Fact Sheet - Mean Potential Evapotranspiration in Kentucky
David Sander Research Assistant and Glen Conner
State Climatologist Emeritus for Kentucky
The hydrologic cycle is the continual interchange of water between the surface and the atmosphere. It is an integral part of weather processes around the planet and is evaluated by means of a water budget. One water budget, developed by Warren Thornthwaite, is an accounting method for moisture gains, losses and the consequent changes in storage that occurs in successive time periods at the surface of the earth. Thornthwaite's water budget is a simple bookkeeping procedure with applications in hydrology, biology, and is also the basis for regional climate indices. The budget considers the amount of available water stored in the soil root zone as well as any change in the amount of this storage. It also calculates any surplus of water that is not evaporated, stored, or transpired. A surplus of water contributes to runoff to streams. Water deficit is calculated as part of the budget, because it represents the additional amount of water that plants could have used if it had been available. While these are important terms associated with the water budget, the primary concept behind the budget is Thornthwaite's simple method of calculating potential evapotranspiration.

For there to be precipitation, water must find its way as water vapor, back into the atmosphere. It does so through evapotranspiration. This is the amount of water transferred back to the atmosphere through means of evaporation from the surface or transpiration from plants. Thornthwaite developed a way to calculate the potential amount of evapotranspiration using temperature and precipitation records for a given area. Estimation of these evapotranspiration rates is important for such things as determining expected rates of stream discharge to controlling irrigation schemes on farms. Potential evapotranspiration can be understood as the maximum possible evapotranspiration.

For Kentucky, potential evapotranspiration varies by several inches across the state. In examining temperature and precipitation records from 1970-1999, the mean annual potential evaporation, using Thornthwaite's equal availability method, was calculated. The eastern regions of the state had a significantly lower potential evapotranspiration, with the least occurring at Grayson. The western regions averaged higher temperatures over longer periods and consequently have a higher potential evapotranspiration. Murray was the highest in the state, with an annual mean potential evapotranspiration of 32.77 inches. One can compare the mean annual potential evapotranspiration to the mean annual precipitation map and see several areas of similarity. For example, the northern and eastern regions of the state have a lower potential evapotranspiration as well as a lower annual precipitation total. The same holds true in comparing evapotranspiration to the mean annual temperature map of Kentucky. The greatest area of potential evapotranspiration is also the area with the greatest mean temperatures (right around Murray).

A map of potential evapotranspiration was created for April, which illustrates relatively the same general pattern seen for the mean annual evapotranspiration. For July, the single highest month for potential evapotranspiration, the pattern was similar, but the amounts were about three times greater. One should note the mean potential evaporation for July. For the entire state, the monthly potential evapotranspiration is well above 5.25 inches, and above six inches in the western half of the state. This is an astounding figure, considering the mean July precipitation ranges from 4-5 inches around the state. The daily potential evapotranspiration ranges from 0.17-0.21 inches across the state. This means that during July, Kentucky could receive one to two more inches of rain to reach its potential evapotranspiration.
Carter, Douglas B. "The Status of the Water Budget in Physical Geography". Prepared for presentation at the 76th Annual Meeting of the Assocation of American Geographers. Louisville, Kentucky, April 12-13, 1980.
Midwestern Climate Center. University of Illinois: Champaign.
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