Tennessee Valley Authority
Drought Update
Weekly Update
The western portion of the Tennessee Valley received some rain last week from Tropical Storm Gustav as it moved northward through the Ohio Valley. The eastern portion of the Valley experienced little to no rainfall.
TVA continues to track hurricanes and tropical storms, including Ike. The track Ike will follow is still uncertain, although it is expected to be in the central Gulf of Mexico later this week.
For the week ending Sept. 5, there was an average of 0.16 inches of rainfall recorded in the Valley. Runoff was 0.07 of an inch. (Note: All rainfall and runoff data included here is as of midnight Sept. 4.)
The forecast as of the end of last week predicted predominantly dry weather through most of this week with normal to slightly below normal temperatures. A frontal boundary was expected to move through late this week and into the weekend, with the possibility of showers and thunderstorms.
Below is updated information on the condition of the reservoir system as of the end of last week.
- For the calendar year-to-date (Jan. 1-Sept. 4), the Valley has received a total of 28.73 inches of rainfall and 11.04 inches of runoff. Rainfall for the calendar year-to-date is 79 percent of normal while runoff is 62 percent of normal. Hydro production for the calendar year-to-date is 62 percent of normal.
- Blue Ridge, Cherokee, Douglas, Fontana, Norris, Chatuge, Nottely, South Holston, Watauga and Hiwassee remain below their respective flood-guide levels.
TVA will schedule flows through the system to conserve water in the tributary projects while meeting downstream flow requirements, maintaining water quality, protecting aquatic habitat and providing for commercial navigation.
Reservoir Operations during Drought Conditions
During drought conditions, TVA operates the river system in a water conservation mode, releasing only the minimum amount of water from tributary dams required to meet downstream needs.
Water is released to meet reservoir-specific minimum flow requirements, which keep the riverbeds below tributary dams from drying out, and to meet system-wide minimum flow requirements, which are needed to maintain the channel depths for navigation on the main Tennessee River mandated by the TVA Act and to protect water quality and water supply along the length of the river system.
TVA generates as much power as possible with the water released to meet minimum flow requirements, but no additional water is released for the specific purpose of hydropower generation—even though it means relying on more expensive coal-fired and nuclear generation or buying power from outside sources
Beginning June 1, TVA is required to provide increased flow through the river system. The amount of flow depends on the total volume of water stored in the tributary reservoir system and varies depending on the time period.
If the volume of water stored in tributary reservoirs remains below the minimum system operating guide level, TVA is required to provide a weekly average flow of 13,000 cubic feet per second (cfs) at Chickamauga Dam from June 1 to July 31, if the dry conditions persist through that period, and a weekly average flow of 25,000 cfs from August 1 to Labor Day. (System-wide flows are measured at Chickamauga Dam, located near Chattanooga, Tennessee, because this location provides the best indication of the flow for the upper half of the Tennessee River system.)
If tributary reservoirs are above their minimum operating guide, flows at Chickamauga will increase weekly through the summer until the unrestricted drawdown begins after Labor Day. See what the different flows are here.
If there isn’t enough rain to meet the minimum flow requirement, TVA has to pull the needed water from the tributary reservoir system, which can cause water levels on tributary reservoirs to drop through the summer.
TVA uses balancing guides for each reservoir to ensure that water is withdrawn in an equitable manner. More water is drawn from reservoirs that are higher relative to their balancing guide levels—which means they have more water—and less from reservoirs that are nearer or below their balancing guide levels.
If it rains enough to fill the tributary system above the minimum operating guide level, the flow requirement at Chickamauga increases on a weekly basis.
Track your reservoir’s elevation. Choose your reservoir from the pull-down menu and then select Operating Guide.
Track Valley rainfall here.