Are Reservoirs Water Consumers or Water Collectors? Reflections on the Water Footprint Concept Applied on Reservoirs

IPCC Special Report on Renewable Energy Sources (2011) revealed potentially very high water consumption rates from hydropower production compared to other renewable technologies, but suffered from few studies and methodological problems. More recent studies present new estimates values far beyond those presented by IPCC, some claiming that hydropower is a large-scale water consumer, but do not provide a more consistent picture of the 'true water consumption of hydropower'. We compiled data from ICOLD's World Register of Dams, considered being the most extensive and complete global dataset of reservoirs and dams larger than 15 m containing description of close to 40 000 dams and reservoirs. We coupled this dataset with water scarcity information about the location of the individual projects and found that only very few reservoirs located in water-scarce areas are used exclusively for hydropower production or have that as their main purpose (fewer than 0.1 %). As the purpose of the majority of the reservoirs located in water-scarce areas are to collect water in the wet season to secure adequate supply of water for irrigation, domestic supply, industry and more purposes in the dry season, we find it fundamentally problematic to assign a water footprint to such an infrastructure, even though the purpose of these reservoirs might also be to produce electricity. Rather opposite - the fact that reservoirs increase the availability of water in the dry season make reservoirs needed. We conclude that assigning water footprint/consumption values of reservoirs will convey the wrong message to decision-makers unless the reservoirs' effect on the availability of local water resources is fully accounted for.