Lake Evaporation in a Changing Climate

Lake evaporation is sensitive to climate change. Two hypotheses have been proposed to predict how lake evaporation may change in the future. According to the first hypothesis, open-water evaporation should increase as air temperature rises at a rate of about 7% K^-1 predicted by the Clausius-Clapeyron equation. The second hypothesis, supported by the universal declining trends in pan evaporation and tied to global diming, states that evaporation variabilities are controlled by variabilities in the surface solar radiation. In this study, we investigated the mechanisms underlying the response of lake evaporation to climate change, using NCAR's CLM4.5-LISSS (Lake, Ice, Snow, and Sediment Simulator) model. We first validated the model against observations at 27 lake in the world. We then simulated future (2005-2100, RCP8.5) lake evaporation using the same lake model. Even though the surface shortwave radiation declines at a rate of 0.8 W m^-2 decade^-1, the global mean lake evaporation increases at a rate of 0.85 W m^-2 decade^-1. Both the observed and modeled lake evaporative fraction (the ratio of latent heat flux to available energy (net radiation minus heat storage)) increase as air temperature rises, at a rate perfectly captured by the Priestley-Taylor model with the model parameter of 1.26. From the energy partitioning perspective, the projected lake evaporation trend is explained by the fact that more radiation energy is consumed by evaporation than by convective heat loss.