DEQ monitors four key hydrologic indicators across the thirteen Drought Evaluation Regions to gauge the presence and severity of hydrologic (water supply) drought. The key indicators are precipitation, groundwater levels at observation wells that indicate regional shallow groundwater conditions, stream gaging stations, and levels at major water-supply reservoirs.  Each indicator has thresholds for drought stage determination. 

A Drought stage indicator other than “Normal” does not indicate that a Drought has been declared!  DEQ and the DMTF weigh all of the hydrologic indicators together, along with other regional scale indexes (see links under Resources) as well as reports from drought evaluation regions before making a decision to recommend a drought declaration.

Abnormal conditions at one or more indicator stations may signify dry conditions in a local area that bear close watching, but do not rise to the level of a drought stage.

Precipitation Indicators & Assessment

DEQ and the DMTF use PRISM precipitation data (very similar to the radar-based estimates you see on TV) to monitor precipitation and compare what we get during each climatic “Water Year” (October 1 through September 30) to long-term “normal” values.

Water years are a natural dividing point for water supply drought, because precipitation that falls in the first six months of a water year is analogous to putting money in the bank. Precipitation that occurs during this six-month period has the potential to recharge groundwater, which will sustain stream flows and support withdrawals from wells during the following six month period when moisture deficits naturally develop as evaporation and plant transpiration generally exceed precipitation. 

Precipitation Indicators for Drought Vary by Month and Percent of Long-Term Normal

Months Analyzed	Normal	Watch	Warning	Emergency
October - December	>75.0	<75.0	<65.0	<55.0
October - January	>80.0	<80.0	<70.0	<60.0
October - February	>80.0	<80.0	<70.0	<60.0
October - March	>80.0	<80.0	<70.0	<60.0
October - April	>81.5	<81.5	<71.5	<61.5
October - May	>82.5	<82.5	<72.5	<62.5
October - June	>83.5	<83.5	<73.5	<63.5
October - July	>85.0	<85.0	<75.0	<65.0
October - August	>85.0	<85.0	<75.0	<65.0
October - September (and previous 12 months)*	>85.0	<85.0	<75.0	<65.0

 

 * Values are carried into October if a deficit exists at the beginning of the water year.

Groundwater and Surface Water Indicators & Assessment

Each day, DEQ compares groundwater levels and streamflow records from “real-time” continuous recording wells and gaging stations across Virginia (see table below) to long-term records (at least 10 years) for the current month.  For groundwater, daily records are compared; for streamflows, the average of the previous seven days’ flow records are compared.

Drought State	Threshold
Normal	> 25th percentile
Watch	Between 10th and 25th percentile
Warning	Between 5th and 10th percentile
Emergency	<5th percentile

“Percentile” refers to the rank of the measurement when set alongside all other measurements for that month collected from the particular well or gaging station.  For example, a groundwater level that falls in the 20th percentile (indicating a Watch stage) during a particular day in August means that 80% of all other August water levels collected from that well were higher than that day’s water level.

Links to Key Groundwater Level and Streamflow Indicator Stations:

Drought Evaluation Region	Groundwater Indicator Well(s)	Streamflow Gaging Station
Big Sandy	Buchanan County SOW222, Wise County SOW223	Clinch River at Cleveland
Chowan	Surry County – SOW 94C	Meherrin River near Lawrenceville
Eastern Shore	Accomack County – SOW110S Northampton County – SOW103A	No gage available
Middle James	Buckingham County – 41H 3, Colonial Heights – 51G 1	Appomattox River @ Farmville
New River	Christiansburg - SOW019	Reed Creek @ Graham Forge
Northern Coastal Plain	Westmoreland County – 55P 9	Mattaponi River nr Beulahville
Northern Piedmont	Orange County – SOW030	Rapidan River nr Culpeper
Northern Virginia	Fairfax County – 52V 2D Loudoun County – SOW022 Prince William County – 49V 1 and 51S 7	Accotink Creek near Annandale
Roanoke River	Bedford County – SOW224 Roanoke – SOW008 Patrick County – SOW010	Goose Creek nr Huddleston
Shenandoah	Clarke County – 46W175 Rockingham County – 41Q1	NF Shenandoah nr Strasburg
Southeast Virginia	Suffolk – 58B 13 Virginia Beach – SOW098A	No gage available
Upper James	Rockbridge County – SOW063	Cowpasture River nr Clifton Forge
York-James	Hanover County – SOW080 York County – SOW184C	Chickahominy River nr Providence Forge

Note that “SOW” = “State Observation Well Network”

Water-Supply Reservoir Indicators & Assessment:

Large reservoirs in Virginia support a wide variety of uses that include water supply storage, electric power generation, and flow augmentation to protect water quality. DEQ evaluates water levels at water-supply reservoirs within the 14 drought evaluation regions based on their estimated number of days of available usable storage:

• Normal conditions:  greater than 120 days of usable storage
• Watch conditions:  90 to 120 days of usable storage
• Warning conditions:  60 to 90 days of usable storage
• Emergency conditions:  less than 60 days of remaining usable storage

DEQ also assesses drought-status conditions at the large multi-purpose reservoirs listed below that record reservoir water levels daily.

Indicator Thresholds for Key Water-Supply Reservoirs:

STATUS	NORMAL	DROUGHT WATCH	DROUGH WARNING	DROUGHT EMERGENCY
Smith Mountain Lake	>793 feet msl	793 to 791.5 feet msl	791.5 to 790 feet msl	<790 feet msl
Lake Moomaw	>1565 Feet msl	1565 to 1562.5 feet msl	1562.5 to 1560 feet msl	<1560 feet msl
Lake Anna	>248 feet msl	248 to 246 feet msl	246 to 244 feet msl	<244 feet msl
Kerr Reservoir	<3 feet below guide curve	3 to 6 feet below guide curve	>6 feet below guide curve	<288 feet msl

msl = mean sea level