A 1‐dimensional surface energy balance model is applied to produce continuous simulations of daily lake evaporation of Upper Klamath Lake, Oregon (UKL) for the period 1950–2005. The model is implemented using observed data from land‐based sites and rafts collected during 2005–2006. Progressively longer, temporally overlapping simulations are produced using observed forcing data sets from sites near UKL. Simulation of the entire 56 years is accomplished using forcing data derived from weather station data and a 1949–2007 regional climate simulation over western North America. Simulated mean annual evaporation for 1950–2005 is 1073 mm. The simulated evaporation estimates are an improvement over existing May–September pan‐derived estimates because the latter are not representative of annual evaporation rates and do not span the multidecadal period of interest over which climate‐driven interannual (and longer) variability is evident. Evaporation and the other components of the water balance display statistically significant trends over the past 56 years that are associated with changes in meteorological forcing over the lake and the radiative and moisture balances at higher elevations of the catchment. Trends in the basin are consistent with and imbedded in regional and hemispheric climate trends that have occurred over the last century.