2010 Washington Metropolitan Area Water Supply Reliability Study

Part 1: Demand and Resource Availability Forecast for the Year 2040

Part one of the 2010 Washington Metropolitan Area (WMA) Water Supply Reliability Study -- Demand and Resource Availability for the Year 2040 -- provides a long-term forecast for water managers in the Washington, D.C. metropolitan region. Water demand forecasts estimate the amount of water required to meet customer demand for a period of time into the future. A reliability and resource availability analysis accounts for the water available to meet these demands and the ability of the system to deliver the water when and where it is needed.

Completed every five years, this iteration of the study indicates a slight upward trend in summertime water use by WMA customers, while population in the region has risen by about 10 percent from approximately 3.9 to 4.3 million people. Model simulations of the current water supply system predict that for the 2030 forecasted demands the system is likely adequate, but might become strained given estimated 2040 demands. By 2040, for a scenario of high demands, model simulations indicate that if the WMA was to experience conditions similar to the worst drought on record (1930) that emergency water use restrictions would be required, water supply shortfalls in portions of the system could occur, and water shortages in the system’s water supply reservoirs could occur. (See figure 1 for a comparison of Washington metropolitan area average annual demand forecasts with current and past studies.)

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Washington, D.C. Metropolitan Area (WMA) Water Supply System

The water supply system for the WMA consists of the Potomac River and a series of reservoirs that release water when required to augment low flow levels (see map 1). The three major WMA water suppliers listed below are the subject of this analysis. (See map 2 for the areas to which each provides water.)

• Washington Aqueduct serves the District of Columbia via the D.C. Water and Sewer Authority (D.C. WASA), as well as portions of northern Virginia - Arlington County, the City of Falls Church, and the Town of Vienna.
• WSSC serves Montgomery and Prince George’s counties in Maryland, and provides a limited amount of water to Howard and Charles counties. Water is also provided on an emergency basis to the City of Rockville and D.C. WASA.
• Fairfax Water serves most of Fairfax County, Virginia, and the following wholesale customers in Virginia: Dulles International Airport, Fort Belvoir, Town of Herndon, Loudoun Water, Prince William County Service Authority, and the Virginia American Water Company (serving the City of Alexandria and Dale City).

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Conclusions of the Demand and Resource Availability Forecast for the Year 2040

1. The WMA’s current water supply system will continue to meet demands through 2030, under a range of hydrologic conditions similar to the 78-year period of historical record, with no water supply shortfalls and no emergency water use restrictions.
2. By the year 2040 the current system may have difficulty meeting the region’s demands during periods of drought without water use restrictions, and/or the development of additional supply resources.
3. Summertime outdoor water use may be increasing in some areas of the WMA, offsetting the benefits of the adoption of more water efficient indoor fixtures and appliances.
4. The system’s largest reservoir, Jennings Randolph, appears to be losing storage capacity due to sedimentation at a higher rate than previously estimated.

Demand Forecasting Method

Forecasts of average annual water demand were developed by combining recent water use information derived from billing data provided by the WMA suppliers and their wholesale customers, information on the current and future extent of the areas supplied with water from the WMA suppliers and local planning agencies, and the most recent demographic forecasts from the Metropolitan Washington Council of Governments (MWCOG). Forecasts were also made for the City of Rockville. Water use data was disaggregated into three categories for forecasting purposes: single family households, multi-family households (apartments), and employees (including commercial, industrial, and institutional use). Two forecast scenarios were developed to address some of the uncertainty involved in forecasting water use.

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Resource Analysis Method

The resource analysis assesses the ability of the current WMA water supply system to meet the forecasted demand discussed above. This analysis is done using the Potomac Reservoir and River Simulation Model (PRRISM) to simulate future water demand and availability based on forecasted demands and the historical hydrologic and meteorological record. PRRISM simulates on a daily basis the processes that govern water supply and demand in the WMA system: flows in the Potomac River; inflows, storage, and releases from the WMA system of reservoirs; and water withdrawals by the three major WMA suppliers.

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Highlights and Reports

2010 Study

Study Overview

System Overview

Conclusions for Year 2040

Demand Forecasting Method

Resource Analysis Method

Figures and Maps

Demand Forecasts (figure 1)

Reservoirs (map 1)

Service Areas (map 2)

Resource Reliability Studies

2010 ICPRB-10-01

2005 ICPRB-05-06

2000 ICPRB-00-05

Access more information on ICPRB CO-OP by returning to the Overview Page.