Assessing the Individual and Combined Impacts of Climate Change and Land Use Change on Streamflow Utilizing SWAT and CMIP6

Shifts in Land Use and Land Cover (LULC), paired with evolving climate patterns, are primary drivers of hydrological volatility—specifically regarding rainfall distribution and river discharge. This research targets the Plasjan River Basin (PRB) in Isfahan, Iran, isolating these variables to project conditions for the 2025–2052 period. Climatic futures were projected using the CanESM5 model (CMIP6), framing the analysis within the bounds of an optimistic mitigation scenario (SSP1-2.6) and a high-emission baseline (SSP5-8.5). In parallel, we forecasted the 2035 land cover landscape via the Land Change Model (LCM), extrapolating from historical shifts recorded across 1987, 2009, and 2017. To decouple and quantify the specific hydrological signature of each stressor, the Soil and Water Assessment Tool (SWAT) was deployed. The model’s predictive rigor was confirmed through a split-sample approach—calibration (1996–2000) and validation (2001–2005)—which yielded Nash-Sutcliffe Efficiency (NSE) coefficients of 0.70 and 0.68, respectively. Projections indicate a distinct shift towards wetter and warmer conditions: annual precipitation is set to rise by 12–14%, with a notable seasonal spike of 13–15% in wet months. Concurrently, thermal metrics are rising, with maximum and minimum temperatures climbing by approximately 2.23–2.80°C and 2.25–2.89°C. Interestingly, the drivers exert opposing forces on hydrology. Climate change in isolation appears to boost annual streamflow by 14–16%. Conversely, predicted land-use alterations tend to attenuate flow, reducing annual discharge by roughly 4% (peaking at ~ 6% reduction in the dry season). Ultimately, the climatic signal overrides the land-use signal; the combined modeling approach suggests a net positive anomaly in streamflow (10–12% increase) for the 2025–2052 window, with the hydrologic surplus being particularly evident during dry seasons.