Remote sensing ET of alfalfa using a surface aerodynamic temperature model

Accurate estimates of spatially distributed evapotranspiration (ET) using remote sensing algorithms could potentially improve crop water management. In addition, seasonal ET values could be used in assessing irrigation project efficiency, water use efficiency, ground water depletion, and the use of water rights over large irrigated areas. In this study, ET was mapped using airborne remote sensing model (SAT-ET) which uses inputs such as surface reflectance and radiometric surface temperature (T_s), and a surface aerodynamic temperature (T_o) sub-model. The SAT-ET model for irrigated alfalfa in Colorado was developed using surface temperature, horizontal wind speed and air temperature profile measurements, and with measurements of turbulent sensible heat fluxes (H) acquired with a Large Aperture Scintillometer (LAS). Estimates of the remote sensing-based ET for a 4.0 hectare alfalfa field, during the 2009 cropping season, were evaluated using a large weighing lysimeter (3.0 x 3.0 x 2.4 m) located at the Colorado State University (CSU) Arkansas Valley Research Center (AVRC) in Rocky Ford, CO. Hourly alfalfa ET was very well estimated with small errors of -5.8 to 1.1% while larger errors were found for estimates of daily ET (-13.5 to 17.8%). The T_o sub-model is very promising; however it needs to be further developed incorporating a wider range of crops and environmental and atmospheric conditions. Furthermore, the mechanism to extrapolate hourly to daily ET values needs to be investigated as well as the method to calculate reference ET for the conditions encountered in eastern Colorado.