Calibration and validation of soil and water assessment tool on an agricultural watershed in upstate New York

Hydrologic (i.e., flow volumes) and sediment loading calibration and validation were completed for a New York City water supply watershed, the Cannonsville Reservoir basin, located in upstate New York. An unusually large amount of data exist for this watershed, with more than 20,000 data points consisting of climate, flow, and sediment measurements for calibration and validation. Hydrology results using the Soil and Water Assessment Tool (SWAT) displayed a Nash-Sutcliffe measure (NS) of 0.74 at the main monitoring station that drains 80% of the 1,200 km(2) basin. Sediment results at this station came within 1% of average loading estimated from data for calibration (with one large storm and snowmelt event removed). Two of the three smaller drainage areas displayed calibrated hydrology NS values above 0.7. Adjustments in parameter values for calibration reflected northeastern conditions related to snowmelt events and fragipan soils. Validation of the hydrology and sediment results was based on an independent data set with no changes in parameter values. These validation results for hydrology showed an NS=0.76 and an 8% difference in average flow volumes. Certain limitations of SWAT are discussed, including the model's approach to snowmelt, sediment erosion, and sediment transport.