Sunday, September 10, 2023

Prince William’s Perennial Streams Drying Up an Ominous Sign

This has been a dry water year (October 1- September 30)- the first one in a decade. The average rainfall in the Potomac River Basin for August was 0.8 inches below normal, but 2.25 inches below normal here in Haymarket. The cumulative rain deficit for the Potomac Basin was about 7.1 inches, but until Friday's deluge here in Haymarket the year-to-date deficit was 11.8 inches. Stream flow across the basin remains below average, and groundwater monitoring indicates below-normal levels.

The Potomac River, its tributaries, reservoirs and the associated groundwater resources are the source of drinking water for the over 6,000,000 people in the Washington Metropolitan area. The Interstate Commission on the Potomac River Basin (ICPRB) coordinates water supply/withdrawal operations for the Potomac River during times of drought and recommends releases of stored water from the jointly owned reservoirs. This is to ensure adequate water supplies for the large Washington metropolitan area water companies and for environmental flow levels.
from Drought Monitor 9/7/2023

Much of the Potomac watershed is currently in D1 drought (beige) according to the U.S. Drought Monitor, though more rain could change this in the usually wet fall. However, current conditions have triggered the Metropolitan Washington Council of Governments (MWCOG) Drought Coordination Technical Committee to consider initiating a “Drought Watch” stage. For now, the Potomac River’s flows are adequate to meet the water demands of the Washington metropolitan area without requiring releases from upstream reservoirs. However, the groundwater, an essential part of our water supply, remains an unknown. Groundwater has very little monitoring and management, but there have been some troubling observations recently.

Round Hill and Purcellville, Virginia whose town water supplies come from a series of wells have in recent days issued water conservation notices to utility customers as dry conditions continued to persist in the region. Round Hill Town staff were concerned that a creek near one of the wells has dried up and that Catoctin Creek in Purcellville had also run dry. Two of the town wells have been pulled off-line in Purcellville to allow them to recharge.

In Haymarket, there were also signs of concern. These pictures were sent to me from the Bull Run Mountain Conservancy last Friday morning. They showed that the perennial streams: Little Bull Run and Catlett’s Branch were dry. Catharpin Creek, another perennial stream, appeared to have been reduced to a series of puddles. This was the driest the Conservancy had seen the streams, ever. This Bull Run watershed is an essential part of the Occoquan watershed that directly supplies water to 800,000 people in Northern Virginia and allows the ICPRB to “ask” Fairfax Water to draw less from the Potomac River in times of need.
Catharpin Creek from BRM Conservancy, M. Kieffer

Catlett's Branch from BRM Conservancy, M. Kieffer

Little Bull Run from BRM Conservancy, M. Kieffer

Generally, groundwater in the Culpeper Basin is renewed each year through precipitation. The water stored in the watershed has always been able to provide adequate water in droughts because historically the withdrawal of water was within the average recharge rate. However,  the only nearby US Geological Survey groundwater monitoring well is no longer stable. The water level has been slowly falling since before the last drought- despite a series of wet years.

USGS Groundwater Monitoring Well 49V1

Properly managed and protected groundwater can be extracted indefinitely and still serve its ecological function. Groundwater recharge through precipitation requires adequate area for infiltration; control of sheet flow created by roads and paved areas, as well as protecting the most geologically favorable infiltration points. In a natural environment much of the precipitation soaks into the ground. Some water infiltrates deep into the ground and replenishes aquifers, which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into rivers, creeks, and ponds through the hyporheic zone.

A stream is a living ecosystem. It includes not just the water flowing between the banks but the earth, life and water around and under it. Beneath a living streambed is a layer of wet sediment, small stones and tiny living creatures called the hyporheic zone. Stream water filters down into this dynamic layer between surface water and groundwater, mixing with the groundwater pushing up to feed the rivers during dry spells. Water in the hyporheic zone cannot push up the groundwater if the groundwater level has fallen too low. The stream becomes disconnected from the groundwater. The level of groundwater falls usually due to overuse and reduced recharge.

Maintaining open areas provides areas of groundwater recharge. According to the U.S. Environmental Protection Agency, impervious cover levels of less than 5%-10% can significantly impact watershed health increasing stormwater runoff and reducing groundwater recharge. When runoff volume increases, runoff velocity increases, and peak storm flows increase and you get flooding with soil erosion, fast moving stormwater carrying contamination and reduced or eliminated water infiltration into groundwater. The groundwater is essential as the base flow to the streams and rivers that feed the Occoquan Reservoir during the dry months. Is this a little hint of the beginning of the end.

This weekend's rain might be some small relief in the region and the long-term forecasts call for 1 to 2 inches of rain from the remnants of hurricane Lee in the coming days. The trajectory of Hurricane Lee is still up the air, but it could bring some significant rain as it heads our way. Nonetheless, bad land use decisions and poor management of our water resources is magnifying precipitation changes due to a changing climate.


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