All of Prince William County Virginia is in severe drought. Usually, rainfall averages approximately 44 inches per year, but varies from year to year. Last year we were about 9 inches short of average and in the first 4 months of this water year we have had about half the usual amount of rainfall. Rainfall in 2023 and 2024 were fairly close to normal though there were dry months.
According to studies commissioned by the Interstate Commission on the Potomac River Basin (ICPRB), climate forecasts are for our region to get wetter with more intense rainstorms and droughts to get more severe. These changes in rainfall patterns are forecast to change our water supply. We have also changed the land use which will also impact the availability of water in our region.
Increasing impervious cover from roads, pavement and buildings as our area continues to build and build and build out (not up) does two things: It reduces the open area for rain and snow to seep into the ground and causes stormwater velocity and quantity to increase. Stormwater runoff increases in quantity and velocity flooding roads and buildings carrying fertilizers, oil and grease, and road salt to our rivers and streams.
Groundwater is an essential part of our water supply. It
serves as the savings account for our rivers and streams, providing the
baseflow during dry periods. Groundwater
has very little monitoring and management, but there have been some troubling
observations in the past few years that seem out of proportion to the rainfall deficit. Generally, groundwater in the Culpeper
Basin is renewed each year through precipitation, but as we cover more of the
region with impervious surfaces this has been changing. 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 very slowly falling for over a decade and a half-
despite a series of wet years.
Here in Haymarket, there have been other signs of concern. The pictures below were sent to me from the Bull Run Mountain Conservancy a little over
two years ago. They showed that the perennial streams: Little Bull Run and
Catlett’s Branch were dry during a dry August (and they have experienced dry periods since). At
that time 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 on Bull Run Mountain, but that may becoming the new normal during dry periods and is a concerning sign of how our watershed is responding to the last round of development at the turn of the 21st century.
The US Geological Survey emphasizes that virtually all surface-water features—streams, lakes, reservoirs, wetlands, and estuaries—interact with groundwater. All water is interconnected, and groundwater flow and storage are dynamic, constantly changing due to human and climatic stress. Altering the land changes both the quality and quantity of groundwater and streamflow.
Land use changes that increase impervious cover beyond 5–10%
from roads, pavement, and buildings have two significant effects: they reduce
open areas where rain and snow can infiltrate the ground and recharge
groundwater, and they increase stormwater velocity, preventing water from
percolating into the earth. This leads to more frequent flooding and less
groundwater recharge.
Over time, reduced groundwater levels transform perennial
streams into ephemeral ones, disconnecting groundwater from the surface water
network (this is what we are beginning to see in Prince William). Once
watershed hydrology is destroyed by development, restoration is extremely
difficult, if not impossible. The Occoquan watershed is essential for the
region’s drinking water supply and the groundwater is an essential part of our streams and rivers.
Properly managed and protected groundwater can be extracted
indefinitely and still serve its ecological function as base flow for streams.. 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 (> 50%). 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 and the life in this ecological zone is destroyed. The level of groundwater falls
usually due to overuse and reduced recharge.
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| Ward backyard credit Steve Ward |
Maintaining natural open areas provides areas of groundwater recharge. According to the
U.S. Environmental Protection Agency, increasing impervious cover levels 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.
A watershed begins to respond to development almost immediately, but long-term ecological and physical changes emerge over 20 to 50 years. Replacing 35–50% of a forested area with impervious surfaces permanently alters how water moves through the landscape. As you develop a watershed, adding impervious coverage, the watershed undergoes profound hydrological and ecological shifts that take years to be seen:
- Hydrological Alterations:
- Runoff reaches streams much faster and in greater volumes, leading to higher peak floods and more frequent flooding. Perennial streams begin to experience dry periods when there is no flow or intermittent flow. Ultimately, streams become ephemeral.
- Impervious surfaces prevent water from soaking into the ground, which can cause the water table to drop and streams to dry up during summer months. The changes from the development/ building boom of 2004 are just emerging.
- Physical
and Water Quality Changes:
- High-velocity runoff during storms causes severe bank erosion, downcutting (incision), and "blowouts" that destroy aquatic habitats.
- Stormwater runoff collects oils, heavy metals, road salts, and nutrients (nitrates/phosphates) from pavement, transporting them directly into waterways without natural forest filtration.
- Rainwater becomes heated as it flows over sun-warmed pavement, raising stream temperatures and stressing or killing sensitive aquatic life.
- Pavement, compacted lawns, roads and buildings prevents groundwater from percolating into the ground which previously served to cool the earth. Land temperatures increase.
It has been over twenty years since the last big building boom in
the county. What we are observing are the cumulative changes from building
during the 1990’s -2007.
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