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United States Department of Agriculture

Agricultural Research Service

Research Project: Development of Water Management Technologies for the Mid-South

Location: Crop Production Systems Research Unit

Title: Evaluation of alternative methods for estimating reference evapotranspiration

Authors
item Fisher, Daniel
item Pringle, H -

Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 31, 2013
Publication Date: August 30, 2013
Citation: Fisher, D.K., Pringle, H.C. 2013. Evaluation of alternative methods for estimating reference evapotranspiration. Agricultural Sciences. 4(8A):51-60.

Interpretive Summary: Evapotranspiration (ET) is an important component in water-balance models and irrigation scheduling routines. A standardized method of estimating reference ET has been developed, but it is a complex method to use and requires a variety of weather parameters to be measured. The required weather data are oftentimes unavailable or unreliable, however, and alternative reference ET methods which are simpler to use and require less data are needed. Agricultural engineers with the USDA ARS Crop Production Systems Research Unit and Mississippi State University in Stoneville, Mississippi evaluated three simpler alternative methods of estimating reference ET under the humid conditions in Mississippi. One method, which requires only measurements of air temperature, provided estimates close to those from the more complex standardized method. This simpler method would be appropriate for use if the complete set of weather data was unavailable, or if a computationally simpler method was desired. This method may be an option and should be evaluated in other similar humid regions as an alternate method for models or irrigation scheduling programs which require reference ET estimates.

Technical Abstract: Evapotranspiration is an important component in water-balance and irrigation scheduling models. While the FAO-56 Penman-Monteith method has become the de facto standard for estimating reference evapotranspiration (ETo), it is a complex method requiring several weather parameters. Required weather data are oftentimes unavailable, and alternative methods must be used. Three alternative ETo methods, the FAO-56 Reduced Set, Hargreaves, and Turc methods, were evaluated for use in Mississippi, a humid region of the USA, using only measurements of air temperature. The Turc equation, developed for use with measured temperature and solar radiation, was tested with estimated radiation and found to provide better estimates of FAO-56 ETo than the other methods. Mean bias errors of 0.75, 0.28, and -0.19 mm, mean absolute errors of 0.92, 0.68, and 0.62 mm, and percent errors of 22.5, 8.5, and -5.7% were found for daily estimates for the FAO-56 Reduced Set, Hargreaves, and Turc methods, respectively.

Last Modified: 7/23/2014
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