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An Examination of the Water Budget in the Middle Potomac-Anacostia-Occoquan Watershed

Hydrology and Water Resources Watershed Report

Mike Stoever

ABSTRACT The Potomac River basin provides the surface and groundwater resources to the Washington, D.C. metro region and is home to approximately 6.1 million people, the majority of whom live within the Middle Potomac-Anacostia-Occoquan watershed. Therefore, in order to ensure that their water supply needs are and will continue to be met, an accurate characterization of the water budget for the latter watershed is essential. This study reviews the existing literature on the topic and utilizes data provided by U.S. Geological Survey (USGS) monitoring stations in the District of Columbia, Maryland, and Virginia in order to begin this process. An intensity-duration-frequency curve and flow duration curve were created to help estimate future annual rainfall and potential locations of heavy runoff and minimal groundwater recharge, respectively, along with a plotting of the relationship between evapotranspiration, precipitation, and discharge in the watershed. When taking into account the geologic and climatic conditions of the watershed, these graphs show that low flows and precipitation levels in the summer months may result in diminished water availability during drought conditions. Further, it is hoped that these results will shine a light on the need to ensure that annual water budget estimates do not obscure water supply problems during these months, as has often proven to be the case relative to seasonal water budget estimates.

INTRODUCTION

The Middle Potomac-Anacostia-Occoquan watershed (Figure 1) covers 833,808 acres (3,374 km2) and encompasses the entirety of the District of Columbia, as well as parts of Maryland and Virginia (EPA, 2015). This watershed, USGS Cataloging Unit (or simply, “HUC”) 02070010, is located at the center of the greater Potomac River basin, HUC 020700 (Figure 2). The latter watershed covers 9,394,427 acres (38,017 km2) and encompasses parts or all of the District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia (EPA, 2015). The Potomac River basin is also home to approximately 6.1 million people, all of whom are reliant upon its surface and groundwater resources for public and domestic water supply, power plant cooling, industrial use, and agriculture (ICPRB, 2015).

Due to its location on the Atlantic seaboard fall line, the northwestern half of the Middle Potomac watershed sits atop the crystalline deposits of the Piedmont Plateau and is home to low to moderate gradient streams, while its southeastern half sits atop the unconsolidated deposits of the Coastal Plain and is home to very low gradient streams (USACE et al., 2013). This watershed is home to both heavily urbanized areas such as Washington, D.C. and its surrounding suburbs, and many forested rural and mountain communities where small-scale agricultural activities take place. As such, this watershed consists of a multitude of soil types, with all four groups (A, B, C,

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Figure 1: Location of the Middle Potomac-Anacostia-Occoquan watershed. SOURCE: EPA, 2016.

Figure 2: Location of the Potomac River basin. SOURCE: Tiruneh, 2007. D) and three dual classes (A/D, B/D, C/D) of soils represented (Tiruneh, 2007; USDA, 2016). As one might expect, the soils found in or near urban areas tend to be more representative of Group C or D soils, while those in rural areas tend to be more Group A or B soils (USDA, 2016). The geologic setting also plays a role here, with the soils overlying the Piedmont Plateau displaying more infiltrative ability compared to the poorly drained, fine textured soils (such as clays) overlying the Coastal Plain (USACE et al., 2013).

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Land use in the Middle Potomac watershed is representative of that in the Potomac River basin as a whole. As depicted in Figure 3, the latter basin is predominantly forested, with smaller areas of agriculture and urban development found within (USGS, 1998). Correspondingly, the Middle Potomac watershed’s western side is mostly forested, its central area heavily agricultural, and its eastern side heavily urbanized (USACE et al., 2013).

Befitting a basin whose drainage to the Chesapeake Bay is second only to the Susquehanna River basin, the namesake Potomac River flows 283 miles in a generally southeastern direction with an average discharge at its mouth of 14,300 cfs (USACE et al., 2013). Tidally influenced for its last 113 miles, flow in the majority of streams in the basin is sustained by groundwater discharge during dry periods, resulting in comparable water quality between stream water and groundwater (USGS, 1998; USACE et al., 2013). However, USGS (1998) is careful to note: “When flows are elevated during storms, streams of the basin typically become concentrated in chemicals that are washed from the land, although streams in some areas may become diluted.” Thus, stormwater runoff is of high concern in this watershed with regards to its potentially negative impacts on water quality.

The Middle Potomac watershed (and Potomac River basin overall) receives a fairly even distribution of precipitation throughout the year, with an area-weighted mean of 42 in/yr over the past 30 years (USGS, 2016). Due to a location in the Mid-Atlantic seaboard, its climate is temperate with well-defined seasonal change and corresponding increases or decreases in precipitation and flow. For example, USACE et al. (2013) found that summer lows in stream flow can primarily be attributed to watershed losses due to evapotranspiration (ET) that occurred between the months of March and September. Further, the preponderance of forests in the region (which cover 63 percent of the basin west of the fall line) drives this evapotranspirative process, which has an area-weighted mean actual ET of 22 in/yr (USACE et al., 2013; USGS, 2016). Flow levels are lowest in the warmer summer months when precipitation is less common and snowmelt has been depleted. However, high flows are seen throughout the region year-round. This is due to their being driven by storm events that are stochastic in nature. Average annual precipitation and evapotranspiration for the Potomac River basin is shown in Figure 4.

Figure 3: Land use types in the Potomac River basin. SOURCE: Tiruneh, 2007.

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a b Figure 4: Average annual precipitation (a) and evapotranspiration (b) in inches in the Potomac River basin. SOURCE: Tiruneh, 2007.

METHODS Due to the sheer volume of monitoring stations within the Potomac River basin, I restricted my analyses to the Middle Potomac-Anacostia-Occoquan watershed (HUC 02070010). Area-weighted mean daily precipitation and monthly actual evapotranspiration data were obtained from the USGS National Water Census Data Portal for 1980-2014 and 2000-2014, respectively. The smaller data set for evapotranspiration was due to data limitations (i.e., no data was available for years prior to 2000). Mean daily and monthly discharge values for Water Year 2015 (10/1/14 – 9/30/15), the most recent Water Year where data were fully available, were obtained from the USGS National Water Information System for three USGS monitoring stations randomly selected from within the HUC. USGS 01648000 represented the District of Columbia and was located at Rock Creek at Sherrill Drive; USGS 01650800 represented Maryland and was located at Sligo Creek near Takoma Park, MD; and USGS 01656000 represented Virginia and was located at Cedar Run near Catlett, VA. These values were then imported into Microsoft Excel for further analysis, detailed below.

IDF Curve Once imported into Microsoft Excel, the area-weighted mean precipitation values for 1980-2014 within this HUC were then summed up to represent annual totals and these values were then converted from millimeters to inches. Next, the data were sorted according to their ranking from largest to smallest and the Weibull method was used to obtain the return period (in years) for the data. For example, the return period for the first ranking was 𝑇 = !!!

!= !"!!

!=

36.0, where T is the return period, y is the total number of events, and n is the rank of the event. This information was then plotted with the return period values on the x-axis and the rainfall amounts on the y-axis. The two axes were then set to a logarithmic scale giving the graph a log-

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log scale, and a trend line of best fit was fitted to the displayed data. The expected annual rainfall for the return periods of 2, 20, and 100 years were then calculated.

Flow Duration Curve

Once imported into Microsoft Excel, the mean daily discharge values (in cfs) for each set of data were ranked by magnitude. The exceedence probabilities for each entry were calculated as 𝐸𝑃 = 100 ∗ !

!!! where EP is the exceedence probability, n is the rank of the event and y is the

total number of events. In order to compare the three streams, the curves were plotted on the same graph using q/A (on a log scale on the vertical axis), where q is the discharge value and A is the drainage area. The drainage area for USGS 01648000 was 62.2 mi2; the drainage area for USGS 01650800 was 6.45 mi2; and the drainage area for USGS 01656000 was 93.4 mi2.

Annual actual evapotranspiration, precipitation, and discharge Once imported into Microsoft Excel, the area-weighted mean actual evapotranspiration (AET) values for 2000-2014 within this HUC were then summed to represent yearly (instead of their original monthly) totals and these values were then converted from millimeters to inches. These values were then plotted against the precipitation values for those years that were calculated when creating the IDF curve. Additionally, the monthly AET values for the year 2014 were plotted in a separate graph against the monthly precipitation values for that year to show the relationship between these two variables in a calendar year. The deviation from Water Year 2015 to calendar year 2014 was due to limited AET data availability for the former. The daily precipitation values for that year were summed up to represent monthly values in the latter graph. Finally, the monthly discharge values for Water Year 2015 from the three monitoring stations were plotted together on a separate graph to show the relationship between discharges at the different stations.

RESULTS

IDF Curve As shown in Figure 5 below, once plotted the trend line of best fit (i.e., one with an r2 > 0.9) was found to be a logarithmic trend line with an r2 of 0.95416. Using the fitted trend line and its equation 𝑦 = 7.7706 ln 𝑥 + 34.512, the expected annual rainfall for a return period of 2 years was calculated to be 39.9 in, 20 years 57.8 in, and 100 years 70.3 in (substituting 2, 20, and 100 in for x).

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Figure 5: Intensity-Duration-Frequency (IDF) curve using the Weibull method.

Flow Duration Curve

Once plotted, the data in Figure 6 (shown below) displayed distinctive steep upper curves at all three sites. However, the Virginia site displayed a steeper lower curve when discharge was equaled or exceeded 60 percent of the time or higher. This implies a difference in amount of urbanization and infiltrative capacity amongst the three sites, particularly between Virginia and the District of Columbia and Maryland sites.

y  =  7.7706ln(x)  +  34.512  R²  =  0.95416  

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1   10   100  

Rainfall  (in)  

Return  period  (years)  

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Figure 6: Flow duration curve for three streams within HUC 02070010.

Annual actual evapotranspiration, precipitation, and discharge When plotted (see Figure 7 below), AET displays little yearly variability from 2000-2014, especially when compared to precipitation over that same period. Further, when values for calendar year 2014 were plotted together on a separate graph (Figure 8), evapotranspiration displays an expected parabolic shape. That is, it is higher in the warmer spring/summer months than it is in the cooler fall/winter months. Precipitation over this period displays a slightly unexpected shape, with diminished amounts in the warmer summer months overall yet a total in August comparable to the wetter spring months. Additionally, when plotted together (Figure 9) the monthly discharge rates amongst the three monitoring stations show noticeable deviation. Monitoring station 01656000 (in VA) showed the greatest range, including the highest flow rate, and monitoring station 01650800 (in MD) was significantly lower than the other two stations and displayed the least similarity to the others.

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Discharge  (cfs/m

i^2)  

Percent  of  time  that  indicated  discharge  was  equaled  or  exceeded  

Station  01648000  (DC)  

Station  01650800  (MD)  

Station  01656000  (VA)  

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Figure 7: AET vs. precipitation in HUC 02070010 for the years 2000-2014.

Figure 8: AET vs. precipitation in HUC 02070010 for the 2014 calendar year.

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2000   2002   2004   2006   2008   2010   2012   2014  

Inches  of  evapotranspiration  /  precipitation  

Year  

Annual  precipitation  

Average  annual  precipitation  

Annual  AET  

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Jan   Feb   Mar   Apr  May   Jun   Jul   Aug   Sep   Oct   Nov   Dec  

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Precipitation  

Evapotranspiration  

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Figure 9: Mean monthly discharge values in HUC 02070010 for Water Year 2015.

DISCUSSION AND CONCLUSION The Middle Potomac watershed and the greater Potomac River basin are unique in that they extend westward from a heavily urbanized area located directly over the Atlantic seaboard fall line to a mostly forested, rural area. Thus, the watershed and its greater basin possess “very distinct geologic, geomorphic, hydrological and landuse/landcover characteristics that vary across the region” (Tiruneh, 2007). These characteristics, along with the climate of the Mid-Atlantic region, play a great role in regulating the fluxes of the hydrologic cycle and water budget of the region, which themselves are subject to varied flow inputs and levels, stormwater runoff amounts, and the aforementioned diverse land uses.

As USACE et al. (2013) note, the deforestation and urbanization that occurs in watersheds such as these increase stormwater runoff and can change “the magnitude, frequency, duration, and possibly the timing of stream flows”. Further, the presence of a high level of impervious surfaces in a watershed can result in flashier, more erosive streams (USACE et al., 2013). These differences in land use and their resultant effects on the water budget and hydrologic cycle of the watersheds are especially noticeable under analysis.

As seen in the flow duration curve depicted in Figure 6, the stations in DC and MD display characteristics typical of flashy, urban watersheds. Curves for such watersheds often have the steep upper and lower ends displayed by the data collected at these two stations, indicating that their discharges are heavily determined by climate and precipitation (shown at the upper end of the curves) and geology and topography (shown at the lower end of the curves). That is, they are likely highly influenced by flashy, short duration storms and are located in areas with an absence of significant groundwater or aquifer storage. This lack of significant subsurface

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Oct  Nov  Dec   Jan   Feb  Mar  Apr  May  Jun   Jul   Aug  Sep  

Discharge  (cfs)  

Month  

Station  01648000  (DC)  

Station  01650800  (MD)  

Station  01656000  (VA)  

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storage results in higher flow variability, often due to the shortened residence time of the contributing water source.

Comparatively, the station in VA looks to be in a more suburban or forested location, as it displays a steep upper end (indicating heavy influence of climate and rainfall) but a flatter lower curve, indicating that it benefits from a different geology and topography than its two counterparts in this study. Thus, a relatively large percentage of its flow is likely coming from storage in groundwater aquifers or frequent precipitation inputs. Knowing that this monitoring station is located on the western side of the fall line, where the underlying substrate is that of the Piedmont Plateau while the other two are located in the Coastal Plain, this may indeed be the case. However, the lowered variability of flow that comes with heavy groundwater input is not seen at this monitoring station when viewing its monthly discharges for Water Year 2015 (Figure 9). As this site is located in a suburban region where urbanization is occurring (it is indeed implied in the name, after all) it is reasonable to see higher variability here relative to a truly forested, rural area. Further, the suburban location of this site may actually help explain the higher than expected variability shown in Figure 9. As it is receiving contributions from both urban runoff and lag-induced groundwater flow, the potential is there for increased highs and lows when its discharge values are plotted. It must be said that the relatively low levels of discharge shown at the MD site may also be attributed to the impaired nature of the water body being sampled. Sligo Creek, which is one of 14 tributaries to the Anacostia River and thus to the Chesapeake Bay, has long suffered from high levels of stormwater runoff and habitat destruction (MDE, 2012). It is listed as a 303(d) impaired water under the Clean Water Act and has been the focus of an extensive, multi-year restoration progress to reduce peak flow discharge and improve water quality, streambed and bank stability, and in-stream habitat (MDE, 2012).

As shown in Figures 7 and 8, seasonality appears to play a factor in the interannual variability surrounding these monitoring locations and the Middle Potomac watershed as a whole. High, regular amounts of precipitation are seen in the fall, winter, and spring seasons, while more stochastic precipitation amounts are observed in summer months. This can likely be attributed to the intense, short duration thunderstorms that often occur in the region during these months. Evapotranspiration responds accordingly, with a parabolic curve representing higher rates during warmer months displayed for the calendar year 2014.

In terms of understanding these hydrologic fluxes, it is important to create IDF curves such as the one shown in Figure 5 to assist in the prediction of expected annual rainfall. For example, Figure 5 enables the prediction of the expected annual amount of this input in 2, 20, and 100 years, such as was detailed in the results section of this study. Additionally, the stability of the relationship between precipitation and evapotranspiration as shown in Figure 8 is important to see. This is especially truthful when taking into account yearly discharge values (Figure 9) when planning for future surface and groundwater withdrawals and uses, such as the reliance of the urban areas within the Middle Potomac watershed and Potomac River basin on waterways for their drinking water supplies.

The author of this study is careful to note that were some drawbacks to its methodology. Primarily, time constraints and limited data availability from USGS for the Middle Potomac watershed resulted in only three monitoring stations being analyzed. While helpful in getting an overall understanding of the water budget for this watershed, sampling from a higher number of

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monitoring stations is recommended in order to get a more detailed, representative view of its hydrologic fluxes and variability.

As the greater Washington, D.C. region continues to grow, the surface water and groundwater resources of the Middle Potomac watershed and Potomac River basin will likely encounter greater stresses. It is imperative that detailed studies take place to fully account for the amount of groundwater recharge taking place, especially in upstream areas where many homeowners rely on aquifers for their water supply. Increased pumping upstream without sufficient recharge may lead to decreased surface water supplies downstream, a problem that may be particularly acute during times of drought. As Tiruneh (2007) noted, “Increasing demand for water associated with population growth and weather anomalies that result in drought may strain the water resources of the [Potomac River] basin.” Further, coordination between groups analyzing seasonal and annual water budgets is strongly recommended. Annual analysis of recharge estimates, which include the high levels of recharge that occur during the fall and winter months, often obscures potential problems related to water supply in the summer months, an issue compounded by the poor ability of the region’s aquifers to store recharge (ICPRB, 2004). Comparatively, seasonal water budget analyses commonly predict lower water availability during summer months than what is estimated by annual recharge (ICPRB, 2004).

REFERENCES EPA (U.S. Environmental Protection Agency). 2015. MyWATERS Mapper. Available online at:

https://map24.epa.gov/mwm/mwm.html?fromUrl=05030101. Accessed November 12, 2016.

EPA. 2016. Surf Your Watershed: Middle Potomac-Anacostia-Occoquan Watershed – 02070010. Available online at: https://cfpub.epa.gov/surf/huc.cfm?huc_code=02070010. Accessed November 14, 2016.

ICPRB (Interstate Commission on the Potomac River Basin). 2004. Annual and Seasonal Water Budgets for the Monocacy/Catoctin Drainage Area. Available online at: https://www.potomacriver.org/wp-content/uploads/2014/12/ICPRB04-4.pdf. Accessed November 17, 2016.

ICPRB. 2015. The Potomac River Basin. Available online at: https://www.potomacriver.org/wp-content/uploads/2014/11/Potomac-Basin-Fact-Sheet_Oct_2015.pdf. Accessed November 17, 2016.

MDE (Maryland Department of the Environment). 2012. Stream Restoration Reduces Peak Storm Flow and Improves Aquatic Life in Sligo Creek. Available online at: http://www.mde.state.md.us/programs/Water/319NonPointSource/Documents/Success%20Stories/md_sligo-success-story_EPA-final.pdf. Accessed November 17, 2016.

Tiruneh, N. 2007. Basin-wide Annual Baseflow Analysis for the Fractured Bedrock Unit in the Potomac River Basin. Available online at: http://www.potomacriver.org/wp-content/uploads/2014/12/ICPRB07-6.pdf. Accessed November 14, 2016.

USACE (U.S. Army Corps of Engineers), TNC (The Nature Conservancy), and Interstate Commission on the Potomac River Basin. 2013. Middle Potomac River Watershed Assessment: Potomac River Sustainable Flow and Water Resources Analysis. Available online at: http://www.potomacriver.org/wp-

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content/uploads/2015/01/MPRWA_FINAL_April_2013.pdf. Accessed November 14, 2016.

USDA (U.S. Department of Agriculture). 2016. NRCS Web Soil Survey. Available online at: http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx. Accessed November 13, 2016.

USGS (U.S. Geological Survey). 1998. Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96: U.S. Geological Survey Circular 1166. Available online at: http://water.usgs.gov/pubs/circ1166. Accessed November 12, 2016.

USGS. 2016. National Water Census Data Portal: Available Water Budget Components. Available online at: http://cida.usgs.gov/nwc/#!waterbudget/huc/02070010. Accessed November 14, 2016.