An Analysis of BMPs and Their Combined Effectiveness at Reducing Phosphorus, Nitrogen, and Sediment Export from the Black Hawk Lake Watershed, Iowa.

Nutrient over-enrichment of surface water is a critical issue affecting Iowa’s water quality. The Iowa DNR’s 2014 Integrated Report on impaired waters concluded that for assessed lakes and reservoirs, the most commonly identified impairments are algal turbidity, suspended sediment, and indicator bacteria (E. coli). These three cases account for 69% of lake and reservoir impairments in the state. Extensive subsurface drainage systems in Iowa’s predominantly agricultural landscape provide an express pathway for nutrients and sediment to enter surface water bodies. Black Hawk Lake watershed, located in Sac and Carroll counties in Iowa on the Des Moines Lobe, is the focus of a 5-year monitoring project. We are monitoring three subwatershed outlets, which allows for comparison of nutrient and sediment export from surface and tile flow. The goal of this study is to identify the distribution and types of best management practices (BMPs) and assess their combined effectiveness in reducing sediment, nitrogen, and phosphorus export when incorporated in a tile-drained, intensively managed, agricultural landscape. BMPs considered in the paired-watershed study are strip till, no-till, cover crops, filter strips, grassed waterways, terraces, and nutrient management plans. Within these subwatersheds, the level of conservation ranges from 22.5%, 30% and 87.5% of the subwatershed area. BMPs change from year to year as more practices are implemented in the watershed. We will present analysis of the BMPs’ combined effectiveness at reducing sediment, nutrients, and runoff volume and peak flow rates in a paired study.