Multi-season lake evaporation: energy-budget estimates and CRLE model assessment with limited meteorological observations

Abstract A lake evaporation study was conducted on a small water-supply reservoir (8.8 km 2 ) in Minas Gerais State, Brazil, during 30 months. Air temperature and humidity, wind speed and direction, sunshine duration and solar radiation were measured continuously, together with monthly water temperature profiles. This allowed the calculation of evaporation by the energy-budget Bowen ratio (EBBR) method, as well as evaporation estimates using a climatological model, the Complementary Relationship Lake Evaporation (CRLE), which is widely used by electricity companies in Brazil. The CRLE is probably the most complete model available for lake evaporation that requires only monthly means of air temperature, air humidity and sunshine duration (or solar radiation), and provides estimates not only of monthly evaporation but of the rate of change of stored enthalpy in the lake's waters and equilibrium temperature, a surrogate of the actual water surface temperature. Since all of the model's variables are amenable to physical interpretation, it is possible to relate them to the actually measured values. We have found that the Priestley–Taylor equation, which is the basis of CRLE's evaporation estimates, performs very well under conditions of positive (upward) sensible heat flux. The model's equilibrium temperature underestimates the actual water surface temperature but follows the same seasonal pattern. The model's estimates of the rate of change of stored enthalpy are generally poor, making the CRLE generate evaporation values that are delayed and damped in comparison with the EBBR method. The CRLE tends to overestimate evaporation slightly (8%) over the 30-month period.