Performance of the ASCE Standardized Reference ET Equation in the Arkansas Valley of Colorado.

The American Society of Civil Engineers (ASCE) standardized Penman-Monteith equation has been recommended for use in the Arkansas Valley of Colorado for estimation of reference evapotranspiration (ET) and subsequent estimation of crop consumptive use.  In 2007, alfalfa was established in a newly-installed weighing lysimeter at Rocky Ford, Colorado to verify the performance and predictive accuracy of the ASCE standardized ET equation for calculating alfalfa reference (tall reference) ET values.  The undisturbed soil monolith is contained in a tank that is 3 m x 3 m in area and 2.4 m deep.  Instrumentation for micrometeorological measurements as well as soil temperature, soil heat flux, and soil moisture measurements have been installed.  This preliminary study aims to present comparisons between daily alfalfa reference ET values obtained from the ASCE standardized equation and from the weighing lysimeter for the 2008 growing season.  Recommendations for improving the accuracy of the ASCE standardized ET equation will be given for local microclimate and soil conditions that exist in the Arkansas Valley.