Potomac Watershed Roundtable- Reliability of Drinking Water Supply

On Friday, October 4th 2013 the Potomac Watershed Roundtable met at the offices of Loudoun Water in Ashburn, Virginia. Curtis Dalpra, Communications Manager for the Interstate Commission on the Potomac River Basin (ICPRB) presented the results of a study to examine what the impacts of Climate Change might be on Drinking Water Supplies in the Washington Metropolitan Area. Let’s back up to understand why and how this study was done.

Turn a tap in most places of the Washington Metropolitan region, and water, primarily from the Potomac River, flows. The system that today provides ample water to the area does not provide unlimited water. The water supply for the region will become less reliable in coming decades, especially during droughts, as the population continues to grow and as surface flow to the Potomac River decreases by changes in the land use of the watershed and changes in climate impact the river.

The Washington Metropolitan region population and development has reached the point that during times of drought, natural flows on the Potomac are not always sufficient to allow water withdrawals by the utilities (including power generation which takes an awesome amount of water) while still maintaining a minimum flow in the river for sustaining aquatic resources. 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 improved reliability of the water supply and ensured maintenance of in-stream flows to meet minimum aquatic habitat requirements. The section of ICPRB responsible for all this is known as the Section for Cooperative Water Supply Operations on the Potomac (CO-OP.

An important aspect of the Water Supply Coordination Agreement of 1982 is periodically forecasting the region’s future needs for water and assessing the current water supply system’s ability to meet those needs. This analysis is conducted every five years to look 30 years in the future to incorporate new data and ideas and allow time for the water utilities to develop new supply or operating parameters in time to meet demand. Careful future planning has ensured and uninterrupted water supply though several significant droughts.

The 2010 Washington Metropolitan Area Water Supply Reliability Study -Demand and Resource Availability for the Year 2040 was done as two studies. The 2010 study estimates that water demand in the Washington Metropolitan region will rise from its current level of approximately 500 million gallons per day to between 610 and 665 million gallons a day by the year 2040. Over this same period, population in this area is projected to increase from 4.2 million to 5.3 million.

According to that portion of the study, the Washington Metropolitan Area’s current water supply system will likely meet demands through the year 2030, under a range of hydrologic conditions similar to those experienced during the past 78-year period of historical record. However by the year 2040, the current system may have difficulty meeting the region’s demands during periods of severe drought without emergency water use restrictions, and/or the development of additional supply resources.

The key assumption for that report was that the future flow of the Potomac River will mirror the hydraulic conditions for the past 78 years. If hydraulic conditions are changing or a 78 year period is inadequate to predict the possible extent of droughts, this could impact the availability of water. So the ICPRB engaged a study for various climate scenarios of water supply availability from Potomac Watershed to determine if the water supply would be adequate to serve the population. The National Research Program of the U.S. Geological Survey (USGS) actually performed the study using six of the global climate models and three atmospheric CO2 scenarios to create 18 separate possible scenarios. The USGS then “downscaled” the 18 global climate predictions to the Potomac River basin and to other areas as part of a separate project on climate change being conducted by the Chesapeake Bay Program Office and the USGS’s Virginia Water Science Center (your tax dollars at work). In addition, the Chesapeake Bay Program’s Phase 5 Watershed Model was used to estimate the impact of changing temperatures and precipitation on Potomac basin stream flows.

There is tremendous uncertainty in projecting the future climate of the earth, especially at the regional scale. Though global climate models are continually being refined and improved, they do not capture complexity of the interrelations of earth’s land, water, and atmospheric systems that we do not yet fully understand. Local nuisances can be lost in the broad sweeps of mathematical modeling of a living system. Scientific confidence in global model projections is higher for temperature than for precipitation, higher for global scales rather than small regional scales, and higher for longer time frames than shorter ones.

Though the modelers have always claimed greater accuracy for temperature forecasts than precipitation, they have not done a good job of forecasting surface temperature for the past decade. The average surface temperature of earth has actually not increased in the past decade and none of the climate models have been able to adequately explain that anomaly. For the period of 1970 to 2000 the median surface temperature as recorded by measurements increased 0.3 ± 0.04°F per year. However, there has been little further warming of the surface of the planet, particularly over the oceans in the most recent 10 to 15 years. Nonetheless, with all those disclaimers, the USGS did get some predictions out of their 18 scenarios.

Results for the 18 climate scenarios fell into three categories: minor impact, moderate impact, and major impact. The biggest impact is the ability of the regional water utilities to continue to supply water on demand during droughts as the climate changes (or not). Six of the scenarios are predicted to have little impact on the system during a moderate drought and the projected population of the region can be supplied with drinking water from the Potomac River and current systems and operations. Six of the climate change scenarios fall into the “moderate impact” category. Under these scenarios the region is predicted to experience more frequent and stricter water use restrictions, but no water supply shortages during a moderate drought. Reservoir levels are predicted to fall to significantly lower levels during a drought than would occur in the absence of climate change with the projected and assumed increase in population.

Sharing Our Water in the Potomac Watershed

The May 1st Potomac Basin Drought Monitor indicated that most (96.8%) of the Potomac River Basin was abnormally dry (D0). Stream flows measured at Point of Rocks and Little Falls were below median levels.  Precipitation levels in the Basin were below normal in April by 1.2 inches. Most of the groundwater monitoring wells were normal to low across the Basin.  I was not the only one wondering if this would be the beginning of a drought, and worried about my own water supply. Then the rains came.

For the past two weeks as thunder storms have rolled through the region, I checked the water level in the nearby U.S. Geological Survey (USGS) well 49 V1 almost daily and watched as the water level has risen. For two weeks the regular downpours have kept my 15 new trees well watered while the groundwater level has risen 3 feet in the USGS well 49 V1! My drinking water well is undoubtedly as flush as 49 V1 and I’m relieved. The water level has gone from the 10th percentile to the 90th in two fairly wet and stormy weeks. My garden is beautiful, and I can with a clear conscience plan to fill the “gator bags” on my new trees during the dry days of summer. I’ve seen first-hand how immediately rainfall and its percolation into the ground directly impact groundwater.

The groundwater and rain also feed the river at the bottom of my land. That water flows into Bull Run at Sudley Springs and onto the Occoquan River. The rain also feeds the tributaries to the Potomac River.  The Washington metropolitan area gets nearly 90% of its drinking water from the Potomac River. The remaining 10% of the region’s supply is from the Patuxent and Occoquan rivers, Goose Creek (a Potomac Tributary that runs through Loudoun County), Lake Manassas (which feeds the Occoquan), the Jennings Randolph and Little Seneca Reservoirs and groundwater resources that serve small community supplies and private wells like mine. Though I fixate on the water resources in my little corner of the region which I have no ability to supplement, the Potomac River is truly the lifeblood of the region. The Potomac is the region’s major source of drinking water, accepts the clean effluent from waste treatment plants, cools power generation plants, and with the C&O Canal, Lake Manassas, and the Occoquan Reservoir provides water recreation and breath taking scenery to our communities.

The Washington Aqueduct Division of the U.S. Army Corps of Engineers (WAD), the Fairfax County Water Authority (FCWA) and the Washington Suburban Sanitary Commission (WSSC) furnish about 95% of the metropolitan region's water. A number of distribution agencies like Prince William Service Authority purchase some or most all of their water wholesale from the big three and distribute that water in their communities. A number of smaller agencies and self-supply portions of distribution agencies supply the remaining 5% of the water.

For more than two centuries the waters of the Potomac seemed unlimited so that the region is not hampered and tied by water allocation agreements created centuries ago that bind many areas of the arid west to fixed and rigid allocations. Instead, the Interstate Commission on the Potomac River Basin, ICPRB, which was authorized by congress in 1940 to address the pollution of the river facilitated the creation of the Potomac River Low Flow Allocation Agreement in 1978 in response to the droughts of the 1960’s and 1970’s.  Back in the days when the ICPRB was formed, raw sewage flowed directly into Four Mile Run, Hunting Creek, Hooffs Run, and the Potomac River. The river tributaries were putrid and clogged, a foul mix of bubbling, decomposing human waste in brown waters. Shorelines were devoid of wildlife, and tests showed dozens of disease-causing pathogens. Water pollution was so bad that propeller airplane passengers from D.C. (filled with the members of congress) could look down and see the sludge. The extent of the problem was documented in 1949 by the Izaak Walton League, one of the first conservation organizations in the U.S., in a film showing water conditions in Alexandria.

The ICPRB was one of the first organizations with a congressional mandate to consider water resources on a watershed basis, rather than along political boundaries. However, now, the focus of the ICPRB has changed. Sewage is not released into the Potomac (unless the combined sewer systems in Baltimore or Washington overflow).  We have reached the point in population density and development that during times of drought, natural flows on the Potomac are not always sufficient to allow water withdrawals by the utilities (including power generation which takes an awesome amount of water) while still maintaining a minimum flow in the river for sustaining aquatic resources. 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 improved reliability of the water supply and ensured maintenance of in-stream flows to meet minimum aquatic habitat requirements as defined by the Maryland Potomac River study in 1981. The section of ICPRB responsible for all this is known as the Section for Cooperative Water Supply Operations on the Potomac (CO-OP), and is formally empowered in its duty by the Water Supply Coordination Agreement of 1982.

This ICPRB is intended to coordinate all the political entities, Maryland, Virginia, Fairfax Water, Washington DC, the federal government and counties and cities within and dependent on the watershed to address the basin’s major challenges, including water quality impairments, water supply and restrictions, flooding, groundwater use, nonpoint source pollution and emerging contaminates. The ICPRB role has been somewhat overshadowed by the recent EPA mandated Chesapeake Bay TMDL, but the ICPRB remains primary in coordinating water supply management and spearheading coordination of effluent water quality issues as they impact drinking water supplies. In their most recent water supply update on June 4th ICPRB assures us that “there is sufficient flow in the Potomac River to meet Washington metropolitan area’s water demands without augmentation from upstream reservoirs.”  After the recent rains it appears unlikely that the Washington metropolitan area’s back up water supply- primarily the Jennings Randolph reservoir helped in by the smaller supply at Little Seneca reservoir will be needed during the summer of 2012.  ICPRB also brings you the Potomac River Watch.
Thanks to Curtis Dalpra Communications Manager at CO-OP for his help.