Valley Rainfall

The Tennessee River watershed has one of the highest annual rainfall totals of any watershed in the United States, with an average of 51 inches of rain a year. The Gulf of Mexico, located to the south, provides a major source of moisture. Wintertime storms provide the most rainfall because they generally cover a larger area and are greater in duration and frequency. The basin also is subject to heavy rainfall resulting from dissipating hurricanes moving across the southeastern U.S.

In over 100 years of record-keeping, the annual rainfall has varied between a high of 65 inches in 1973 and a low of 30.65 inches in 2007. The greatest extremes occur in the eastern half of the Tennessee Valley region. The average annual precipitation at several locations in the mountains of the French Broad, Hiwassee and Little Tennessee River watersheds exceeds 80 inches. In contrast, other locations in the French Broad and Holston River watersheds have annual averages of less than 40 inches.

Average monthly rainfall/runoff comparison

The monthly average rainfall in the Tennessee Valley region ranges from 3.0 to 5.5 inches. March, July and December are typically the wettest months, and September through November are typically the driest. Of course, in any year the wettest and driest months may be different.

The average annual runoff in the region — the amount of water that ends up in the river system after it rains — is about 23 inches, or 44 percent of the average annual rainfall over the watershed. The monthly average runoff varies from a high of almost four inches in March to a low of less than one inch in August, September and October. Generally, runoff is heaviest in the winter and early spring, when the vegetation is dormant and the ground is saturated. As a result, heavy storms moving across the Tennessee Valley region between December and early May become potential causes of major floods.