Climate Effect on Water Quality in a Small Arid Basin with Scarce and Weak Observed Data

The main objective of this study is to enhance the understanding of the physical and chemical processes operating in a still understudied basin and to establish methodologies for assessing the impacts of climate change in an arid region of southern Spain. The work aims to identify areas that are vulnerable, or potentially vulnerable, to climate change and to evaluate the system’s response in terms of both water quantity and quality. To this end, we analyze the evolution of streamflow, suspended sediments, and nitrates, using the SWAT (Soil and Water Assessment Tool) model. A clear lack of observed data was the main limitation improving river flow calibration; however, the validation process showed a very satisfactory coefficient of determination (R2) for the two stations considered (R2 = 0.78 and R2 = 0.70). Due to the limited water quality dataset, the calibration and validation of nitrates and suspended sediments were performed using the LOAD ESTimator (LOADEST) program. Satisfactory results were obtained at both stations during the validation period for nitrates (R2 = 0.52 and R2 = 0.92) and suspended sediment (R2 = 0.83 and R2 = 0.95) load. Finally, the model was applied under two climate change scenarios (Representative Concentration Pathways, RCP 4.5 and RCP 8.5). Reduced precipitation, combined with temperature increases exceeding 1 °C in some areas, leads to decreased flows along the main channel, affecting suspended sediment concentrations. Nitrate levels generally decrease across the basin, although they increase from October to April at the river mouth. This area emerges as highly vulnerable to climate change, particularly regarding alterations in water flow and nitrate concentration.