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January’s Rainfall was better – but more is needed for recovery

U.S. Army Corps of Engineers
Posted: 2/2/2017

U.S. Army Corps of Engineers

The first month of 2017 brought a welcome change in rainfall; all three sub-basins exceeded average precipitation for the first time in almost five months.

 

Hartwell received 104 percent of normal rainfall; Russell, 115 percent of normal; and Thurmond cashed in at a healthy 138 percent of normal – just over 6 inches of rain.

 

Even better, much of the rain we received came in the form of high intensity rainfall events. Hartwell and Russell each had one major rain event that brought about 2.5 inches over three days (from Jan. 21-23). Thurmond observed two major rainfall events making up most of its total rainfall. The first occurred Jan. 1-3 with about 2.5 inches and the second Jan. 21-23 with just under 2.5 inches.

 

Although this is better news, it’s not great news.

 

Runoff (and not rainfall) is the best indicator of recovery. The distinction between rainfall and runoff is very important. Rainfall deficits of the previous 12 months caused the ground to dry out significantly. Dry ground greatly reduces the runoff ratio to rainfall. This explains why Thurmond can receive 138 percent of normal rainfall, yet runoff for the same time period remains at 68 percent of normal. Dry soil is a relentless creditor. On “payday” it demands a full settlement on outstanding debt.

 

As mentioned in a previous post, to recover from this drought by the 2017 recreation season, rainfall must come in the form of high-intensity events. The reservoirs respond much better to an inch of rain when it falls all at once; thus increasing the runoff ratio. Contrarily, that same inch of rain received over many days will absorb into the dry soil. It will never become runoff in the current condition.

 

Furthermore, we said recovery would require at least two high intensity events across the upper basin each month through spring. January rainfall did not deliver (fully) on this. Hartwell and Russell had one major event. Thurmond had two.

 

Therefore rainfall trends in the next several months will prove crucial in determining reservoir levels by summer.

 

A look at the below 10-week projection for Hartwell offers a sober prediction where levels will peak if inflows don’t return to 100 percent or more of normal from now through spring.

 

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