It rained a bit over the weekend, but we remain in drought and a significant precipitation deficit. .
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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.
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graph from David Ward, Loudoun County Presentation 10/2024 |
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Location of the Wells |