January 29, 2009 — Precipitation has been not very far below normal levels for this winter to date, but a University of Virginia climatologist says considerably more rain is needed to recharge the groundwater, which is critical to ending the area's persistent drought.
The region has been in a drought situation for several years, prompting conservation measures and concern about water availability. Cold weather months are critical for groundwater recharge because plants are not consuming water and lower temperatures reduce evaporation, said Philip J. Stenger, University climatologist.
Winter rains are a boon to the groundwater levels, which are generally quite low right now. Local reservoirs are currently about full, but as temperatures rise, both evaporation and water demand will go up. During warmer months, groundwater is usually critical to keeping the reservoir levels up.
Official records at the University show that there have been 5.7 inches of precipitation in the past two months. Normal precipitation for that time would be about seven inches. The recent ice and snow event only contributed about a half-inch to that total.
"Stream flows in Albemarle County are on the low end of the normal range," Stenger said. "Monitoring-well levels have been on the rise, but are generally still very low for this time of year."
He cautioned that continued rainfall is needed to restore water levels. "Over the long-term the groundwater levels have been running much lower then the historical averages," he said.
While winter is the best time to recharge groundwater, in some years the region can receive an extra large dose of rainfall during the tropical storm and hurricane season.
"Averaged over a large number of years, 15 to 20 percent of the normal rainfall late in the growing season comes from tropical systems," Stenger said.
"So far this winter, we have been seeing a number of winter storms that have tracked through the region on a consistent basis. That bodes well for receiving additional precipitation." Those storms coming from the Midwest carry less water than those tracking up the coast, which can bring in ample moisture from the Atlantic Ocean, Stenger said.
The area experienced a rainfall shortage from the winter of 1998-99 through the winter of 2002-03, followed by the two consecutive "very wet years," 2003 and 2004.
But shortfalls in precipitation in the winter of 2006-07 (70 percent of normal) and the winter of 2007-08 (64 percent of normal) have helped bring about long-term moisture problems again.
"The last winter with precipitation of at least normal levels was 2002-03," Stenger said.