What to expect with this year’s winter drawdown and refill

If we were to use the current conditions of the Savannah River Basin as indicators, it would seem reasonable to conclude we are facing imminent drought.

First, the upper basin has experienced below average rainfall for the last three months. Thurmond fared better than Hartwell by receiving about 86 percent of average rainfall in the past 90 days. Southeast droughtBut in the same period Hartwell has only received about 69 percent of average.

Second, the result of this rainfall deficit has left portions of the basin abnormally dry, and a few places in moderate drought, as shown in the image released Aug. 6 by NOAA here.

Third, inflows are currently dismal. At Hartwell inflows are 22 percent of normal, and at Thurmond we are observing a painful -19 percent of normal. The negative inflow at Thurmond means evaporation is greater than local inflows to the reservoir. Even assuming inflows improve to 75 percent of normal, our 10 week projection has us brushing up against Drought Level 2 in October.

Projection

If the projections are close to the mark, reservoir levels will reflect a sharper naturally-induced winter drawdown that hangs in the Drought-Level 1 vicinity, and maybe in Drought Level 2.

But by December we have good reason to expect a turn of events. We may even be dealing with the opposite challenge: high water.

The most recent ENSO forecast shows all models remain equal to or above +0.5ºC through spring 2016, and most predict a +1.5ºC or higher, as shown below.

ENSO Forecast

This means El Niño conditions are now present and we are facing a near-certain “strong El Niño” for the winter. El Niño conditions are warmer-than-average air currents in the Pacific that typically translate to above-average precipitation in the southeast, as indicated in the winter outlook map below.

DecJanFeb outlook
As a reminder, the cooler winter weather significantly increases the runoff-to-rainfall ratio. Historical data demonstrates reservoir levels are very responsive in the winter, which is why successful refills occur even with below-average rainfall.

In fact, I spoke with Todd Hamill, a hydrologist with NOAA’s Southeast River Forecast Center, and he said the last time we experienced a comparable El Niño was the winter of 1997-98. In October of 1997 both Hartwell and Thurmond were about a foot away from Drought Level 2.

Starting in December that year we began receiving above average rainfall. In January 1998 Hartwell received more than 9 inches and more than 7 inches the following month. As a result we were dealing with high water issues beyond June of that year.

“We got rain every three days [that winter],” Hamill said. “There are other factors that can come into play [besides El Niño] but it looks like we’re facing a chance of flooding – which is a bigger problem than people think.”

If the ENSO forecast is a reliable prediction of what’s to come in the winter, we may experience a refill that exceeds expectations earlier than usual – perhaps even causing a high-water event.

As always, we welcome your comments.

~Russell Wicke, Corporate Communications Office

About US Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multi-million dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on Facebook.com/SavannahCorps
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