Quantifying Potential Long-term Changes in Erosion, Discharge, and Total Suspended Solids Resulting from Agricultural Land Use Change in South Dakota

South Dakota is a mosaic of grasslands, wetlands, and cropland. A continuing shift from grassland to cropland has occurred over the past decade and is expected for the next 50 years. Rate of future conversion may vary greatly in response to regulatory, economic, and social factors. Concern has risen over environmental consequences associated with land conversion, which include but are not limited to changes in rill and sheet erosion rates from cultivated soils, stream and river discharge, and water quality. Quantifying future changes for these three externalities is important to understand the possible long-term consequences of complex grassland conversion decisions such as soil loss, flooding or drought, and diminished water quality. Systems Thinking and System Dynamics (SD) methodology was used to model complex land use and soil-related factors over time. The SD model replicated historic annual erosion rates (metric-tons/ha), discharge [million cubic meters (MCM)], and average total suspended solids (TSS; mg/L) from 1947 to 2012 with relative accuracy and precision in four South Dakota watercatchments, which included the Big Sioux, James, Bad, and Belle Fourche rivers. The SD model was utilized to forecast future annual and cumulative erosion [million metric-tons (Mt)], discharge (MCM), and TSS (mg/L) change under different potential future grassland conversion rates and conservation and conventional tillage from 2012 to 2062. Forecasted environmental externalities increased for policy scenarios that promoted grassland conversion but decreased for scenarios that limited grassland conversion to cropland or promoted grassland restoration. Policy implementation is likely to have the same general impact toward the reduction or increase of erosion, discharge, and TSS as cumulative estimates were 70 ̶ 77%, \u3c 1 ̶ 10%, and 70 ̶ 76% greater for the worst-case scenario compared to the best-case scenario estimates, respectively. However, externality change was greater in western verses eastern water-catchments. Results may provide producers, policymakers, and other stakeholders more specific quantitative estimates to assess the future impact of grassland conversion decisions. Additionally, comparisons between these estimates provide support that addressing grassland conversion issues and cultivation practices are important in order to preserve and conserve soil and water resources