Estimation of evaporation in Andalusian reservoirs: Proposal of an index for the assessment and classification of dams

Abstract

Study region: This study focuses on Andalusia (Southern Spain), a mediterranean region of 87,270 km2. Andalusia experiences significant water stress, making effective water management crucial. Study focus: Evaporation is a key element of the water budget, particularly in arid and semi-arid climates, where losses from reservoirs represent a significant fraction of the volume annually regulated and compromise the efficiency of these infrastructures. This study aims to (i) estimate the annual volume of water evaporated from large dams (> 5 hm³) in Andalusia and (ii) propose an index to classify reservoirs based on evaporative losses. New hydrological insights for the region: Data from 76 large dams in Andalusia, including their descriptive characteristics, morphometric details, and hydrological information, were retrieved from institutional sources. The monthly average flooded area was calculated from the Area- Volume-Elevation (AVE) curve, while monthly average evaporation rate was calculated through the FAO Penman-Monteith equation using meteorological data. The combination of both variables has allowed to estimate the mean monthly volume of water evaporated in each reservoir. Results were validated with isotopic content (¹⁸O and ²H) from selected reservoirs. The annual average volume evaporated from Andalusian reservoirs was estimated at 547 hm3/year (8.0 % of the inflows). Evaporative losses represent 1.1 % of inflows in the most efficient reservoirs, while in others, losses exceed 30 %, reaching 54 % in the most extreme case.