Linking Field-Scale Phosphorus Export to a Watershed-Scale Model

Abstract

Agricultural runoff is an important non-point pollution source in many Wisconsin watersheds including southwestern Wisconsin’s Fever River. The Fever River (a tributary to the Galena River Watershed) was recognized as affected by nonpoint source pollution (sediment and phosphorus) and served as one of Wisconsin’s first non-point pollution control sites (WIDNR 2001). Controlling the sources of nutrients from the landscape is particularly complex because end-of-pipe monitoring is not available and simulation tools are usually necessary. Management practices were originally installed to mitigate sediment and phosphorus loading in the Fever River to protect its aquatic ecosystem. The excellent smallmouth bass fishery resulted in the Fever River being recognized as part of Wisconsin’s exceptional resource waters (ERW) in 1995. Since the ERW classification, uncontrolled non-point source pollution within the Fever River Watershed has resulted in the deterioration of the waterway for recreation and a sustainable fishery. Currently within the headwaters of the Fever River Watershed, extensive water quality monitoring is being conducted to determine the effectiveness of alternative management practices. To understand and eventually control phosphorus loading from nonpoint sources into the Fever River, the Soil and Water Assessment Tool (SWAT) model approach was used to simulate the influence of land management on phosphorus transfer at different spatial scales within the headwaters of the Fever River. Runoff volume and composition was measured for four years from alfalfa and corn fields of the University of Wisconsin – Platteville Pioneer Farm in the southern portion of the 7.8 km2 Upper Fever River Watershed. Runoff volume and composition data was also collected from the URFW outlet. SWAT was applied at the field and watershed-scales on an event basis to be consistent with field collection efforts. The results show that SWAT can be used at the different spatial scales. Simulating field-scale watersheds was challenging because SWAT does not incorporate variations in precipitation intensity with its daily time step. Nevertheless, SWAT was successful simulating the field runoff events. The watershed simulations were also successful, but there were differences in the calibration between the field and watershed. The differences in calibrated parameter model values appear to be the result of a delivery disconnect between fields and perennial waterways in SWAT. In both field and watershed simulations, statistical variation for discharge and water quality was likely the result of using individual measured storm events rather than monthly or yearly average as historically has been done. The calibrated field-scale simulations were then used for comparison with a tool for phosphorus loss risk at the field-scale. The research showed a general agreement between SWAT and the Wisconsin Phosphorus Index.