The Potomac River flow fluctuates with season and weather. The Interstate Commission on the Potomac River Basin (ICPRB) helps the water companies manage the river’s water resources. The ICPRB was born out of the severe and extended drought in the 1960's when water withdrawals by the three major water companies from the Potomac reduced flows to such an extent that the River practically ran dry, leaving only mud between Great Falls and the tidal river. Ultimately (after more than a decade) the ICPRB was created and the Jennings Randolph Reservoir was built to manage the use of the Potomac River and to ensure that there is enough flow for essential services like wastewater assimilation and habitat maintenance. The ICPRB monitors river flows and water withdrawals to ensure the 100 million gallons per day minimum flow at Little Falls.
The minimum flow levels have been maintained since the early 1980's, but during times of drought, natural flows on the Potomac are not always sufficient to allow water withdrawals by the utilities while still maintaining the minimum flow in the river. The ICPRB allocates and manages water resources of the river through the management of the jointly owned Jennings Randolph Reservoir (built in 1981), Potomac River Low Flow Allocation Agreement (1978) and the Water Supply Coordination Agreement in 1982 which designated a section of the ICPRB as responsible for allocating water resources during times of low flow. These steps were part of a water management scheme developed by scientists at Johns Hopkins University and successfully improved reliability of the water supply for decades. The tools available to the ICPRB are to have members utilize their in system storage or the shared system storage and reduce their water withdrawals.
Fairfax Water has a reservoir on the Occoquan River, which is outside the freshwater drainage area and is supplied to a large extent by recycled wastewater. The reservoir’s current storage capacity is estimated at 7.6 billion gallons. Water from the Occoquan Reservoir can only supply the Griffith treatment plant which predominately serves the customers in the eastern portion of Fairfax Water’s service area and Prince William County. Fairfax Water has only a limited ability to transfer water from the Griffith plant to the western portion of its service area.
WSSC operates two reservoirs the Tridelphia Reservoir and T. Howard Duckett Reservoir (sometimes referred to as Rocky Gorge Reservoir). These reservoirs are operated in series and have a combined total storage capacity of about 10.0 billion gallons. Loudoun water is building not only a water treatment plant, but 1 billion gallons of water storage in an old quarry to supply their community during times of low flow. Finally there is the upstream jointly owned water storage. The Little Seneca Reservoir located in Black Hill Regional Park in Montgomery County, Maryland. The storage capacity of Little Seneca Reservoir is 3.9 billion gallons. The Jennings Randolph Reservoir is located in the far northwest corner of the Potomac River basin, bordering Garrett County in Maryland and Mineral County in West Virginia. Its storage capacity is 29.3 billion gallons. Finally, there is the Savage Reservoir with a storage capacity of 6.1 billion gallons.
An important aspect of the Water Supply Coordination Agreement of 1982 is for the ICPRB to forecast the region’s future needs for water and assessing the current water supply system’s ability to meet those needs. Reservoirs and other water supply solutions are very long term capital projects so the ICPRB performs an analysis every five years to look 25 years in the future to incorporate new data and ideas and allow time for the water utilities to develop new water supply or operating strategies in time to meet demand. Careful future planning has ensured and uninterrupted water supply though several significant droughts.
Recent ICPRB forecasts have overestimated water use by not accounting for water savings and also overestimating population growth. Over time the water demand projections have grown flatter and hopefully more accurate. Now, ICPRB has released their 2015 report that assumes daily per capita water use will decrease by an additional 25%, incorporates various climate and weather scenarios but still uses the hockey stick projection of population growth provided by the Washington Council of Governments who forecast that the residential population is expected to grow by 23% and the workforce is expected to grow by 36% by 2040. They also looked carefully at the impact the climate change might have on water supply.
Historically, a key assumption was that the future flow of the Potomac River will mirror the hydraulic conditions for the past 79 years. If hydraulic conditions are changing or a 79 year period is inadequate to predict the possible extent of droughts, this could impact the availability of water. So, a couple of years ago the ICPRB engaged a study that created a model for various climate scenarios of water supply availability from Potomac Watershed to determine if the water supply would be adequate to serve the population. They used this model to examine the water supply adequacy of the current study.
The ICPRB found that the existing water supplies can meet demands of the forecasted population levels through the Year 2035, by implementing mandatory water restrictions during severe droughts. However, as the population and water demand continue to grow in the last five years of the projection period, the water supply is no longer certain. As the population continues to grow beyond 2035 forecast levels, the current supply system including the Potomac River and all current and planned reservoirs and water storage would not be adequate to supply all needs during a severe drought made worse by increased temperatures. The models showed that even with more stringent use restrictions, some of the reservoirs could be exhausted and the Potomac River water supply would still not meet total demand. The modeling showed that during a severe 2040 drought the flow of the Potomac River could drop below the environmental flow guideline of 100 million gallons per day (mgd) to prevent water supply failure even after using all the reservoirs to supplement flow and implementing water use restrictions.
Opposition to constructing reservoirs the late 1970s, lead to the research at Johns Hopkins University that developed the cooperative water management system used today for the Potomac. This research indicated that the management of Jennings Randolph Reservoir in coordination with the existing Occoquan and Patuxent reservoirs could meet the region’s projected demand and maintain adequate flow in the Potomac River through about 2020. That proved a successful plan, but it will not carry the region indefinitely. ICPRB recommends that it is time to beginning planning additional water storage facilities now before they are needed.