Climate change impacts on hydrological alteration, water quality and environmental flows in the Jucar River basin

Study Region The study is conducted in the Júcar River basin, located in eastern Spain. As a highly regulated Mediterranean river basin, its hydrological and environmental stability faces significant challenges driven by climate change. Study Focus This paper analyzes the impact of climate change on river hydrology, water quality, and environmental flows. Hydrological and water quality models were jointly used to compare historical baseline simulations with CMIP6 climate projections under the SSP2–4.5 and SSP5–8.5 scenarios. New Hydrological Insights for the Region Projections reveal a severe reduction in future water resources, marked by an average 47.8% decrease in total annual inflow across the basin. This decline leads to a deterioration of hydrological status, where an average of 68.5% of water bodies register increased hydrological alteration, and the number of water bodies classified as very altered more than double compared to historical conditions. The analysis shows that reduced dilution capacity and temperature-driven metabolic acceleration (specifically impacting BOD5 and Ammonium) increase the risk of water quality failures by an average of 117%. Furthermore, the findings highlight a critical trend in environmental flow performance with average deficits increasing by up to 17.7%. A significant drop in the coefficient of variation for these deficits confirms a regime shift from episodic drought events to a state of chronic water stress and loss of ecological resilience.