Valley Rainfall

The Tennessee Valley is one of the rainiest regions in the United States. Managing flooding takes special expertise. TVA is up to the challenge.

Did you know that the Tennessee Valley region averages 51 inches of rain a year? The Gulf of Mexico is a source of this moisture, as are the dissipating hurricanes and tropical storms moving across the Southeast.

December through early May is the major flooding season in the Tennessee Valley. Winter storms provide the most rainfall because they are generally more numerous, last longer and cover the largest areas. Average rainfall ranges from 3.0 to 5.5 inches per month. Runoff, the amount of water that ends up in the river system, is about 23 inches, or 44 percent of average rainfall.

Generally, runoff is heaviest in winter and early spring when vegetation is dormant and the ground is saturated. As a result, heavy storms moving across the Tennessee Valley between December and early May become potential causes of floods—necessitating a winter drawdown to provide flood storage.

 
  Rainfall Runoff
Jan 4.73 3.05
Feb 4.53 3.17
Mar 5.33 3.79
Apr 4.35 2.97
May 4.25 1.95
June 4.09 1.18
July 4.68 1.01
Aug 3.9 0.78
Sep 3.43 0.65
Oct 2.95 0.67
Nov 3.76 1.06
Dec 4.78 2.06

Rainfall and Runoff

The amount of water in the Tennessee River system and its reservoirs depends on rainfall and runoff. Runoff is the portion of rainfall that doesn’t soak into the ground but drains from the surrounding lands into streams and reservoirs.

In early spring, we need plenty of rain to fill the tributary reservoirs, and summer storms help keep reservoir levels up and adequate water flowing through the system. However, meeting flow requirements can be a challenge during the summer. Summer storms generally bring less rainfall than winter ones, and more rainfall is absorbed by the roots of green plants before it can drain into streams, rivers, and reservoirs.

Even in summer, however, flooding is still a concern. The higher reservoir levels that enhance summer recreation also mean that less flood-storage space is available.

In the winter, when more runoff reaches the river system, reservoir levels are kept lower so there’s room to store the water from heavy rains.

Rainfall and runoff