Keeping an Eye on Drought

from Drought Monitor May 2, 2017

Last year was a dry year. By last fall, the beginning of the water year almost 85% of Virginia was experiencing abnormally dry conditions. There was very little snow around here this winter. Spring continued to deliver less rain than normal in the region and large areas of moderate drought began to build in north and central Virginia covering most of Fairfax, Loudoun, Prince William and Fauquier Counties. The rain and snow are essential. The good news is that spring rains have been falling, since the date of the drought report data (last Tuesday) it rained 1.5 inches as measured a my rain gauge.

Rainfall and snow melt are the water that flows to the rivers and streams of our region ultimately feeding the Potomac River, Occoquan Reservoir and Lake Manassas, but also percolates into the ground and recharges the groundwater that serves private drinking water wells and community wells that draw their water supply from groundwater and contributes to streams; some of the water flowing in rivers comes from seepage of groundwater into the streambed. Groundwater supplies a significant portion of Prince William, Loudoun Counties and almost all of Fauquier County with drinking water, in addition Virginia is still a very rural state that needs the rain for agriculture.

The water level in a groundwater well usually fluctuates naturally during the year. Groundwater levels tend to be highest in the early spring in response to winter snowmelt and spring rainfall when the groundwater is recharged. Groundwater levels begin to fall in May and typically continue to decline during summer as plants and trees use the available shallow groundwater to grow and streamflow draws water. Natural groundwater levels usually reach their lowest point in late September or October when fall rains begins to recharge the groundwater again. The natural fluctuations of groundwater levels are most pronounced in shallower wells and those in fractured rock systems that are most susceptible to drought.

Unless there is an earthquake or other geological event groundwater changes are not abrupt and problems with water supply tend to happen slowly as demand increases with population growth and recharge is impacted by adding paved roads, driveways, houses and other impervious surfaces. Slowly, over time groundwater levels respond to these changes. The U. S. Geological Survey (USGS) collects real time groundwater monitoring data from over 160 groundwater wells in Virginia. Many measure groundwater conditions daily and can be viewed online. The USGS Water Resources system has undergoing a transition to a new data management system that allows you to play with the data and highlights provisional data.

One of the Virginia wells, 49V1 is just up the road from me in the same groundwater basin and serves as the proxy of the condition of my well. The time span that I have been tracking the well data is somewhat short, but it does appear that there is a slight downward trend despite the data beginning at the end of a drought. This is a sign that the present groundwater use may not be sustainable. No studies have been done that attempt to quantify what the total available water is within the county or what the current demand is, so it’s hard to know how significant this is. The nine years of data do clearly show the natural annual fluctuation of the water level within a groundwater well which is cool to see.