-term water levels, while the HMLL represents the expected 10th percentile (P10) of long-term water levels. To determine the status of minimum levels for Crews Lake or minimum flows and levels for any other water body, long-term hydrologic data or model results must be used.

Table 1. Proposed Minimum Levels for Crews Lake.

Proposed Minimum Levels Elevation in Feet

NGVD 29

High Minimum Lake Level 52.4

Minimum Lake Level 51.0

D. Status Assessment

The goal of a minimum levels status assessment is to determine if long-term lake levels are fluctuating in accordance with criteria associated with adopted or proposed levels, i.e., to determine whether or not the minimum levels are being achieved. Tools to assess lake status can include review of lake models, comparison of long-term water levels with adopted or proposed levels, review of groundwater modeling updates, review of rainfall records and, if necessary, investigation of other factors that could explain lake level fluctuations.

Since historical observed measurements are available, an empirical approach was used to compare observed Crews Lake data to adopted levels to assess whether the MLL and HMLL are currently being achieved. Because the MLL and HMLL represent long-term exceedance percentiles for the P50 and P10, respectively, full assessment of the MLL and HMLL using observed percentiles requires evaluation of long-term data. Status was evaluated by reviewing both long-term percentiles (P50 and P10) and long-term cumulative percentiles (P50 and P10) of the observed data for periods with varied

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start dates. A first approach for evaluating the lake status used the observed water level record. A second approach used a sample of the observed water levels in an attempt to reduce the weighting effect of variable data collection frequency.

First Approach

The period of record for Crews Lake water level elevations begins in March 1964, about 16 years prior to the start of groundwater withdrawals at the nearby Cross Bar Ranch Wellfield. While the period beginning in 1964 was evaluated, a more conservative approach that begins about the time Cross Bar Ranch Wellfield withdrawals started was also used to compare water levels to the proposed MLL and HMLL. If measured lake stages that occurred after groundwater withdrawals began and include a period of groundwater withdrawals greater than future anticipated withdrawals exceed the MLL and HMLL, it could be concluded that the proposed minimum levels are met and should be continued to be met. A period that starts about the time that Cross Bar Ranch Wellfield withdrawals began and coincides with the beginning of daily stage data (May 1979) was used to evaluate the lake relative to wellfield startup conditions. A 1979 start date was considered reasonable, since it coincided with the beginning of daily data collection in May 1979 and its inclusion would maximize the data used to calculate long-term levels without significantly pre-dating wellfield startup. Groundwater withdrawals have varied through time, and the assessment period includes withdrawals that are greater than future anticipated withdrawal rates at the Cross Bar Ranch Wellfield.

Groundwater withdrawals began at the Cross Bar Ranch Wellfield in 1980 and incrementally increased to a peak 12-month moving average withdrawal quantity of approximately 31 mgd in 1992 (Figure 3). The peak monthly withdrawal quantity from the wellfield was approximately 41 mgd in 1993 (Figure 2). Groundwater withdrawals have declined significantly since they peaked in the early 1990s and have recently been at a 12-month moving average quantity of about 12 to 15 mgd. Current withdrawal quantities are similar to the quantities withdrawn when the wellfield began operation in 1980.

The cumulative median (P50) and cumulative P10 surface water elevations were calculated for the observed data starting in March 1964 and May 1979, ending in October 2015. Cumulative medians (P50s) for the evaluated start dates ended with values above the proposed MLL. Similarly, cumulative P10s for the evaluated periods ended above the proposed HMLL (Figures 7 and 8). This evaluation using empirical data suggests the proposed MLL and HMLL for Crews Lake are being achieved. Because the evaluation included periods of groundwater withdrawals at the Cross Bar Ranch Wellfield that are greater than those anticipated for the twenty-year planning period that must be evaluated for minimum flows and levels per Section 373.0421, F.S., the proposed minimum levels are also expected to be achieved for the planning period.

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Figure 7. Crews Lake observed data cumulative P50 (dark purple line) and cumulative P10 (dark green line) water levels starting in 1964 (beginning of measured water level elevation record) compared to the proposed MLL of 51.0 (horizontal light purple line) and HMLL of 52.4 feet, NGVD29 (horizontal light green line).

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Figure 8. Crews Lake observed data cumulative P50 (dark purple line) and cumulative P10 (dark green line) water levels starting in May 1979 (near beginning of Cross Bar Ranch Wellfield startup) compared to the proposed MLL of 51.0 (horizontal light purple line) and HMLL of 52.4 feet, NGVD29 (horizontal light green line).

Percentiles of the empirical data for the same time periods used to evaluate cumulative percentiles were also calculated, as shown in Table 2. The first long-term period analyzed started when lake level measurements began in March 1964, and incorporates 16 years of water levels prior to groundwater withdrawals at Cross Bar Ranch Wellfield. The second period started in May 1979, near the time withdrawals began at Cross Bar Ranch Wellfield. The P50 and P10 percentiles for each time period exceed the proposed minimum levels, while including groundwater withdrawals that were greater than those anticipated for the planning period. Based on this, long-term levels that include peak groundwater withdrawals exceeded the proposed MLL and HMLL.

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Table 2. Comparison of Crews Lake stage exceedance percentiles derived from each time period assessed compared to the minimum and high minimum levels proposed for the lake.

Percentile

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Proposed Minimum Levels


P10 53.7 53.6 52.4

P50 51.7 51.6 51.0

a Period of record observed lake stage. b Lake stage record starting near the time when withdrawals began at Cross Bar Ranch Wellfield. Second Approach

An alternate method for evaluating lake status was investigated due to the variability in the frequency of observed data. Figure 9 shows that data collection was more frequent (nearly daily) for the period May 4, 1979 to September 30, 1990 and less frequent for other periods in the data record. The average number of days between measurements for the entire period of record was approximately 16 days. Given the average daily data gap, data were sampled every 16 days in an attempt to reduce the weight of more frequent data collection periods on the cumulative median and P10 while maximizing the use of available data points to calculate long-term percentiles. Where data were unavailable exactly on the 16 day increment, the missing 16-day increment was infilled with an average of the five previous days, if data were available, to maximize use of available data.

Samples in 16 day increments were evaluated for periods with two different start dates. The first period began in March 1964 and corresponds to the first observed Crews Lake water level. The second sample period began in in May 1979. A May 1979 start date was considered reasonable, since it coincided with the beginning of daily data collection, and its inclusion would maximize the data used to calculate long-term levels without significantly pre-dating wellfield startup. It is likely that groundwater pumping and testing for the new wellfield would have begun around 1979 when the frequency of water level data collection increased.

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Figure 9. Crews Lake water-level data (1964 – 2015).

The cumulative median (P50) and cumulative P10 surface water elevations were calculated for the 16-day sample series of observed data starting in March 1964 and May 1979, ending in October 2015. Cumulative P10s for all evaluated periods ended above the proposed HMLL (Table 3). Cumulative medians (P50s) for the start dates evaluated ended with values above the proposed MLL for the period starting in 1964, and slightly below the MLL for the period starting in 1979. Previous District investigations of lake stage percentiles indicate that cumulative medians based on a long-term period that spans 60 years or longer typically would exhibit small deviations from the MLL, if structural alterations or changes in water withdrawal quantities did not occur during that 60 year period (unpublished data). The Crews Lake 36 year evaluation period that starts in 1979 is much shorter than a 60-year or longer period, but was used specifically to evaluate lake levels after wellfield withdrawals began, during periods that included peak pumping rates. Given that the evaluation period is slightly greater than half the length of an optimal long-term 60-year period, more variation of the cumulative median is expected with a 36-year cumulative median than typically occurs for a 60-year cumulative median.

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Table 3. Comparison of Crews Lake stage exceedance percentiles derived from a 16-day sample of each assessed time period to minimum and high minimum levels proposed for the lake.

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Proposed Minimum Levels


P10 53.6 53.3 52.4

P50 51.1 50.7 51.0

a Period of record observed lake stage. b Lake stage record starting near the time when withdrawals began at Cross Bar Ranch Wellfield.

The cumulative P10 for the 16-day sample series of observed data from March 1964 through October 2015 exceeded the proposed HMLL as shown in Figure 10. The cumulative P50 shown for this same time period also exceeded the MLL (Figure 10). Figure 11 shows the cumulative P10 for the period May 1979 through October 2015 exceeded the proposed HMLL. The cumulative P50 for the same period remained above the proposed MLL from 1979 into late 2010, and then it dips slightly below the proposed minimum level until the end of the evaluation period. It should be noted that both periods analyzed included peak pumping (Figures 2 through 6) and are considered conservative, since there have been significant reductions in area groundwater withdrawals. Given the significant reductions in groundwater pumping since the early 1990s, additional analysis was considered to determine if factors other than groundwater withdrawals may have caused the cumulative P50 for the shorter assessment period to fall below the proposed MLL.

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Figure 10. Crews Lake 16-day sample of observed data cumulative P50 (dark purple line) and cumulative P10 (dark green line) water levels starting in 1964 compared to the proposed MLL of 51.0 (horizontal light purple line) and HMLL of 52.4 feet, NGVD29 (horizontal light green line).

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Cumulative P50 and P10 was calculated based on a 16-day sample of observed data. Missing observed data on the sample day, was infilled with an average of readings from the previous 5 days when available. A sample of the data was used as an alternate method to assess status due to variable observed data frequency. A sample frequency of 16 days was used based on the average number of days between observed data records over the period of record The intent was to maximizing the number of observed data points used to calculate the cumulative P50 and P10.

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Figure 11. Crews Lake observed data cumulative P50 (dark purple line) and cumulative P10 (dark green line) water levels starting in May 1979 compared to the proposed MLL of 51.0 (horizontal light purple line) and HMLL of 52.4 feet, NGVD29 (horizontal light green line).

Groundwater Withdrawals

Information concerning groundwater withdrawals was investigated to assess factors that may have led to recent low water levels in Crews Lake. Groundwater withdrawals began at the Cross Bar Ranch Wellfield in 1980 and incrementally increased to a peak 12-month moving average withdrawal quantity of approximately 31 mgd in 1992 (Figure 3). The peak monthly withdrawal quantity from the wellfield was approximately 41 mgd in 1993 (Figure 2). Groundwater withdrawals have declined significantly since they peaked in the early 1990s and have recently been at a 12-month moving average quantity of about 12 to 15 mgd. Current withdrawal quantities are similar to the quantities withdrawn when the wellfield began operation in 1980.

The relationship between total monthly groundwater withdrawals at Cross Bar Ranch Wellfield and average monthly Crews Lake stage is shown in Figure 12. The graphic shows that pumping is not well correlated with groundwater withdrawals. The effects of

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Cumulative P50 and P10 was calculated based on a 16-day sample of observed data. Missing observed data on the sample day, was infilled with an average of readings from the previous 5 days when available. A sample of the data was used as an alternate method to assess status due to variable observed data frequency. A sample frequency of 16 days was used based on the average number of days between observed data records over the period of record The intent was to maximizing the number of observed data points used to calculate the cumulative P50 and P10.

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groundwater withdrawals were further considered by evaluating the results of the Integrated Northern Tampa Bay Model.

Geurink and Basso (2013) evaluated changes in aquifer levels with the Integrated Northern Tampa Bay Model. Upper Floridan aquifer drawdown based on the average of Integrated Northern Tampa Bay model cells that include the lake south of the earthen dike, where the staff gage is currently located, is predicted to be less than 0.9 feet for the model scenario that simulates 90 mgd of groundwater withdrawals from the Central System Facilities. Groundwater withdrawals from Cross Bar wellfield in the 90 mgd scenario were 16.1 mgd. Actual 6-year average Cross Bar withdrawals from 2008-2013 were 14.2 mgd. Therefore, the model results for the 90 mgd scenario are showing slightly more drawdown than under current pumping conditions. In comparison, drawdown in the vicinity of Crews Lake is relatively small compared to other areas in the model domain.

Reported and estimated groundwater use in the vicinity of the lake has decreased significantly over time, as shown in Figures 2 through 6. For example, groundwater use within four miles of the lake has decreased about 20 mgd between 1992 and 2012. There have been significant reductions in groundwater withdrawals near the lake since 1992. Given the significant reduction in reported and estimated groundwater withdrawals, reduced withdrawals from Cross Bar Wellfield, the relatively small predicted drawdown by the Integrated Northern Tampa Bay Model, and the lack of correlation between groundwater withdrawals and lake stage, groundwater withdrawals do not appear to explain the more recent low observed lake levels.

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Figure 12. Relationship between monthly Crews Lake water level elevations (feet above NGVD 29) and Cross Bar Ranch Wellfield groundwater withdrawal quantities (mgd).

Rainfall

Low rainfall in recent years is one possible factor that could have resulted in more recent low lake levels. Mean annual rainfall for the period 1930 to 2014 based on rainfall gage measurements near Crews Lake was 55.5 inches. Departure from mean rainfall for each year in the assessment period is shown in Figure 13. Since 1930, the three years with the lowest departure below the mean occurred in the recent record in 1999, 2000, and 2006. The three years with the highest departure above the mean are in the earlier part of the record in 1945, 1953, and 1975, showing a difference in rainfall patterns between the early and late record. For the period evaluated, the longest sustained departure below the mean lasted 9 consecutive years from 2005 to 2013, also in the recent record. Prior to record departure below the mean starting in 2005, there were three years above the annual mean (2002-2004) and three years below the annual mean (1999-2001), two of which were the first and second largest departure below the mean for the entire period. Annual rainfall was below the annual mean for 21 of the 35 years since the wellfield began operation in 1980. CH2MHILL (2011) notes that based on a water balance model that they completed for Crews Lake, the lake response to drought conditions is variable. CH2MHILL referenced Keesecker (2010) who attributed the differential response to the porous hydrogeology of northwestern

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Pasco County, and the need for the hydrologic system to receive above-normal rainfall to recover from extended dry periods. Occurrences of above normal rainfall are rare in recent years in comparison to the magnitude and frequency of below normal rainfall conditions.

Figure 13. Crews Lake area annual total rainfall departure from mean for the period 1930-2014.

The moving 10-year annual average rainfall for the period 1930 through 2014 is shown in Figure 14. This figure shows, similar to rainfall departure (Figure 13), that rainfall in the early period was significantly higher than the more recent period. The moving 10- year annual average peaks in 1950 at 62 inches and declines to a low in 2013 of 48.6 inches. Starting in 1993 and continuing through 2014, the 10-year moving annual average is below the average for the period 1930 through 2014. This decline corresponds with the decline in the lake stage cumulative median.

Considering rainfall departure from the mean for the period 1930 through 2014 and the moving 10-year annual average, rainfall in the recent period is significantly lower than rainfall in the early part of the record. Rainfall appears to have significantly contributed to the lower lake stages observed in the recent record.

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Figure 14. Crews Lake area moving 10-year average annual rainfall and mean for the period 1930-2014.

Other Considerations and Limitations

There are other known and unknown considerations that could have contributed to lower water levels in the recent Crews Lake stage record. They include sinkholes, data record issues, undocumented water withdrawals, and land use changes.

Development and closure of sinkholes during the evaluation period may have been discernible if a daily stage record was available for examining the rate of lake level decline; however, daily stage data is not available for most of the record. There is a documented history of sinkhole and subsidence occurrence in the area surrounding Crews Lake. Given this history, it is plausible that sinkholes may have altered the lake such that the lake stage seen in the early record could not be achieved during the more recent period.

The status assessment for Crews Lake was complicated, in part, due to data record issues. The data record used to evaluate status was collected by multiple agencies at variable frequencies from gages that were relocated several times. The stage data record includes several gaps, up to several years in length. Some water level records were surveyed, rather than recorded at a consistent staff gage location. There could be other, unidentified issues with the data, as well. This status assessment is a best effort based on the best available data.

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Undocumented water withdrawals other than those reported or estimated may have occurred in the evaluation period. However, no information is available regarding this factor and its potential effect on the lake’s water levels. If land use changes in the watershed occurred, they also could have affected lake levels. No analyses were, however, conducted to evaluate the magnitude of the effect of potential land use changes.

E. Conclusions

Two empirical approaches were used to evaluate the status of Crews Lake with respect to the proposed MLL and HMLL. The long-term P10 and P50 based on observed data exceeded the proposed HMLL and MLL, respectively, when percentiles were calculated starting in 1964 (beginning of the lake stage record) and 1979 (about the time Cross Bar Ranch Wellfield began operation) through October 2015. An alternative approach was also considered, given that the frequency of data collection was variable over the period of record. The goal of the second approach was to maximize use of available data while balancing the frequency of data used to calculate the percentiles. A sample of data was taken every 16 days. Where data were unavailable exactly on the 16-day increment, the missing 16-day increment was infilled with an average of the five previous days, if data were available. Based on this alternative assessment, the long-term P10 exceeded the proposed HMLL. The long-term P50 was 0.3 foot less than the proposed MLL. The range of variation below the MLL by 0.3 foot for the cumulative median starting in 1979 was not unexpected, given that the evaluation period was only slightly greater than half the length of an optimal long-term 60-year assessment period. To assess factors that may have affected the cumulative median, groundwater withdrawals, annual rainfall, and other issues were considered.

Based on the weight of evidence and limitations described in this technical memorandum, it is concluded that groundwater withdrawals do not appear to be the primary cause of lake stage decline at Crews Lake in recent years. Reported/estimated area groundwater withdrawals have declined significantly, since they peaked around 1992 and groundwater impacts estimated by the Integrated Northern Tampa Bay model are relatively small. Conversely, rainfall deficits occurred for many years over the same time period that groundwater withdrawals declined. Record departures below mean rainfall occurred in 1999, 2000, and 2006. The longest sustained departure over the period 1930 to 2014 occurred in recent years from 2005 to 2013, a period of 9 years. Other considerations that could have contributed to lower water levels in recent years could include occurrence of sinkholes, issues with the available data, existence of unknown water withdrawals, and possible land use changes.

Minimum flow and level status assessments are completed on an annual basis by the District and on a five-year basis as part of the regional water supply planning process.

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In addition, Crews Lake is included in the Comprehensive Environmental Resources Recovery Plan for the Northern Tampa Bay Water Use Caution Area (40D-80.073, F.A.C). Therefore, the analyses outlined in this document for Crews Lake will be reassessed by the District and Tampa Bay Water as part of this plan, and, as part of Tampa Bay Water’s Permit Recovery Assessment Plan (required by Chapter 40D-80, F.A.C. and the Consolidated Permit (No. 20011771.001)). Tampa Bay Water, in cooperation with the District, will assess the specific needs for recovery affected by groundwater withdrawals from the Central System Facilities. By 2020, if not sooner, an alternative recovery project will be proposed if it is determined that Crews Lake is not meeting its adopted minimum levels. The draft results of the Permit Recovery Assessment Plan are due to the District by December 31, 2018.

F. References

Ardaman and Associates, Inc. 2015. Floodplain Justification Report for the Bear Creek and Pithlachascotee River Watersheds within Pasco and Hernando County, Florida. Orlando, Florida. October 2015.

CH2MHILL. 2011. Crews Lake Natural Systems Restoration Feasibility Study. Prepared for Pasco County. March 2011.

Geurink, J.S. and R. Basso. 2013. Development, Calibration, and Evaluation of the Integrated Northern Tampa Bay Hydrologic Model. Prepared for Tampa Bay Water and Southwest Florida Water Management District. Clearwater and Brooksville, Florida. March 2013.

Keesecker, D. 2010. Crews Lake Investigation. Memorandum to Warren Hogg, Tampa Bay Water. June 18, 2010.

Leeper, D.A., Y. Ghile, and T. McBride. 2016. Proposed Minimum and Guidance Levels for Crews Lake in Pasco County, Florida. Southwest Florida Water Management District. Brooksville, Florida.

McBride, T. 2016. Technical Memorandum to Douglas A. Leeper, Subject: Draft Crews Lake, Rainfall Regression Model, Historic Percentile Estimations and Assessment of Minimum Levels Status. Southwest Florida Water Management District. Brooksville, Florida.

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