A Comparison Between Measurements Using Surface Renewal, Eddy-Covariance, and Radiant Temperature Based Aerodynamic Methods, and Satellite-Based Remotely Sensed Evapotranspiration Estimates for Fallow and Cropped Fields

Abstract:

We measured Evapotranspiration (ET) from multiple fallow, tomato, and alfalfa fields in the Sacramento-San Joaquin Delta region in 2018. Measurements included (1) direct eddy-covariance with sonic anemometry and infrared gas analyzers, energy budget residual with (2) eddy-covariance and (3) surface renewal, (4) infrared thermometry based aerodynamic methods, and (5) estimates from 5 different satellite-based remote sensing based models. Differences were found between the different measurements; the measurements and model estimates; and in variables linked to the models, such as between the satellite-based remotely sensed surface temperatures and field based infrared thermometry. Surface renewal, originally developed for sensible heat and other scalar exchange from plant canopies, is shown to work well for fallow fields, with appropriate time-of-day or stability based adjustments to the calibration coefficient alpha. The calibration coefficient alpha was more constant for the cropped fields, with 2 different values, one for daytime and one for nighttime. Energy budget residual estimates were improved by enhancing the ground heat flux/storage term. NDVI was shown to help assess the presence and status of weed growth on fallow fields, and subsequently the dependence of ET values on vegetation. We quantify the uncertainty between satellite-based remotely sensed methods and the different field measurements.