A National Water Stress Index for Over-Allocation of Water Use Rights in Peruvian River Basins

Peru's National Water Authority (Autoridad Nacional del Agua, ANA) has granted water use rights to 532,751 registered users nationally, totalling 99,039.76 hm³/year, but this administrative allocation is not routinely set against renewable water availability at the scale at which it is managed. We compute a Water Stress Index by over-allocation (IWS), the ratio of granted water volume to mean annual water availability, for each of Peru's 14 Autoridades Administrativas del Agua (AAA), from ANA's own registry of water use rights and its official water-balance estimates for 2012–2022. IWS is computed for all granted rights and, separately, for consumptive uses only, since non-consumptive rights such as hydropower concessions return water downstream without depleting the resource. Consumptive rights total 32,525.24 hm³/year, 33% of granted volume. The consumptive IWS identifies five coastal, arid AAAs (Jequetepeque-Zarumilla, Cañete-Fortaleza, Chaparra-Chincha, Huarmey-Chicama and Caplina-Ocoña) as moderately stressed (0.43–0.65), while the seven Amazon-slope AAAs remain below 0.11 despite holding a nominal majority of granted volume through hydropower concessions. Without separating uses, Cañete-Fortaleza reaches 2.38 and Mantaro 1.78, both classified as over-allocated on hydropower rights alone; Mantaro's non-consumptive entitlements exceed its consumptive ones by 14.7:1. Recomputing against four alternative denominators leaves the ranking and the coastal/Amazon contrast unchanged, but Jequetepeque-Zarumilla reaches 1.22 once transboundary inflow, 46.8% of its stated availability, is excluded, and 0.93 when 30% of the total is reserved as environmental flow. The registry reports actual use for 0.22% of granted volume.