Impacts of combined land-use and climate change on streamflow in two nested catchments in the Southeastern United States

Stream restoration aims to improve hydrologic, geomorphic, and ecological processes and provides an opportunity for ecological design in an urban context. Urban stream restoration and stormwater management involve conventional and low impact development strategies that may employ ecological engineering techniques. Urban flood-risks may increase rapidly due to the combination of land-use and climate change. This is a major research frontier because the changes are poorly understood, especially for the combined impacts of climate and land-use change in small catchments. Climate and land-use change may affect hydrologic systems in a complex, nonlinear, and non-additive manner. Streamflow simulation models can measure the relative magnitude of land-use and climate change on hydrologic response to precipitation events. Based on a simulation model calibrated with instrumental rainfall and streamflow data, this study compares simulated urban stormflow response to projected land-use and climate change in two highly urbanized catchments in Columbia, South Carolina, USA, using the U.S. Environmental Protection Agency (EPA)'s Storm Water Management Model (SWMM). Runoff responses for moderate-magnitude storms are contrasted using three land-use change scenarios with differing imperviousness and three climate-change scenarios. Land-use effects on runoff were proportionally greater in the smaller of the two catchments. At the headwater and downstream gauges, a warm/wet climate-change scenario caused larger increases than land-use changes in peak discharges and total event runoff volumes from late spring to early fall. The maximum monthly increase in peak discharge and total runoff volume from all simulations of climate change was for the warm/wet climate scenario in both catchments. However, the combined climate and land-use change scenario produced a 117.9% increase in peak discharge, a slightly (up to 8.6% in September) greater increase in peak discharge than the sum of changes from the individual scenarios (82.6% from climate change and 26.7% from land-use change) at the headwater gauge for the far term (2045-2074). These results suggest a complexity and nonlinearity of the impacts of combined land-use and climate change on hydrological processes, and have implications for stream restoration projects, integrated climate-impact assessments, urban planning, and policy decisions related to storm runoff and water quality.