Determining Nutrient and Sediment Critical Source Areas with SWAT: Effect of Lumped Calibration

Abstract

In many watershed modeling studies, due to limited data, model parameters for flow, sediment, and nutrients are calibrated and validated against observed data only at the watershed outlet. Model parameters are adjusted systematically for the entire watershed to obtain the closest match between the model-simulated and observed data at the watershed outlet (lumped calibration). It is hypothesized that the relative loadings of pollutants and/or sediments contributed by each computational unit are not affected by this calibration procedure. In other words, areas generating relatively higher pollutant loads with an uncalibrated model will still generate relatively higher loads after calibration. This study explored the effect of lumped calibration of the Soil and Water Assessment Tool (SWAT) on locations of sediment and nutrient critical source areas (CSAs). Two watersheds in Alabama with differing size, topography, hydrology, and land use/cover characteristics were used to study the variations in locations of sediment, total phosphorus (TP), and total nitrogen (TN) CSAs identified by calibrated and uncalibrated SWAT models. Identified CSAs for sediment, TP, and TN were mostly the same with and without the calibration of the model in both watersheds. This study thus concluded that lumped calibration of the SWAT model using data at the watershed outlet has little effect on the locations of CSAs. Based on the results from these two watersheds, it was further concluded that SWAT can be used without calibration for identification of CSAs in watersheds that lack sufficient data for model calibration, but not for all other modeling purposes. More studies are encouraged to support these findings.