From Forest to Farmland in the Straight River Watershed: What Has Changed Since 1850?

Land-use change from natural vegetation to agricultural systems significantly affects watershed hydrology and water quality. This study assesses the long-term effects of historical land-use change on hydrologic processes and nitrogen transport in the Straight River watershed, Minnesota, USA, using the Soil and Water Assessment Tool Plus (SWAT+) model. Three land-use scenarios were created to assess changes in water balance and nitrate levels. These scenarios represent the reconstructed pre-settlement conditions from 1855, established agricultural development from 2006, and current conditions from 2022. Results show a significant increase in water percolation and groundwater recharge. Percolation more than doubled, increasing from about 118 mm under reconstructed pre-colonial conditions to over 256 mm in 2022. Streamflow increased to 2.1 m3s−1 in 2022, indicating improved hydrologic connectivity and groundwater contributions. Nitrate leaching increased from about 1.14 kg N ha−1 to more than 32 kg N ha−1 (1850s–2022), and nitrate export increased by >2000%, indicating strong nitrate loading. The significant increase in nitrate compared to water fluxes points to agriculture as the primary source of groundwater pollution and downstream nutrient loading. These findings highlight the importance of land-use change in affecting water balance and nutrient behavior. They also point out the need to include a historical baseline in watershed assessments. The results show the importance of better land and nutrient management strategies to reduce nitrate losses and protect water resources in intensively managed agricultural areas.