Precipitation Deficits

Precipitation deficits are monitored for each drought evaluation region by comparing current precipitation amounts with historical precipitation values as a percent of normal long-term average values.   Normal long-term average precipitation is defined as the mean precipitation for a thirty-year period of record for the area and time period being evaluated.

Precipitation amounts are evaluated based on the water year (beginning October 1). Water years are a natural dividing point for water supply drought, as 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. If a precipitation deficit outside of the normal range exists at the end of a water year, the precipitation records are carried forward until a normal condition is reached (i.e. if a precipitation deficit exists on October 1, precipitation records for the previous twelve months are evaluated until the twelve month deficit is eliminated).
 

If a precipitation deficit does not exist at the beginning of a new water year, the precipitation records are not carried over.  Therefore, new precipitation deficits may appear during October if the first days or weeks of the new water year are dry because the current precipitation totals are being compared to normal long-term average values for October rather than those from the previous water year.  Because the significance of a precipitation deficit changes as the water year progresses, drought response stages will trigger at different percentages of normal depending upon the date of evaluation.

 

 

Months analyzed

normal

(% of Normal Precipitation)

Watch

(& of normal precipitation)

Warning

(& of normal precipitation)

Emergency

(& of normal precipitation)

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
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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000


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