Hydrological response of semi-arid watersheds to isolated land cover–land use transitions: loss of natural regulation and increased flood risk

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Introduction The hydrological regime of a watershed can be substantially altered by transitions in its land cover-land use (LCLU), particularly in arid and semi-arid regions, where changes in soil and vegetation cover directly influence surface runoff generation. Accordingly, this study investigated the hydrological response to isolated LCLU changes in the Gorganrood Watershed (GW), located in the semi-arid environment of northeastern Iran, using the Soil and Water Assessment Tool (SWAT). Methods To attribute such hydrological variations specifically to long-term LCLU transitions, the SWAT model was configured with four historical LCLU datasets (1975, 1988, 2001, and 2014), while all other input data, including climate, were held constant. This modeling framework enables a robust attribution of hydrological changes solely to LCLU transitions, addressing a key limitation of previous studies that assessed combined climate and LCLU effects. Results The SWAT model performed satisfactorily in simulating daily discharge across the GW and its five sub-basins (Galikash, Gonbad, Haji-ghushan, Tamar, and Tangrah) during both the calibration (1986–2000) and validation (2001–2010) periods, with R 2 values ranging 0.73–0.82 and 0.69–0.76, respectively. The significant LCLU transitions across the GW over 1975–2014 involved large-scale expansion of farmlands, contraction of rangelands, forest loss, and progressive urbanization. In response, substantial increases of 6.69% and 16.30% in simulated mean and maximum daily discharge, respectively, reflect a shift toward a more surface runoff-dominated hydrological regime within the watershed. Spatially, steep upland sub-basins (e.g., Tangrah) were more sensitive to LCLU-driven discharge changes than the lower-elevation sub-basins (e.g., Gonbad), which showed the highest absolute flows. Discussion These findings provide hydrological evidence of degraded natural regulation capacity under LCLU transitions and emphasize the importance of integrating LCLU management strategies, including Nature-Based Solutions (NbS) such as soil structure restoration and reforestation, into climate adaptation planning to reduce flood risk in semi-arid watersheds worldwide.

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