August/September, 2016 streamflow
Source: US Geological Survey (USGS), Water Watch
August/September, 2016 streamflow
Source: US Geological Survey (USGS), Water Watch
The depth and width of stream channels in Virginia have been shaped by the normal flow of water, resulting from the normal runoff from an average year. However, rainfall in Virginia on any particular day or for any particular year is rarely "average." Averages (including the mean, median, and mode) come from recording rainfall for decades and then using statistics to derive a reliable pattern.
Across Virginia, the average rainfall is 43" per year. That statewide number varies by location, season, and year.
Ski operators care about precipitation in the winter. Every day of snow might incentivize urban residents to plan ski trips, even if the slopes in Virginia are covered with manufactured snow. Grape growers are sensitive to August/September rains. If harvest time is dry, juice in grapes is concentrated, the sugar percentage increases, and the winemaking is easier. If it rains just before harvest, grapes can absorb so much water they may even split their skins; more skill is required to bring in the grapes and create a great wine from a wet harvest.
Northern Virginia gets lots of rain in the summer from short, intense thunderstorms. In the winter, lightning/thunder is rare and three days of gray, overcast skies are common. At the start of the Spring Semester after the holiday break, students at Virginia Tech in Blacksburg get tired of seeing gray skies above a campus filled with equally-gray limestone buildings.
since 1900, rainfall in Virginia has ranged from 25 inches/year to over 60 inches ("average" is 43 inches/year)
Source: National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center, Climate at a Glance
A predicted impact of climate change, with increased energy in the atmosphere, is an increase in the intensity of weather events. Rainfall and snowfall will be heavier within short periods of time, triggering greater flooding and erosion.
Storms bring strong winds and heavy rain that can damage human-built infrastructure. More-intense storms will, over time, alter the shapes of natural floodplains and stream channels, as they accommodate new pattterns of fast/heavy rainfall runoff. In the four decades after 1981, Virginia has experienced storms with more than 1" of precipitation for an additional day each year.1
intense storms dropping at least 1" of rain in Virginia are increasing as the atmosphere warms
Source: Urban Sustainbility Directors Network, Climate and Hazard Mitigation Planning (CHaMP) Tool
Droughts, in contrast to storms, evolve slowly. High-altitude air currents such as the Jet Stream move north in the summer. That allows high pressure systems to remain in place longer, blocking rainfall for weeks at a time.
Damage to human-built infrastructure by a drought is rare; roofs do not blow off and roads are not blocked. Television stations shift into continuous "the sky is falling" weather coverage during hurricanes and snowstorms, with colored maps based on Doppler radar data for precipitaton and wind, but droughts are less visual and receive far less coverage.
The Virginia Drought Monitoring Task Force declares official droughts within one or more of 13 defined regions. A drought declaration can trigger calls for voluntary water conservation, and then mandatory meaures when necessary. Declarations are based on four factors: precipitation, groundwater levels, streamflow, and reservoir levels.
A drought "watch" may be declae when two of the four indicators reach a threshold below "normal." If water supply reservoirs drop below a 120-day supply of useable storage and streamflows fall between the 10th and 25th percentile, a "watch" may be declared. A "warning" may be announced if water supply reservoirs drop to a 60-90-day supply and streamflows fall between the 5th and 10th percentile. A drought "emergency" may be triggered if water supply reservoirs drop to less than a 60-day supply and streamflows fall below the 5th percentile. Comparable thresholds are defined for consideration of precipitation and groundwater level indicators as well.2
Drought Region Indicator Map on July 1, 2024
Source: Virginia Department of Environmental Quality (DEQ), Drought
The worst drought of the 20th Century in Virginia was in the 1930's, the same period of the Dust Bowl in Oklahoma. In 2002, Virginia experienced another record-setting drought that was followed in 2003 by record-setting rainfall.23
 in 2002, Virginia rivers were at record low flow levels (red dots) |
 in 2003, rivers were at above-average high flow levels (blue dots) |
The Rapidan River went dry in 2002, down to to 0.45 cubic foot per second (cfs). That forced the water system in the Town of Orange into emergency status. All public restrooms were locked, and portajohns decorated the parking lots of even the fast food restaurants.
The police chief captured condensation from the air conditioner in town hall in order to wash police cars. At home, his basement dehumidifier and bath water provided what he required to flush toilets.4
Source: USGS Water Resources, USGS 01667500 RAPIDAN RIVER NEAR CULPEPER, VA
In the 1950's and again in the 1960's, other droughts had lowered the flow of the Rapidan River below 10cfs.
Rapidan flows since the 1930's
Source: USGS National Water Information System (NWIS) -
USGS 01667500 RAPIDAN RIVER NEAR CULPEPER, VA
A cubic foot of water is close to the size of a basketball, about 7.5 gallons. When the Rapidan River flow dropped to one cubic foot of water per second (cfs), that means about 7.5 gallons per second were flowing past the river gauge on the Orange County-Culpeper County line.
If every drop of Rapidan River water were diverted to the water system, leaving nothing for the fish in the river and letting them die in the sun, then the Rapidan River flowing at 1cfs could supply only 450 gallons per minute (7.5 gallons per second x 60 seconds in a minute). A single house can use 5 gallons/minute, so the drought was a crisis.
Source: Drought Monitor - 2002
In 2003, the rainfall was above-average. In October, 2003, Hurricane Isabel showed that flooding is not the only problem with storms. Tree roots in Virginia's saturated soil could not withstand the long period of strong winds. Massive power outages occurred as trees fell on power lines that were not buried underground.
Storms named Gaston and Ivan made September 2004 far wetter than average, and created dramatic flooding in some places. In 2011, the trigger for flooding and power outages was Tropical Storm Lee.
 Young's Branch on Manassas National Battlefield Park at "bankfull" level, before rising higher and flooding adjacent fields (the "floodplain") in August, 2003 |
 patterns of Virginia streamflow in mid-September, since 1999 |
 Youngs Branch in Manassas Battlefield (September 6, 2008) |
 Youngs Branch one week later (September 13, 2008) |
Droughts and floods are common experiences across Virginia. Land use planners and disaster management specialists pay close attention to development within the 100-year floodplain, the area where there is a 1% chance of a flood at any time, but the potential lack of water is equally serious.
Water distribution has two facets - how Mother Nature provides it, and how humans capture and distribute it. One is a science story, the other is political. Two droughts in 1998 and 2002 were sufficient for the General Assembly to require all localities to create a long-range plan to ensure an adequate water supply consistent with planned population growth.
After the 2002 drought, the General Assembly passed legislation requiring all counties/cities to prepare water supply plans, and to prepare to meet future demand. in 2003 the Drought Response Technical Advisory Committee delivered a Virginia Drought Assessment and Response Plan. It encouraged regional planning for water management, though local jurisdictions were allowed to create long-range plans if they chose not to partner with their neighbors.
Some communities proposed new sources of supply, while others emphasized conservation. Charlottesville debated for nearly a decade whether to raise the height of the Ragged Mountain dam and flood adjacent property vs. dredge the reservoir to increase its capacity. In the end, Charlottesville chose to increase supply by building a new dam that was 42' higher, rather than relying upon existing reservoirs and water conservation.5
to ensure adequate water supply, the Rivanna Water and Sewer Authority chose to expand the Ragged Mountain Reservoir
Source: ESRI, ArcGIS Online
A single year of drought is just a short-term problem. A string of drought years back-to-back can have substantial impact. Studies of tree rings, where the growth of bald cypress trees from southeastern Virginia can be measured back into the 1500's, suggest the English settlers landed at Jamestown during a severe drought. The saltiness of the water in the summertime at Jamestown peaked then, perhaps debilitating the colonists through subtle, slow salt poisoning.
The Roanoke Island colonists also were unfortunate in their timing. A scientist examining bald cypress tree rings dating back to 1185 CE (Common Era) determined:6
modern bald cypress tree at Jamestown and bald cypress knees at James Smith's Fort
Starting in 1997, the faucet ran dry again, and Virginia received below-average rainfall for five of the next six years. In 1999, the multi-year drought in southeastern Virginia was interrupted by major flooding from Hurricane Floyd, but the rest of the state continued to suffer.
 rainfall was below average statewide between 1997-2002 |
 in September 1998, the Palmer Drought Index showed all of Virginia in moderate to extreme drought |
Source: National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center, Drought - Summer 1999 | |
Streamflow in the Fall often varies based on a mix of hurricanes and drought:
 average Virginia streamflow in mid-September, 2008 | |
 average Virginia streamflow in mid-September, 2007 |
 average Virginia streamflow in mid-September, 2004 |
 average Virginia streamflow in mid-September, 2003 |
 average Virginia streamflow in mid-September, 2002 |
the Appomattox River basin is at a higher risk of water shortage than other watersheds
Source: Water Resources Institute, Water Risk Atlas
English colonists arrived at Jamestown during a record-setting drought, measured by the Palmer Drought Severity Index (PDSI)
Source: North American Drought Atlas
