In arid zones, water comes mainly from snow, glaciers and ice-rich permafrost landforms. We introduce the Normalized Cryospheric Index (NCI), which combines ice storage and snow permanence to rank sub-watersheds where no regulatory framework exists. Applied to the Copiapó basin (Atacama, Chile), the NCI identifies the Montosa, Del Potro, Manflas and Estero Come Caballos sub-watersheds as priority targets for sustaining runoff in Chile's national water resources strategy.