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August 20, 2008


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Crop Coefficients

Potential Evapotranspiration (PET) is an estimate of the water requirements of a 4-inch grass in a deep soil growing under well watered conditions. The water requirements of specific crops are determined from pET through crop coeffecients (Kc). The following equation is used:

PET x Kc = crop water requirement

Crop coefficients vary for different crops, as well as for the region crops are grown in. In addition, they change based on the growing stage of the crop. Unfortunately, for Texas, we only have verified crop coefficients for the North High Plains. These coefficients were developed by the North Plains pET Network Project Team.

FAO (Food and Agriculture Organization of the United Nations) has published generalized crop coefficients which are used throughout the world where local values are not available.

Irrigation Efficiency - To adjust for irrigation system efficiency, we use the follwoing equation

PET x Kc / Eff = crop water requriement

FAO Crop Coefficients

North Plains Crop Coefficients for Sorghum

North Plains Crop Coefficients for Cotton

North Plains Crop Coefficient for Corn

North Plains Crop Coefficient for Winter Wheat





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The TexasET Network is partially supported through a federal program, the “Rio Grande Basin Initiative,” and administered by the
Texas Water Resources Institute of the Texas A&M University System, Cooperative State Research, Education and Extension Service, U.S.
Department of Agriculture under Agreement No. 2005-34461-15661 and Agreement No. 2005-45049-03209.