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

This study focuses on Andalusia (Southern Spain), a mediterranean region of 87,270 km 2 . Andalusia experiences significant water stress, making effective water management crucial. 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. 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 hm 3 /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. • Annual evaporation losses are estimated for 76 large Andalusian reservoirs (<5 hm 3 ). • For the set of reservoirs average losses amount 547 hm³ /year, (8 % of inflows). • A combination of indexes is proposed to classify reservoirs by their efficiency. • The most inefficient reservoirs lose 30–54 % of inflows due to evaporation. • Isotopic analysis confirms evaporation estimates in selected reservoirs.