Modeling the impacts of climate change on streamflow and nitrate export in a Mediterranean agricultural watershed in Spain

Nitrate pollution adversely affects water quality, making it unsafe for human consumption and contributing to increased eutrophication. Nitrate exportation in agricultural areas is inevitable; however, climate change introduces great uncertainty into an already very complex problem. Thus estimating the effects of climate change on streamflow and nitrate dynamics would significantly contribute to the management of the affected areas. This research aimed to predict the impacts of climate change on streamflow and nitrate exportation in a Mediterranean rainfed agricultural watershed using the Soil Water Assessment Tool (SWAT). The model was first evaluated for its suitability to simulate streamflow and nitrate loads under rainfed agricultural conditions in the 477 km² Cidacos River Watershed in Navarre, Spain. The model was then used to analyze the climate change impacts on streamflow and nitrate load in the short-term (2011-2040), medium-term (2041-2070), and long-term (2071-2100) climate projections compared to a historical baseline period (1971-2000) using the RCP4.5 and RCP8.5 CO₂ emission scenarios. The model evaluation showed a good model performance during calibration (2000-2011) and validation (2011-2020) for streamflow (NSE = 0.82/0.83) and nitrates load (NSE = 0.71/0.68), indicating its suitability for adoption in the watershed. The climate change projection results showed a steady decline in streamflow and nitrate load for RCP4.5 and RCP8.5 in all the projections, with the long-term projection scenario of RCP8.5 significantly affected. Autumn and winter saw the greatest seasonal declines compared to spring and summer. The decline in streamflow was attributed to the projected decrease in precipitation and increase in actual evapotranspiration due to increasing temperatures, while the nitrate load decline was consistent with the projected streamflow decline. Based on these projections, the long-term projection scenarios of RCP8.5 indicate severe situations requiring urgent policy changes and management interventions to minimize and mitigate the negative consequences. Therefore, better agricultural management practices are needed to ensure sustainable water resource utilization and efficient nitrogen fertilizer application rates in the watershed to reduce pollution.