Comparison of the Performances of APEX and SWAT Models in Simulating the Impacts of Alternate Grazing Management Practices on Hydrology and Water Quality

Abstract:

Comparison of hydrologic and water quality models is useful to contrast merits and demerits of each model and enable user to select an appropriate model for an intended purpose. In this study, the performances of APEX and SWAT models in evaluating the effects of alternate grazing management practices on water balances, and sediment and nutrient losses in a rangeland-dominated Clear Creek watershed (763 km²) located in north central Texas, were compared. Three grazing management practices including the light continuous (LC), heavy continuous (HC) and planned multi-paddock (MP) grazing were simulated. Measured data on vegetation, soil physical and hydrological properties, and grazing management at four study ranches within the study watershed (two under planned MP and one each under LC and HC grazing management), was used to parameterize both APEX and SWAT models. Both models were calibrated and validated using the measured streamflow (1980-2013), sediment (1994-2009), total nitrogen (TN) and total phosphorous (TP) (1986-2009) load data at the watershed outlet. Preliminary results indicated that in general, both models performed equally well in terms of hydrology, but the APEX model performed better in predicting sediment and nutrient losses. At the watershed level, shifting grazing management from the baseline HC scenario to planned MP grazing decreased annual streamflow by 29% and 27%, and TN load by 47% and 35%, according to the APEX and SWAT models, respectively. The simulated reduction in sediment load with the change in grazing management from the baseline HC to the planned MP grazing was about the same (40%) according to both models.