Assessing Hydrological Variability and Water Scarcity in a Mountainous River Catchment under Climate and Demographic Change

Climate change is intensifying hydro-climatic variability in mountainous regions, where complex terrain, limited data availability, and rising anthropogenic pressures already challenge sustainable water-resources management. This research analyses the combined influence of extreme weather, drought variability, water-resource components (blue and green water), and demographic change on the future water security of a mountainous catchment. An integrated framework was applied using downscaled CMIP6 projections to examine future changes in hydro-meteorological extremes under 2°C and 3°C global warming levels (GWLs), as well as near-future (2021-2050) and far-future (2071-2100) periods. A calibrated hydrological model was used to quantify blue water (surface water), green water (soil moisture and evapotranspiration), and the Streamflow Drought Index (SDI). These indicators, combined with future population projections, were incorporated into a Fuzzy Analytic Hierarchy Process (FAHP), enabling identification of sub-catchments that are most vulnerable to future water scarcity. Results indicate a significant increase in hydro-climatic extremes. Under SSP5-8.5, extreme temperature indices (TXx, TNx) rise sharply, and annual rainfall increases by up to 31% by century’s end, accompanied by a 50-77% increase in very heavy precipitation events (R95p, R99p). High flows (Q95) exhibit notable amplification (+30%), while low flows (Q05) decline across most sub-catchments, highlighting increasing hydrological variability. Blue water availability is projected to rise by 22-44%, whereas green water flow increases moderately (+15-28%). In contrast, green water storage shows minimal or negative trends, suggesting declining soil moisture resilience despite higher rainfall. Hydrological drought analysis shows a basin-wide shift toward negative SDI values, indicating the rise of mild to moderate drought conditions even under wetter climates, driven by altered runoff timing, intensified evapotranspiration, and declining low-flow. Integrating eight hydro-climatic, hydrological, and socio-demographic indicators, the fuzzy AHP framework identifies four out of nine sub-catchments as highly vulnerable under SSP2-4.5, driven by rapid population growth (up to +40%), rising drought stress, and limited green-water buffering. Under SSP5-8.5, although higher rainfall reduces vulnerability in some areas, intensified extremes and shifting hydrological regimes expand moderate-risk zones across the basin. Findings reveal rising rainfall yet declining effective water, worsening low flows, and drought, highlighting the need for catchment-specific, climate-resilient water-management strategies.