Future Quantitative Water Security Under Climate and Demographic Change in Selected Peruvian Tributaries of the Lake Titicaca Basin: A Spatially Explicit SWAT–CMIP6–SSP Assessment

Climate change and demographic dynamics may jointly alter future water availability and demand in high-Andean watersheds, yet their combined spatial effects remain insufficiently quantified in the Lake Titicaca Basin. This study integrated the Soil and Water Assessment Tool (SWAT), bias-adjusted CMIP6 climate projections, Shared Socioeconomic Pathway (SSP) population projections, and a network-consistent demand-to-availability Water Stress Index (WSI) to assess quantitative water security in three selected Peruvian tributary basins of Lake Titicaca—the Ramis, Coata, and Huancané—during 2025–2100. SWAT simulations were forced with ACCESS-CM2 and MPI-ESM1-2-HR under SSP1-2.6 and SSP3-7.0, while spatially explicit FuturePop projections were used to estimate domestic water demand across 177 sub-basins. Projected water availability showed substantial interannual and inter-model variability, with Huancané exhibiting the highest supply, Ramis intermediate conditions, and Coata the lowest availability. Under SSP1-2.6, population declined by 2100 in all three basins, whereas SSP3-7.0 increased population by 64.5% in Ramis, 17.2% in Coata, and 66.9% in Huancané. Coata exhibited the narrowest basin-scale supply–demand margin and reached a slight deficit of −0.29 hm3 year−1 by 2100 under SSP3-7.0. However, the network-consistent basin-outlet WSI remained below the critical threshold of 1 in all three basins. Spatially, 98.3–98.9% of the 177 sub-basins were classified as very low stress across the representative years and scenarios under the principal 10% non-allocable streamflow reserve assumption, while WSI ≥ 1 hotspots remained limited to 0–2 sub-basins and were concentrated in Coata. Increasing the non-allocable reserve from 10% to 30% changed hotspot occurrence by no more than one sub-basin at any evaluated horizon. These results show that quantitative water stress is highly localized and sensitive to the spatial correspondence between accumulated demand and routed-streamflow availability, emphasizing the importance of network-consistent supply–demand assessments for long-term water-resource planning in high-Andean systems.