Abstract <p>The paper proposes a general calculation scheme for assessing the water losses due to an additional evaporation from the flood zones of large river reservoirs and considers methodological techniques for determining its individual components depending on the availability of initial data, the properties of a reservoir, as well as the natural zone in which it is located. The results of testing the proposed calculation scheme are presented by the example of the Tsimlyansk Reservoir. The problems associated with the implementation of the proposed methodological approaches are discussed, including the results of their application for assessing the water losses due to an additional evaporation from the flood zone of the Tsimlyansk Reservoir in 2021.</p>