A SWAT model depicts the impact of land use change on hydrology, nutrient, and sediment loads in a Lake Michigan watershed

Abstract High levels of nutrient loads in a catchment indicate the presence of pollution sources that must be identified and quantified. These loads in surface and groundwater have been a major concern that impacts water quality in the Midwestern US, including the Great Lakes Basin. In this study, we use the Soil and Water Assessment Tool (SWAT) to assess the impact of land use changes on hydrology, nutrients, and sediment loads for the St. Joseph River Basin (SJRB), which drains an area of 12,200 km 2 in Southwest Michigan/Northwest Indiana and is a primary source of pollutant to Lake Michigan. The SWAT models were developed to simulate streamflow, baseflow, total suspended solids (TSS), total phosphorous (TP), dissolved reactive phosphate (DRP), total nitrogen (TN), and nitrate (NO 3 -N), using data from two stream gauges. The calibrated models accurately simulated the studied variables across the SJRB. The simulated average annual baseflow for Niles and Paw Paw subwatersheds were 153 and 190 mm, respectively. The impacts of land use change on variables from the SJRB were also explored. The impact of land use change on water quality over time was statistically significant but trends were not linear. Hydrology, sediments and nutrients were also quantified at the subbasin level. Subbasins with heavy urbanization or agricultural drainage infrastructure, showed more substantial increases in sediment and nutrient loads as well as decreased groundwater recharge. This study will aid in the evaluation of historical and future water resources for Midwestern rivers, enabling stakeholders to prepare for future impacts, and to execute conservation and management to sustain the SJRB.