Assessing the Effects of Potential Climate and Land Use Changes on Annual and Seasonal Hydrologic Processes of a Large-Scale Agriculture Dominated Watershed.

Land use and climate are two major factors that affect water availability governing agricultural production of a region, specifically in large-scale agriculture dominated watersheds, so separation of their effects is crucial for future land use planning and water resource management. To assess the effects of potential land use change and climate variability on hydrologic processes of the Big Sioux River (BSR) watershed, we used Soil and Water Assessment Tool (SWAT) model. Future climate change scenarios were simulated using average output of temperature and precipitation data derived from Special Report on Emission Scenarios (SRES) (B1, A1B, and A2) for 2090s. Land use change was modeled spatially based on historic long-term pattern of agricultural transformation in the basin, and included the expansion of corn cultivation by 2, 5, and 10%. We calibrated the model using seven years (1990-1996) historic discharge data and found surprisingly good agreement between measured and model predicted values with NSE = 0.70, PBIAS = -3, and r² = 0.75 for monthly simulation. We estimated higher surface runoff in all potential land use scenarios with maximum increase of 4% while expanding 10% corn cultivation in the basin. Annual stream discharge was estimated higher with maximum increase of 72% in SRES-B1 attributed from higher groundwater contribution of 152% in the same scenario. We assessed increased precipitation during spring season but the summer precipitation decreased substantially in all climate change scenarios. Similar to decreased summer precipitation, discharge of the BSR also decreased potentially affecting agricultural production due to reduced future water availability during crop growing season in the basin. However, combined effects of potential land use change with climate variability enhanced for higher annual discharge of the BSR. Therefore, these assessments might help for better future land use planning and water resources management required for sustainable agricultural production of the basin.