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Small agricultural reservoirs support water demands during dry spells. However, evaporative losses that are often overlooked in water accounting and management diminish the storage efficiency of these popular but un‐inventoried resources. We developed a predictive framework to identify the spatio‐temporal extent of small reservoirs (900–100,000 m²) and quantify their evaporative losses using a physically‐based model. Focusing on water‐stressed regions of Europe (Italy, Spain, and Portugal), our results indicate that the total number and cumulative area of small reservoirs in drier areas of Europe almost doubled in two decades from about 6,200 reservoirs with the cumulative area of 46 km² in 2,000 to 11,800 reservoirs with the cumulative area of 93.5 km² in 2020. We observed climate‐driven trends in the expansion of agricultural reservoirs and their evaporative losses which exceeded 72 million cubic …