Now We Have Drought

In their weekly report of October 1, 2019 the U.S. Drought Monitor published by the National Oceanic and Atmospheric Administration (NOAA) and Department of Agriculture reported that 59.2% of Virginia is in drought conditions. In addition, 37.3% of Virginia was categorized as “Abnormally Dry.”

The Potomac watershed upstream of Washington DC got less than an inch of rain in September, and there is no major rain predicted in the near future. Yet, the Interstate Commission on the Potomac River Basin (ICPRB) has released its October Water Supply Outlook, which forecasts adequate water to meet drinking water demands for the metropolitan area this year without releasing water from the reservoirs.

The dry September weather has caused such low flow at the Point of Rocks stream gauge that it has triggered daily monitoring of the water supply. A recent news story noted that area of the river was low enough to allow people to easily walk across the river. This is a big change from last year’s rain and there has been much discussion at the ICPRB about whether these extremes of rain are a product of climate change. The ICPRB has been researching the possible effects of climate change on water supplies and included a section on its possible effects in "2015 Washington Metropolitan Area Water Supply Reliability Study: Demand and Forecast for the Year 2040."

Though the report will be updated next year, the 2015 version of the Washington Metropolitan Area Water Supply Study indicates that by 2035, the current water supply system could experience considerable stress during a severe drought, not accounting for the impacts of climate change. “By 2040, in the event of a severe drought, there would be a chance that storage in Little Seneca Reservoir would be depleted and that flow in the Potomac River would drop below the minimum environmental flow level of 100 million gallons per day at Little Falls dam...”.

The variability of  potential future changes in stream flows due to a changing climate were obtained from a model developed using the Chesapeake Bay Program Watershed Model and climate change projections. This model linked potential changes in climate to changes in stream flows and found that “According to projections from climate models, temperatures in the Potomac basin will rise whereas precipitation could rise or fall. Both temperature and precipitation have an impact on stream flows, and the range of available climate projections lead to a wide range of potential changes in water availability in the basin.” In other words when the last projection was done, they were not sure what would happen. Thus, as climate modeling and water demand improves, updates to the assessments are necessary.