Field-scale calibration of crop-yield parameters in the Soil and Water Assessment Tool (SWAT)

Accurate modeling of crop growth within watershed hydrological models is essential, yet most studies pay little attention to parameterizing crop-growth sub-models or validating their performance. This study evaluated crop sub-model parameters of Soil and Water Assessment Tool (SWAT), a widely used, physically based, hydrological model. Baseline SWAT crop parameters were calibrated at the model hydrologic-response-unit-scale using 10 years of replicated field-scale data at one site and validated using 5 years at a second site for corn and grain sorghum, and new parameters were developed and tested for sweet sorghum (bioenergy crop) using 4 years of unreplicated field data. Calibration of crop yields focused on four parameters: lower harvest index (WYSF), harvest index for optimal growing condition (HVSTI), radiation use efficiency (BIO_E), and maximum leaf area index (BLAI). Calibration improved model performance and resulted in slight changes to SWAT default values for four parameters for corn and sorghum. These results provide important preliminary parameters for modeling sweet sorghum in SWAT; both BIO_E and BLAI were greater than default values for grain sorghum. Calibrated parameters improved model performance in validation of corn but not grain sorghum, which was heavily influenced by drought conditions and possibly other management differences at the validation site. Results of this study support use of site-specific, rather than default or off-site, calibration of crop-model parameters to minimize effects on model performance of different soil, water, and nutrient management conditions. Watershed-specific, field-scale crop-yield calibration methods demonstrated in this study are recommended to reduce the plant-growth-related systematic error component of larger-scale hydrological processes (such as streamflow).