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

Abstract

[eng] Reservoirs are crucial for managing water resources but they may also promote enhanced waterevaporation inland. Here we analyze the historical trends, causal attribution, and future projections of regionalevaporative losses across 362 Spanish reservoirs, representing 94% of the nation's storage capacity, in one of themost heavily dammed countries in the world. There is a consistent annual increase in evaporation of27.7 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 researchdemonstrate that the impact of new reservoir construction and fluctuations in available water surface area havebeen much more influential (22 and 7 times greater, respectively). Anticipated evaporative losses by the end ofthe 21st century under a high greenhouse gases emissions scenario are expected to be 35% higher than thoseregistered during the observational period. Our projections suggest that warming will increasingly driveevaporation, yet the available water surface will remain a critical determinant.