Why Use Reference Evapotranspiration to Calibrate Satellite-Based Energy Balances?

Satellite-based surface energy balance has become a very useful tool for determining actual evapotranspiration (ET) over large areas. Major uncertainties in derived ET exist, however, due to error and bias introduced to the surface energy balance and ET by components of the energy balance. These biases include biases in atmospheric correction, albedo calculation, net radiation calculation, thermal band and surface temperature, air temperature gradient used in sensible heat flux calculation, soil heat flux function, and extrapolation to 24-hour and longer periods. At the University of Idaho, we use calculated hourly reference evapotranspiration (ET_r) to calibrate the surface energy balance during processing to remove a majority of these biases. The ET_r method uses alfalfa reference ET calculated with the ASCE-EWRI standardized Penman-Monteith equation. Experience and field verification with this method has created confidence in using ET_r for image calibration rather than using available energy only. In addition to its use to calibrate the energy balance, ET_r is used to extrapolate ET images to 24-hour and longer periods. The use of ET_r provides equivalency and congraency with ET as estimated using the traditional crop coefficient x ET_r approach. With Landsat images, full-cover alfalfa or other high leaf area vegetation can be identified, and ET from these fields is expected to be near the value of alfalfa ET_r. Comparisons between ET by METRIC, ET by lysimeter and ET by traditional methods suggest that METRIC or similar methods hold promise as efficient, accurate, and inexpensive procedures to predict actual evaporation fluxes throughout a growing season.