MODELING HYDROLOGIC PROCESSES AND NUTRIENT RUNOFF IN THE LITTLE CHAZY RIVER WATERSHED USING THE SOIL AND WATER ASSESSMENT TOOL (SWAT)

The Soil and Water Assessment Tool (SWAT) is a watershed-scale hydrologic model that is often used to study the impact of land management practices on hydrologic processes and water quality. High-resolution stream discharge and water quality monitoring data collected from 17 gaged subbasins form the basis for SWAT calibration in the Little Chazy River watershed, a medium sized (146 km²) watershed in the Champlain lowlands of northeastern New York. Agriculture accounts for approximately 17% of land-use in the basin, but is concentrated in large- to medium-sized CAFO (Concentrated Animal Feeding Operations) farms in a 6 km-long reach on the main channel between the villages of West Chazy and Chazy and in the lowermost 7 km of the mainstream where it empties into Lake Champlain. Other possible nutrient sources in the watershed include a small municipal sewage treatment plant and rural septic systems. As a result of high nutrient loading, the watershed was identified on the 1996 Lake Champlain Basin Waterbody Inventory/Priority Waterbodies List Report as a Class C river with possible impaired fish survival. A well-calibrated SWAT streamflow model serves as the foundation for future modeling of nutrient transport from the watershed and for assessment of potential changes to land-use management practices and their impacts on water quality.

Preliminary attempts at calibration using the ArcGIS SWAT extension have been promising. Inputs include high-resolution digital elevation models (DEMs) and SSURGO soils data. To improve flow estimates in smaller subbasins, field-verified watershed boundaries and drainage channels were applied to the model, rather than using automatic delineations based on DEMs. Meteorological data from weather stations within the watershed allow for sub-daily (hourly) time steps. High-temporal-resolution water-quality analyses conducted during high runoff events in October 2010, demonstrate that the highest nutrient and sediment concentrations and loads occur during rainfall events and are closely related to rainfall intensity. Consequently, calculated values for infiltration and runoff at sub-daily time steps could be useful for accurate modeling of nutrient loading in the watershed.