Biological Systems Engineering


Date of this Version



University of Nebraska-Lincoln Extension EC765


© 2009, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension. All rights reserved.


Reference evapotranspiration (ETref ) is used to determine the actual water use (ETa) rate for various crops and is one of the key variables that needs to be determined accurately for effective irrigation management. This extension circular provides information on the magnitude and temporal distribution of daily, monthly, annual, and growing season alfalfa-reference evapotranspiration (ETref)) in south central Nebraska. It also discusses why long-term average ETref values may not represent non-average years or be accurate enough for irrigation management.

The most common way to calculate actual crop water use (ETa) for a given agronomic crop is to use the ETref and crop coeffi cient (Kc) approach (i.e., ETa = ETref x Kc). This is described in detail in the UNL Extension NebGuide Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients (G1994). This NebGuide also presents information about sources of local ETref and crop coefficients. ETref can either be measured directly using advanced evaporative flux measurement systems, including atmometers (ETgage™), or calculated from weather variables such as solar radiation, air temperature, relative humidity, and wind speed. (For more information on ET gauges see Using Modified Atmometers (ETgage™) for Irrigation Management, UNL NebGuide G1579.)