Evaluating the combined effects of climate and land use change on hydrological response in a mixed land use watershed.

Communities increasingly face challenges from changing hydrological patterns caused by the combined effects of climate change and land use/land cover (LULC) change. However, the hydrological response of mixed land use watersheds to these combined effects remains poorly understood. To address this gap, we developed an automated platform to retrieve projected climate and LULC data and input them into the Storm Water Management Model (SWMM) and the Soil and Water Assessment Tool Plus (SWAT+). These models were coupled to assess the impacts of climate and LULC change on the Stroubles Creek watershed in Montgomery County, Virginia. Climate projections were obtained from CMIP6 and combined with LULC projections from the Integrated Climate and Land Use Scenarios (ICLUS) data source. The joint impacts of climate and LULC change were assessed over three 30-year periods, baseline (1981-2010), mid-century (2031-2060), and late-century (2071-2100), using projections from two Shared Socioeconomic Pathways (SSP245 and SSP585). Results indicate that temperatures are projected to increase under both scenarios in the mid- and late-century periods, with a maximum rise of nearly 9 °C by the late century under SSP585. The warming is most pronounced in winter, while spring and summer exhibit comparatively milder increases. Precipitation projections indicate a shift toward more intense rainfall events under most scenarios, with the exception of mid-century SSP585. In the mid-century SSP245 scenario, precipitation increases by up to 21.6 %. Expanding urban land use, together with these higher-intensity storms, leads to a sharp rise in urban peak flows, with 25-year flood magnitudes increasing by up to 100 % in the mid-century SSP245 scenario. In contrast, the decline in mean annual precipitation, coupled with the expansion of impervious surfaces, leads to substantial reductions in 7Q30 in urban areas, with decreases reaching nearly 45 % under the late-century SSP585 scenario. These findings underscore that urban areas are particularly vulnerable to the compounded effects of climate and land use change, facing amplified flood risks and reduced drought resilience compared to rural portions of the watershed.