Fitting measured evapotranspiration data to the FAO56 dual crop coefficient method

Authors

R G. Allen, University of IdahoFollow
Ayse Kilic, University of Nebraska-LincolnFollow
Andrew Suyker, University of Nebraska - LincolnFollow
Jane A. Okalebo, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2015

Citation

2015 ASABE/IA Irrigation Symposium: Emerging Technologies for Sustainable Irrigation Proceedings of the 10-12 November 2015 Symposium, Long Beach, California USA Published by ASABE St. Joseph, Michigan, USA Publication date November 10, 2015 ASABE Publication No. 701P0415

Abstract

The FAO-56 publication of the UN Food and Agriculture Organization contains guidelines on constructing and applying a ‘dual crop coefficient’ method to characterize the behavior of evapotranspiration (ET) on a day to day basis. The dual crop coefficient (Kc) method substantially improves the ability to fit simulated with measured data, as compared to the ‘single’ Kc method, by partitioning evaporation from soil (Es) from transpiration from vegetation. This permits the separate estimation of Es when there are known wetting events from precipitation and irrigation and assists in explaining behavior of measured data. The application of the dual Kc method is relatively straight forward, especially when applied using the straight-line segment method for the basal Kc curve, Kcb. Illustrations are given on fitting the dual Kc method and Kcb curve to daily ET data for irrigated and rainfed corn crops near Mead, Nebraska measured by eddy covariance and sensitivity to various soil and root zone parameters. Assessment of transferring Kcb curve parameters to other fields and years indicates that soil and root zone parameters are relatively transferrable with little modification, whereas lengths of the four crop growth stages do vary from year to year due to differences in cultivar type and possibly differences in weather.