Monday, October 7, 2013

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.

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