Development and Application of Coupled Optimization-Watershed Models for Selection and Placement of Best Management Practices in the Mackinaw River Watershed

Abstract

Agricultural non-point source (NPS) pollution remains to be one of the biggest challenges in the Midwest due to extensive farming practices and the use of fertilizers to increase agricultural productivity. Excess sediment and nutrient loadings such as nitrogen and phosphorous are major causes of non-point source pollution in rivers and streams. The Mackinaw River watershed, which is one of the tributary watersheds of the larger Illinois River basin, covers a total drainage area of about 2950 square kilometers. Since 1990, this watershed has been one of The Nature Conservancy’s (TNC) conservation sites, considered to be an area of biological significance in the Upper Mississippi River basin. The Mackinaw River watershed plan prepared by The Nature Conservancy in 1998 indicates that altered hydrology and sedimentation are the primary threats to the Mackinaw River. The plan provided recommendations to improve river hydrology and reduce sediment yields through changes in the landscape. Conservation practices serve as crucial control measures in reducing NPS pollutants from agricultural watersheds. The 2008 Farm Bill provided more than $7 billion for promoting agricultural production and environmental quality by supporting implementation of structural or non-structural management practices under its Environmental Quality Incentives Program (EQIP) (Cowan and Johnson, 2008). Successful implementation of such programs, however, requires sound watershed management plans. Watershed management plans involving implementation of best management practices (BMPs) can help reduce pollution from agricultural sources. BMPs are structural or non-structural control measures that can be implemented in watersheds to control pollutant loads at their source or their transport to receiving water bodies. Implementation of these BMPs should focus on critical source areas that may contribute large amounts of pollutant loads. Identifying areas for the placement of BMPs should take into account both ecological benefits and associated implementation costs. The objectives of this study include (1) developing watershed models for Mackinaw River and two of its tributary watersheds, namely Bray Creek and Frog Alley, to simulate streamflows and water quality constituent loads, and (2) developing a coupled optimization-watershed model for cost-effective selection and placement of BMPs in Bray Creek and Frog Alley watersheds to reduce nonpoint source pollutants such as sediment and nutrient loads to the streams. An integrated modeling approach that involves interfacing a simulation model with an optimization algorithm has been employed to develop the coupled optimization-watershed model. Such integrated modeling approaches have been demonstrated in solving complex, realistic problems in the areas of watershed management, reservoir operations, groundwater monitoring design, and others. In this study, the coupled optimization-watershed model was developed by interfacing a watershed model known as the Soil and Water Assessment Tool (SWAT) with Non-dominated Sorting Genetic Algorithm II (NSGA-II), a multiobjective optimization algorithm. Figure 1 shows the location map of the Mackinaw River watershed.published or submitted for publicationis peer reviewe