An Easy Method of Estimating Potential Evapotranspiration

This report is brought to you for free and open access by New Prairie Press. It has been accepted for inclusion in Kansas Agricultural Experiment Station Research Reports by an authorized administrator of New Prairie Press. Copyright 1997 Kansas State University Agricultural Experiment Station and Cooperative Extension Service.

Stress from lack of moisture reduces crop yields in Kansas more than any other factor. Even irrigation farmers lose potential yields to such stress, particularly by corn, sorghum, and soybeans, usually during July and August.
To help irrigation farmers tell how much moisture is being used and lost by plants, we have developed an easy way to estimate potential evapotranspiration. Potential evapotranspiration (PET) is evaporation from a wet surface that is limited by the energy it can absorb. The more energy it absorbs, the higher evaporation is. So it is much higher on a sunny than on a cloudy day.
PET depends primarily on energy from the sun. Various methods of estimating PET require data on solar radiation, temperature, humidity, and wind speed. Suppose you wanted to know the potential evapotransp iration for corn. The solar radiation was 600 cal cm·2day·1, maximum temperature was 30°C (86°F) and minimum temperature was 25°C (77°F). You would calculate the average temperature as (30 + 25)/2 = 27.5°C (81 °F).
Look on Fig. 1 (wheat-corn) and select the appropriate point between the 30 and 20°C lines. The PET value is approximately 0.24 inch of water. If maximum and minimum tempetatures are not available, use the noon temperature.

Actual Evapotranspiration
Under a full crop cover and water not limiting (plants not severely stressed), actual evapotranspiration (ET) and PET are approx imately equal. Therefore, under normal cropping conditions ET equals PET for an extended period during the summer-from pretasselling to blister stages in corn and from flowering to seed filling in soybeans. Under situations of low crop cover, (e.g. poor stand development, early and late In the growing season, and with winter wheat in western Kansas), actual evapotranspiration can be considerably less than PET. Then, evaporation from the soil surface begins to play an important role. The Evapotranspiration Laboratory has developed a computer program to handle situations of incomplete cover and water stress.
While the above procedure provides only an approximation of daily PET, It allows one to make a quick estimate of daily water loss from several crops during much of July and August in Kansas when irrigation to avoid stress is most important.
Scheduling of irrigation has been deter-· mined by the amount of water In the root zone .~ig. 3. Potential evapotranspiration for soybean as a function of solar radiation and mean temperature. and by the growth stage of the crop. Generally, the reproductive stages (heading, pod-forming, tassel-silking) are more sensitive to water stress than the vegetative stage. In Kansas, the 'imount of water in root zone is primarily conrolled by precipitation/irrigation and evapotranspiration. To maintain the root zone at an optimum soil water content, evapotranspiration losses must be matched by rain or irrigation. For example, 2 inch irrigation on corn can be used up in eight hot days (8 x .25 inch).
Publications by the Kansas Agriculture Experiment Station are available to the public regardless of race, color, national origin, sex, or religion.

NOTICE OF NON·DISCRIMINATION
Kansas State University Is comml1ted to a policy of non-<llscrlllllnatlon on the basis of race, sex, national orlgin, or other non-merit reasons, in admissions, educational programs, or activities and employment all as required by applicable laws and regulations, including Title IX of the Education Amendment of 1972. 1nquirles may be addressed to: Director, Affirmative Action Office, Kal,sas State University,207 Fairchild Hall,Manhattan,Kansas 66506, or Director, Office of Civil Rights, HEW, Washington, D.C. 20201. This publication from Kansas State University Agricultural Experiment Station and Cooperative Extension Service has been archived. Current information: http://www.ksre.ksu.edu.