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The aim of this study was to assess the magnitude of evaporation loss from the agricultural water reservoirs (AWRs) for irrigation at a regional scale and to analyze its impact on water storage efficiency. To this end, we identified the extant AWRs for irrigation in the Segura River Basin (SRB) in southeastern Spain, and calculated the water loss from each AWR per month and year. In order to accomplish this, we determined the monthly and yearly values of the pan coefficient, Kp, taking into account the geometric dimensions (area and depth) of the AWRs and local climate conditions through a function of air vapour pressure deficit (VPD). AWR areas were identified by interpreting aerial images, while climate conditions were assessed using daily meteorological data obtained from 74 automated agro-meteorological stations located in irrigated areas. Regional evaporation losses were estimated using aggregation GIS techniques. A total of 14,145 AWRs covering 4901 ha were identified, which represents 0.26 and 1.81% of the total area of SRB and the irrigated land, respectively. Results indicated that annual water loss at a basin scale reaches 58.5 × 106 m3, which corresponds to 1.404 m of water depth over the flooded area and to 8.3% of irrigation water use in the basin. This quantity is higher than the industrial demand and similar to the environmental demand, and is equivalent to 27% of the domestic water use in a region with approximately two million inhabitants. The method used, based on annual Kp, appears the most straightforward to assess regional evaporative losses from AWRs, and can be extended to other regions and climates, provided that the VPD-dependent function that gives the pan coefficient is available.
