Evaporation from a reservoir’s surface is an important component of water balance calculations. Using long-term (1955–2020) climate data from Alberta Climate Information Service, we estimated gross and net lake evaporation from fifteen reservoirs in the South Saskatchewan River Basin of Alberta based on Morton’s complementary relationship-based model. We ranked the reservoirs based on various criteria to identify the reservoirs with significant evaporative losses. The net maximum annual evaporative loss from fifteen reservoirs ranges from 659 cubic decameters (dam3) to 21,251 dam3. Our analysis reveals that there are six reservoirs with significant net evaporative losses (e.g., > 70% of total annual maximum net loss). We also performed additional evaporative loss calculations on these six reservoirs using the Prairie Farm Rehabilitation Agency (PFRA)-modified Meyer’s method. We compared the differences between the two evaporation models. Our results show that Morton’s model shows better consistency than PFRA-Meyer’s model considering data availability and the sensitivity of evaporation estimates. We also applied Morton’s shallow lake and deep lake models for those six reservoirs to analyze the seasonal variabilities in evaporative losses. Although on an annual scale, both models provide similar results, we demonstrated that the deep lake model is able to capture monthly variabilities better than the shallow lake model.