HYDROLOGICAL MODELING OF A SMALL URBAN WATERSHED IN DUTCHESS COUNTY, NEW YORK

The U.S.D.A. Soil and Water Assessment Tool (SWAT) hydrologic model is being used to simulate streamflow in a small urban watershed in Poughkeepsie, New York as part of a teaching exercise for a future hydrology course at Vassar College. The 13 km long Casperkill stream drains an area of ~31 km² and runs through forested areas as well as residential and commercial neighborhoods before emptying into the Hudson River. For part of its length, the stream crosses through the Vassar campus, including the college's 225 hectare ecological preserve. A meteorological station at the preserve field station houses a tipping bucket rain gauge and air temperature probes used to determine daily rainfall and daily maximum and minimum air temperatures for input to the SWAT model. These instruments and a pressure sensor used to determine stream stage are wired to Campbell Scientific dataloggers powered off of car batteries and a solar panel. Stream gauge pressure readings have been converted to measurements of stream discharge via the method of salt dilution (Day, 1976). In addition to meteorological information, requirements of the SWAT model include soils and landuse information, for which we are using the Soil Survey Geographic Data Base (SSURGO) and U.S. Geological Survey landcover database for New York state. SWAT simulated flows, produced by applying the measured meteorological variables to the topography, will be compared to actual flows from the stream gauge and model parameters adjusted to achieve the best possible calibration. Once calibrated, the model will be used to assess the impacts of climatic change and of further urbanization on flood frequency and magnitude.