Losing Water by Storing It: The Oversighted Side of Intensive Water Regulation and Damming

Reservoirs are crucial for managing water resources but they may also promote enhanced water evaporation inland. Here we analyze the historical trends, causal attribution, and future projections of regional evaporative losses across 362 Spanish reservoirs, representing 94% of the nation's storage capacity, in one of the most heavily dammed countries in the world. There is a consistent annual increase in evaporation of 27.7 hm³ year⁻¹ from 1961 to 2018. This trend resulted in an accumulated evaporative loss of 114,000 hm³, averaging 2000 hm³ year⁻¹. While climate dynamics and warming have played a role in this trend, our research demonstrate that the impact of new reservoir construction and fluctuations in available water surface area have been much more influential (22 and 7 times greater, respectively). Anticipated evaporative losses by the end of the 21st century under a high greenhouse gases emissions scenario are expected to be 35% higher than those registered during the observational period. Our projections suggest that warming will increasingly drive evaporation, yet the available water surface will remain a critical determinant.