Better Than Last Year: Reservoir operations update

It’s all about rainfall, says TVA’s Chuck Bach. “We’ve gotten some much needed rain this spring, and it’s had a positive impact on reservoir levels. But we’ll need continued rain to reach—and maintain—summer target levels.”

 

Cherokee Dam
 

As manager of River Scheduling at TVA, Bach keeps a close eye on rainfall and runoff—the amount of water that flows over the ground and reaches the river system when it rains. “Rainfall in the western part of the Tennessee Valley is basically back to normal. We’ve been able to fill the reservoirs along the main Tennessee River to summer target levels with local inflows. The situation in the eastern Valley has improved, as well. But that area is still in a severe drought, which has made it harder to fill tributary reservoirs, especially reservoirs in east Tennessee and western North Carolina.”

 

From January through May 20, the eastern Valley—the area above Chattanooga—received 16.8 inches of rain, which is 81 percent of normal. Runoff was further behind—55 percent of normal—since much of the rain is absorbed by the dry ground or used by growing vegetation before it reaches the reservoir system.

“This time of year an inch of rain will normally produce about seven-tenths of an inch of runoff,” says Bach. “But this spring, when we’ve been lucky enough to get some rain, we’ve only gotten about three-tenths of an inch of runoff per inch of rainfall.”

 

Low runoff is a result of the prolonged drought. Since January 2007, the rainfall deficit in the eastern half of the Tennessee Valley is 22.2 inches. Normal to higher-than-normal rainfall will be required over a period of many months for the region to fully recover.

 

As the table below shows, they were an average of five feet below June 1 target levels on May 20.

 

 

The exceptions are Watauga, South Holston, and Boone. “Those reservoirs are part of the upper Holston River system, which has been impacted especially hard by the drought,” says Bach. “All three reservoirs were behind at the start of the spring fill, and the situation has been slow to improve because of the dry ground and the lack of local rainfall.”

Last year on May 20, tributary reservoirs were an average of eight feet below June 1 target levels.

 

The difference, says Bach, is due to rainfall. “We are operating the same this year as we did last year. We started operating in a water-conservation mode in February 2007, and we’re still operating in that mode. Fortunately, tributary reservoir levels are higher this year because we’ve had a little more water to work with.”

 

To conserve water in tributary reservoirs, TVA is continuing to release only enough water to prevent the riverbeds below tributary dams from drying out and protect downstream water quality.

 

“We generate as much power as we can with the water we release to meet minimum flow requirements,” says Bach, “but—even though it means relying on more expensive coal-fired and nuclear generation or buying power from outside sources—no additional water is released for the purpose of hydropower generation.” TVA hydropower generation from January through May 20 was 56 percent of normal, and most of the generation came from dams along the main Tennessee River.

 

TVA will continue to conserve water in tributary reservoirs through the summer, but Bach wants reservoir users to understand the situation. “Beginning June 1, our system minimum flow requirement increases. We will need to provide a flow of at least 13,000 cubic feet per second at Chickamauga Dam to ensure that enough water is flowing through the system for commercial navigation, water quality, water supply, and other needs. Hopefully, we’ll get enough rain to meet this requirement. But, if we don’t, we will have to begin pulling the needed water from the tributary reservoir system.

 

“Our policy guidelines ensure that water is withdrawn in a balanced manner. If we have to draw water from the tributary reservoir system, we’ll draw more from reservoirs that are close to their balancing guides—which means they have more water—and less from reservoirs that are below their guide levels.”

 

The best scenario, according to Bach, is above-normal rainfall across the eastern Valley in the weeks ahead. “In that case, reservoir users probably wouldn’t even notice the increase in releases. But if the dry weather continues, there’s a good chance that all the tributary reservoirs will be impacted.”

 

To track your reservoir’s elevation, go to TVA’s Reservoir Information Web page. Choose your reservoir from the pull-down menu and then select Operating Guide. You’ll see a graph with a red line labeled 2008 Observed Midnight Elevations. To track Valley rainfall, go here.