Climate Change and Drought

 It rained a bit over the weekend, but we remain in drought and a significant precipitation deficit. . 

Data from rain gauge PW VA-15, 5.5 miles N of Haymarket

The long predicted impacts of climate change are arriving. Not only is Earth’s temperature increasing, polar ice melting, and the northern U.S. is seeing increased summer heat. We have all seen the impacts of wildfires. Hurricane intensity has increased. Droughts are becoming more intense. 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.

Nonetheless about a decade ago, the Interstate Commission on the Potomac River Basin (ICPRB) engaged the U.S. Geological Survey to  complete a study of water supply availability and the health of the Potomac Watershed for various climate scenarios. The focus of their study was the Potomac River, which supplies water to the Washington Aqueduct, Washington Suburban Sanitary Commission (WSSC), and Fairfax Water who all funded the study. 

The climate models predict that precipitation will increase on a global scale, when dealing with only the Potomac River basin, the models differed on whether precipitation will increase or decrease. The models project that the total annual precipitation varies from plus 9% to minus 9% meaning that rainfall will stay within 4 inches of that average. It is to be noted, that year to year weather variations in rainfall in the region are very large, precipitation has varied from over 80 inches to below 20 inches in the past century.

Though annual rainfall increases in half of the climate change scenarios, flow in the Potomac River falls in most scenarios. Changes in both temperature and precipitation affect stream flows. Changes in rain or snow affect the amount of water that runs off the land surface and enters streams during rainfall. Precipitation also affects the amount of water recharging groundwater aquifers, which are the primary source of stream flow during dry weather periods. Increasing temperatures will cause more rain to be lost to evaporation from the soil, streams, and will increase transpiration, the water released to the atmosphere by plants. These increases in evaporation and transpiration will tend to reduce flow in streams which in turn reduces flow in the Potomac.

With rising population and land use and land cover changes -cutting down streamside vegetation and woodland buffers that once slowed and absorbed rains. Covering the ground with compacted and/or impervious surfaces.  The faster flow of storm water will not only reduce groundwater availability for private well owners like me, but reduce stream and river flow, cause floods and property damage. This will in turn require more and larger reservoirs to store the flood waters and  supply  water to the region, and additional water treatment to make the floodwaters suitable as water supply.

The study also predicted that average annual basin-wide evaporation and transpiration is predicted to increase by 6-8%. Groundwater recharge decreases under 83% of the climate scenarios.  The groundwater monitoring well up the road has experienced slow but continuous decline of one foot per decade over the last 20 or so years. The monitoring well east of Leesburg which has seen more development has fallen about 3 feet per decade over the same period after the change from open space to development. Land use and land cover change may be the cause. 

According to study results, the seasonal pattern of groundwater recharge does not change significantly under climate change, with January, February, and March remaining the months of greatest recharge. However, the average annual amount of groundwater that provides base flow to streams and the water in my well, is predicted to decrease in 16 out of the 18 scenarios by as much as 34% in one case. Looking below at what is happening over the last 20 years in Loudoun county is not a comforting picture. 

The U.S. Drought Monitor shows 62.39% of Virginia is experiencing abnormally dry to moderate drought conditions. Northern Virginia groundwater wells continue to exhibit much below normal water levels. The Shenandoah and Northern Virginia regions remain in emergency status for groundwater indicators, and the Roanoke and Northern Coastal Plain regions’ groundwater indicators were in warning status.

graph from David Ward, Loudoun County Presentation 10/2024

Location of the Wells