Rhode Island Water Resources BoardWater Availability

An Overview of Water Supply and Water ResourcesMay 5, 2011

Strategic Planning

• Introduction

• Organizational Mission

• Plan Mission

• Past Studies and Investigations

• Workshops

Water Resources

Major Public Water Supplies

Study Areas47 Existing

Interconnections

Current Capacity

Water Supply Area(Supplemental Water Studies)

Total Capacity (Sources + Purchased Water)

(MGD)

Providence 83

Northern Rhode Island 42.5

East Bay Area 28.87

West Bay, Central and Southern 63.88

Richmond Water Supply System 0.87

Westerly Water Division 7.16

Block Island Water Company 0.29

TOTAL 226.57

Groundwater and Surface

Water Resources

• 7 basin studies• Watersheds (HUC 10

and 12)• Resource Inventory• Hydrology• Water budget• Summer Baseflow

– Subbasin level– By study area

(basin)Study Areas10 Digit HUC

WRB Inventory of Water Resources

Watersheds

• Boundaries define surface water drainage to a point.

• Follow the highest ridgeline around the stream channels and meet at the lowest point, outflow.

• Vary in size• Watershed and basins are

interchangeable terms

Resource Inventory

What the Glaciers Left Behind

•How do hydrology and geology impact our options?

•Interpreting and applying the studies

Water BudgetTotal Precipitation

Basin StudyTotal

Precipitation (MGD)Blackstone 452.17

Woonasquatucket/Moshassuck 171.26

Pawtuxet/Quinebaug 692.20

East Narragansett Bay 289.50

West Narragansett Bay 270.04

Pawcatuck 715.20

South Coastal 135.20

Total Statewide 2,725.57

Water BudgetTotal Water Resources

Basin StudyTotal Inflow/

Outflow (MGD)Blackstone 815.83

Woonasquatucket/Moshassuck 175.29

Pawtuxet/Quinebaug 723.10

East Narragansett Bay 292.10

West Narragansett Bay 455.30

Pawcatuck 723.10

South Coastal 136.40

Total Statewide 3,321.12

Safe Yield

• RIGL 46-15.7-2 defines safe yield as a sustainable withdrawal that can be continuously supplied from a water source without adverse effects throughout a critical dry period with a one percent (1%) chance of occurrence, or one that is equivalent to the drought of record, whichever is worse.

Safe YieldWater Use and Availability Studies

• Safe yield information for surface water reservoirs

–Based on drought of record

• Developing a methodology to determine groundwater safe yield

–Based on baseflow

What is Baseflow ?

• The flow in a channel sustained from groundwater discharge in the absence of direct runoff.

• During most of the year, streamflow is composed of both groundwater discharge and runoff.

• When groundwater provides the entire flow of a stream, baseflow conditions exist.

• Only that portion of flow attributable to baseflow was considered available for use.

• In some areas baseflow can be negative (losing streams).

BaseflowWater Use and Availability Reports

• Estimate baseflow, gross yield, at the 25th, 50th, and 75th percentiles of baseflow – June, July, August, and September

• These estimates were made at the subbasin level but can be summarized by study area

• How much of baseflow is available for use?

Why is Baseflow Important?

• Sustainable flow in the channel

• Temperature

• Biological considerations

• Water quality considerations

Time of year and variations among years affect water availability

Flow Standards

• Fixed flow standards–Aquatic Baseflow (ABF)–7Q10 –Natural 7Q10

• Variable flow standards and depletions–RI Aquatic Baseflow (RI-ABF)–RI Stream Depletion Methodology (RI-SDM)

Aquatic Baseflow (ABF)

• Aquatic Base Flow, U.S. Fish and Wildlife Service

• Need at least 25 years of unregulated flow• DA at least 50 sq. mi.

– ABF is often lower than streamflow during August and September

• Example Forestdale gage – ABF = 38.8 ft3/s (25.1 Mgal/d) – 83.6-percent flow duration

7Q10

• Definition

• The 7-day low flow

• Commonly used to assess the capacity of a river to carry pollutants.

• Example Forestdale gage –7Q10 = 11.9 ft3/s (7.9 Mgal/d)–99.5 percent flow duration (flow is lower 0 .5%)

Natural 7Q10

• Theoretical 7-day low flow assuming little to no human effect on flows. –Forestdale, little difference between the 7Q10 and

the Natural 7Q10

• Flows downstream of an impoundments (reservoir)– the natural 7Q10 may < measured 7Q10

• Basin with heavy pumping and no impoundments – the natural 7Q10 > measured 7Q10

RI specific modifications to the US Fish and Wildlife Aquatic Base Flow (ABF) methodology to better represent the climate and streamflow conditions found in RI.

How is RI-ABF Calculated?

• Forestdale in the Eastern Highlands data set

• DA for the Forestdale gage is 91.2 sq. mi. • To calculate the RI-ABF

– multiply DA by the RI-ABF flow values (table 5.1)

• Example Forestdale gage – June RI-ABF = 74.8 ft3/s (48.4 Mgal/d)– 65.6-percent flow duration

DRAFT Stream Depletion Methodology (RIDEM, 2010)

1. Classification based on watershed and other characteristics

2. Links seasonal flow variations and ecology needs

3. Quantifies allowable stream flow depletions as a percentage of the natural 7Q10

------------Percent Remaining----------Date Streamflow Mgal/d Baseflow Mgal/dJan-97 92.4 90.3Feb-97 91.9 91.1Mar-97 90.9 89.3Apr-97 95.2 92.9May-97 92.3 91.0Jun-97 86.8 86.6Jul-97 81.0 79.5Aug-97 83.2 79.5Sep-97 82.5 77.0Oct-97 79.1 73.5Nov-97 87.7 82.5Dec-97 84.6 81.9

Example Forestdale gage

Conclusions

• Several Flow Standards have been considered in the past–  * ABF, 7Q10 and Natural 7Q10 

• More recent standards have evolved to mimic the natural hydrograph – *RI-ABF, DRAFT SDM methodology

• What standard will support future water supply, economic development and population needs?– * Alternate SDM, % of baseflow

Policy Considerations

• Any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere).

• Public policy issue is to understand the tradeoffs between ground-water use and changes to the environment and set a threshold for what level of change becomes undesirable.

Scituate Reservoir