Bringing crop model evapotranspiration up to standard: dual crop coefficient in DSSAT

While methods for estimating reference evapotranspiration (ET) and subsequent crop ET via crop coefficient and dual crop coefficient methods have been standardized since 2005 and 1998, respectively, the current version of the DSSAT cropping system model has not been updated to fully implement these methods. The model was updated to (1) calculate grass and alfalfa reference ET according to the ASCE Standardized Reference Evapotranspiration Equation and (2) determine potential ET following a combination of FAO-56 procedures and using simulated leaf area index (LAI) to determine a basal crop coefficient (Kcb) for potential transpiration. Previously published datasets of full and limited irrigation cotton in Arizona and maize in Colorado were used to demonstrate functionality of this new model. Simulations of grass based ET (ETo) calculated by the new model had better agreement with RefET software than prior “FAO- 56” versions of DSSAT. By comparing crop coefficients for transpiration (Kcb), evaporation (Ke), and overall ET (Kc) the new model showed improved theoretical behavior for both crops than the prior DSSAT versions. The most notable improvements include spikes in Ke due to irrigation and rainfall events and overall dynamic behavior of Kcb as a function of canopy growth. Additional information will emphasize the importance of using the standardized methods as a basis of comparison: while comparisons to observed results should be a priority, comparison against standardized ET baselines can give tremendous insight into model behavior, especially considering the effects of water stress.