Water loss in the Potomac River basin during droughts

The Washington D.C. metropolitan area’s (WMA’s) Potomac River water supply intakes are located downstream of the Potomac’s Point of Rocks gage and upstream of the Little Falls gage. In early July of 2002, it was noted that there was an apparent water loss between these two gages. Little Falls flow w as estimated based on Point of Rocks gaged flow, with adjustments for intervening withdrawals and inflows. The estimated Little Falls flow was as much as 150 million gallons per day (MGD) higher than the recorded flow at the Little Falls gage. This discrepancy implied that water was being lost between the Point of Rocks and Little Falls gages. Management of streamflow in the Potomac River is critical during drought conditions to ensure an adequate water supply in the WMA, which is the main source of water to 3.6 million people and to ensure recommended environmental in-stream flow. Water is released from several key reservoirs to maintain minimum flows and to supply water for the WMA during times of drought. These releases are managed carefully in order to maintain sufficient reservoir storage ; therefore, water loss is a major concern. Gage error, permitted withdrawals, evaporation, and transpiration by trees along the river were investigated to account for the loss. The Interstate Commission on the Potomac River Basin (ICPRB) calculated potential gage error and examined permits to determine permitted levels of consumptive withdrawals from the Potomac. Gage error may account for some loss but was thought unlikely to explain a significant portion of the loss . In total, about 24 -MGD of the loss could be attributed to permitted withdrawals during the summer.