Quantifying the impact of climate change and land use change on surface-subsurface nutrient dynamics in a Chesapeake Bay watershed system.

Nutrients such as nitrogen can be harmful to aquatic organisms in excessive amounts. Climate change, through possible increases in temperature and variable rainfall, may cause changes in nutrient loading patterns from watersheds. This study assesses the potential impact of climate and land use change on nitrate (NO) loading in the Nanticoke River Watershed (NRW), Chesapeake Bay region, USA, using an updated version of SWAT + watershed model that simulates groundwater nitrate fate and transport in a process based spatially distributed manner. The model was calibrated for the 2000-2015 timeframe and tested against measured streamflow and in-stream nitrate loadings, as well as groundwater head measurements from monitoring wells. After calibration and testing, the model simulated hydrological and nitrate (NO) flux changes under two future climate scenarios-Representative Concentration Pathways (RCP) 4.5 and 8.5 alongside projected land use changes by the FOREcasting SCEnarios of Land-use Change (FORE-SCE) model. The simulations suggest that under RCP 4.5, streamflow could decrease by 18-34 % and NO in-stream loading by 4-22 %, while under RCP 8.5, the projected decreases are 22-33 % for streamflow and 4-11 % for NO in-stream loading. Streamflow decrease is due to higher temperatures resulting in higher evapotranspiration during summer months, offsetting increases in precipitation. In-stream NO loading is influenced by a decrease in NO runoff loading, but an increase in groundwater loading due to increased leaching as plant uptake decreases due to higher surface temperatures. Compared to the influence of climate, land use change results in a minor decrease in NO loading. These insights can be used for nutrient management in similar landscapes. Additionally, we show that the updated SWAT + model can be a useful tool in quantifying and investigating NO fate and transport in surface-soil-aquifer-channel systems.