Coupling SWAT and CMIP6 Climate Projections to Assess Runoff Variability and Water Security in the Semi-Arid Colorado River Basin

Climate change poses an existential threat to water security in the semi-arid Colorado River Basin (CRB), a critical source of water for over forty million people. This study evaluates the future impacts of climate change on runoff variability by coupling the Soil and Water Assessment Tool (SWAT) with downscaled climate projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6). The model was calibrated and validated using historical streamflow data from 1990 to 2020, achieving high predictive accuracy with a Nash-Sutcliffe Efficiency (NSE) of 0.82. Hydrological projections were simulated for the mid-century period (2025–2060) under two Shared Socioeconomic Pathways: SSP2-4.5 (moderate emissions) and SSP5-8.5 (high emissions). Results indicate a projected decrease in mean annual runoff by 11.4% under SSP2-4.5 and 17.8% under SSP5-8.5, primarily driven by rising temperatures and enhanced evapotranspiration. Furthermore, we identify a critical temporal shift in the seasonal hydrograph, with peak runoff occurring approximately two to three weeks earlier in the spring due to accelerated snowmelt. This shift leads to a severe reduction in late-summer flows of up to 35%, exacerbating water scarcity during periods of peak agricultural and municipal demand. These findings highlight the urgent need for adaptive reservoir operations and updated water allocation policies to mitigate risks to water security in the basin.