A GIS-based variable source area hydrology model

E€ective control of nonpoint source pollution from contaminants transported by runo€ requires information about the source areas of surface runo€. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runo€ in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland ̄ow (i.e., surface runo€), and inter ̄ow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of NewYork State and observed stream ̄ow hydrographs and soil moisture measurements were compared to model predictions. Stream ̄ow prediction during non-winter periods generally agreed with measured ̄ow resulting in an average r of 0.73, a standard error of 0.01 m/s, and an average Nash-Sutcli€e eciency R of 0.62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. Themodel results were most accurate for non-winter conditions. Themodel is currently used formakingmanagement decisions for reducing non-point source pollution frommanure spread ®elds in the Catskill watersheds which supply NewYork City's drinking water. Copyright # 1999 JohnWiley &