While summer doesn’t officially start until June 21, to most of us in the Savannah River Basin, it feels like summer is already here. Temperatures are heating up, the ground is drying, and the trees are lush and green as they enter their peak season.
So what does that mean for reservoir levels? As the summer progresses, we will likely experience a dip in lake levels.
“Even with 100 percent of normal rainfall, the pools rise and fall at certain times of the year,” said Stan Simpson, a hydrologist with the Savannah District. “This is evident in the pool plots when you look at the average elevation line.”
Simpson said normal inflow in the summer is typically insufficient to hold pools at a steady elevation, but the opposite occurs in the winter and spring—when conditions are typically cooler, the ground is more saturated, and evaporation rates are lower. During those periods, the reservoirs normally rise with normal rainfall.
“A point to keep in mind is that rainfall does not vary significantly throughout the year—therefore other environmental considerations such as temperature, evaporation, and transpiration play a larger role in pool elevations,” Simpson said.
In the Savannah River Basin, evaporation alone accounts for up to 1,200 cubic feet of water loss – every second. Transpiration is much more difficult to measure, but in the summer more water is lost from transpiration than evaporation. According to the U.S. Geological Survey website, a large oak tree can transpire 40,000 gallons per year. The most tangible way these two phenomena can be recognized is observation of lake-level trends in the summer. Despite significant amounts of above-average rain in the summer, this warmer season is almost always associated with falling lake levels.
Additionally, human needs vary throughout the year. The coldest months and the hottest months tend to increase demand for electricity, which results in greater hydropower generation through increased releases.
When operating in normal conditions, our protocols include being responsive to the hydropower needs of the Southeastern Power Administration, who is under contract with their customers. According to our water control manual, the reservoirs are in normal conditions during the summer when water levels are within the first four feet of conservation storage.
Check the latest weekly projections every Wednesday by visiting our water management website at http://water.sas.usace.army.mil and click the “Declarations” button on the left menu.
As always, we welcome your questions and comments in the comments section below. Thanks for reading us!
~Tracy Robillard, public affairs specialist