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In this study, two different versions of the Soil and Water Assessment Tool (SWAT) model were used to simulate the hydrology and biogeochemical response of the Cannonsville Reservoir watershed, in New York. The first version distributes overland flow in ways that are consistent with variable source area (VSA) hydrology driven by saturation excess runoff, whereas the second version is the standard version of SWAT. These two models were each calibrated for streamflow (Flow), particulate phosphorus (PP), total dissolved phosphorus (TDP), and sediment (Sed) against measured data from the 1,200 km2 Cannonsville watershed. The standard version of the model yielded an r2 between the measured and simulated data of 0.85, 0.73, 0.70, and 0.72 for Flow, Sed, TDP, and PP, respectively. The VSA version yielded an r2 of 0.84, 0.69, 0.72, and 0.53 for Flow, Sed, TDP, and PP, respectively. The two models were then used to determine the maximum upper bound on the reduction in phosphorus loading by removing all of the corn in the watershed. The average reductions between the two models were 65 and 37% for PP and TDP, respectively. The VSA version was also used to estimate the effect of moving corn land in the watershed from the wettest, most runoff prone areas to the driest, least runoff prone areas, which cannot be done directly with the standard SWAT model.

Abstract:In this study, two different versions of the Soil and Water Assessment Tool (SWAT) are used to simulate the Cannonsville Watershed. These versions are referred to as SWAT_TS and SWAT_VSA. The SWAT-VSA model distributes overland flow in ways that are consistent with variable source area hydrology, while the SWAT_TS model does not. Since many water quality models, such as SWAT, use some form of the curve number equation to predict storm runoff, these models may fail to accurately describe the effects of variable source areas (VSAs), which are mostly important in rural, humid areas, such as upstate New York. These two models are compared by calibrating each for flow, phosphorous and sediment against measured data. Introduction:

Assessing best management practice (BMP) effectiveness in large basins is complicated by hydrologic variability, ecosystem complexity, and issues of scaling. A traditional approach to assessing BMP effectiveness in large watersheds is to forecast several BMP scenarios and compare their performance relative to a baseline scenario. While this approach may provide insight into the effectiveness of future BMP scenarios, it does not quantify the impact of BMPs that have already been implemented. The authors propose that a continuous water quality model may be used as a pseudo-control watershed in order to determine BMP effectiveness in large catchments. Preliminary results using this approach suggest that agricultural BMPs have reduced dissolved phosphorus loads to the Cannonsville Reservoir in Upstate New York. Future work will involve the implementation of sophisticated analyses to confirm the statistical significance of reductions in dissolved phosphorus loads to the reservoir.