Testing and improving evapotranspiration and soil water balance of the DSSAT crop models

Crop models have proven to be useful in establishing strategies to improve production under water-limiting conditions; however, this requires that models have accurate water balance. The objective of this paper was to evaluate various potential evapotranspiration (EO) equations and different ways of partitioning EO between soil evaporation and crop transpiration within the DSSAT models and particularly the CROPGRO faba bean (Vicia faba L.) model. The default DSSAT EO options of Priestley-Taylor (PT) and Penman-FAO (P-FAO24) as well as several equations based on Penman-Monteith theory, and an alternative EO partitioning function were evaluated against measured time-series data on soil water content, actual evapotranspiration (ET), and crop biomass accumulation of faba bean grown under rainfed conditions in southwest Spain. We conclude (i) the PT option is reasonable but tends to overpredict ET, especially in early season, and its ET predictions are significantly improved when the extinction coefficient is reduced from its default value of 0.85 to 0.5; (ii) P-FAO24 is the least adequate to simulate ET and biomass; (iii) the Penman-Monteith reference method (PM-REF based on FAO no. 56 manual) gave good predictions for faba bean but tended to underpredict ET in other locations with soybean [Glycine max (L.) Merr.]; and (iv) dynamic forms of the Penman-Monteith option that use crop-model-predicted leaf area index and height effects on aerodynamic and surface resistance to water vapor transport also gave good predictions. For this crop and environment, ET prediction accuracy with all the ET options except the PM-REF was improved using a 0.5 extinction coefficient for net radiation.