The existing water supplies that serve more than four million residents can adequately meet demands through the Year 2035, according to a new study by the Interstate Commission on the Potomac River Basin (ICPRB). Drought conditions as severe as the worst drought on record will likely require some form of mandatory use restrictions, however.

Looking further to 2040, the current supply system could experience considerable stress requiring stronger use restrictions, and some reservoir resources may be exhausted. During a severe 2040 drought there also is a small probability that the flow of the Potomac River could drop slightly below the environmental flow guideline of 100 million gallons per day (mgd) as measured at Little Falls Dam.

The study also assessed possible effects of climate change on the system. The impact on water supply varies dramatically depending on the change in stream flow that may result from changes in precipitation and temperature. Results from this study indicate that in the event of a severe drought with 2040 forecasted demands, the following range of potential impacts on the system could be expected.

• If summer flows fall by 10 percent or more: During a severe drought, most system reservoirs would be drained and on some days the system would be unable to meet water supply demands and the 100 mgd environmental flow-by at Little Falls.
• If summer flows change by 0 to +10 percent: the moderate increase in flows would not be enough to prevent some water use restrictions from occurring during a severe drought; some reservoir storage could be seriously depleted.
• If summer flows rise by 20 percent or more: a substantial increase in flows would increase metropolitan area water supplies sufficiently to allow the current metropolitan area system to meet forecasted 2040 demands.

The study makes several recommendations, including,

• The region’s water suppliers should continue their efforts to identify and evaluate potential new water supply storage facilities and conduct an evaluation of the relative benefits of new storage facilities, non-structural changes in operations, and other options. Enhance flow forecasting abilities to allow more precise operation and avoidance of shortages.
• Further develop ICPRB’s database and model of Potomac basin water withdrawals and consumptive use to provide a sound foundation for basin-wide water supply planning and for a planned basin-wide comprehensive plan.

The study is performed every five years by the ICPRB Section for Water Supply Operations on the Potomac River (CO-OP). The CO-OP works with the metropolitan area water suppliers to coordinate normally independent water supply operations during droughts, and studies the water supply system to improve operations and plan for its future reliability.

You can download the complete study on the ICPRB website.

The ICPRB CO-OP works closely with the Washington metropolitan area water utilities, Fairfax Water, the Washington Suburban Sanitary Commission, and the U.S. Army Corps of Engineers Washington Aqueduct Division, which collectively provide the bulk of potable water to the region’s residents. The suppliers usually operate independently, but work together with CO-OP during drought conditions to assure that all demands are met equitably. The CO-OP also provides important services, including annual drought exercises an analyses of potential changes to the existing system.