Snowmelt and Agricultural Runoff: Load Quantification and Event Timing

Non-point source pollution of surface waters with sediment, nutrients, and other agrichemicals is a growing public and regulatory concern. Pollutant export from agricultural fields is often estimated using computer models or simulated runoff experiments, rather than with actual field-scale measurements. Pioneer Farm, a 174 hectare (430-acre) mixed-livestock farm associated with the University of Wisconsin-Platteville is intensively instrumented with flumes and automated samplers on subwatersheds ranging from 1 to 28 hectares (3 to 70 acres) in size. The field devices were modified to quantify winter runoff events, which are notoriously difficult to monitor and are rarely reported in scientific literature. Through monitoring, it was determined that winter runoff volumes can be a significant fraction of total annual runoff. Runoff from four years of monitoring is quantified and the relative importance of winter runoff to the annual export of sediment, nitrogen and phosphorus is highlighted. In many cases, the majority of the annual pollutant export occurs during a relatively brief time period between mid-February and mid-March. The impact of the variability in winter precipitation and field conditions, and the timing of spring season warmer temperatures and rainfall is demonstrated.