Integrating Standardized Reference Evapotranspiration and Dual Crop Coefficient in the DSSAT-CSM.

Although the DSSAT-CSM series of crop models have been used for decades, new focus has been put on improving evapotranspiration (ET) simulation in crop models. Current DSSAT models calculate potential ET, then partition that potential in to transpiration and soil evaporation based on leaf area index (LAI). A new ET module was added to the model code to calculate ET, which combines the ASCE Standardized Reference ET (both grass and alfalfa reference) with the dual crop coefficient described by FAO-56. This new standardized method has three advantages: 1) reference ET is calculated according to recently standardized procedures, 2) basal crop coefficient procedure adequately gives a dynamic vegetative transpiration demand as the season changes, and 3) potential surface evaporation due to early season irrigation and rainfall events is calculated appropriately. Model results using previous ET modules are compared with the new module, including corn in semi-arid Colorado and cotton in arid Arizona.