Magnitude and Trends of Reference Evapotranspiration Rates in South Central Nebraska: Daily, Monthly, Growing Season Total, and Annual Total

Authors

Suat Irmak, University of Nebraska-LincolnFollow

Date of this Version

2009

Citation

University of Nebraska-Lincoln Extension EC765

Comments

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

Abstract

Reference evapotranspiration (ET_ref ) is used to determine the actual water use (ET_a) 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 (ET_ref)) in south central Nebraska. It also discusses why long-term average ET_ref values may not represent non-average years or be accurate enough for irrigation management.

The most common way to calculate actual crop water use (ET_a) for a given agronomic crop is to use the ET_ref and crop coeffi cient (Kc) approach (i.e., ET_a = ET_ref x K_c). 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 ET_ref and crop coefficients. ET_ref 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.)