The influence of evaporation and rainfall on the reservoir water balance equation

Evaporation has a major significance in the water balance of a reservoir. Usually, recorded data for the evaporation on the free surface of a reservoir are not available. There are numerous empirical relationships for the assessment of the evaporation that can be implemented into the water balance equation. In this paper, for Vidraru, one of the largest reservoirs in Romania, the Hargreaves method is used to estimate the evaporation values that are compared with recorded data obtained from Meteoblue archive. Recorded precipitation and evaporation data are used in mathematical model for water balance to find the answer to the question: can the evaporation and the directly water surface rainfall be neglected in the monthly/annual water balance of a reservoir? Daily meteorological values for the minimum and maximum temperature, evaporation and precipitation measured in the Vidraru reservoir area are used in this work. The main conclusion of the paper is that although in the summer months, on the surface of the lake, the amount of water lost through the evaporation is greater than the amount of water from the precipitations, on the time horizon of one year, the two components of the water balance of the reservoir have close values. Thus, for an accurate application of the water balance equation for a reservoir, the two variables, evaporation and precipitation, can be both considered or both neglected.