Daily evapotranspiration estimates from extrapolating instantaneous airborne remote sensing ET values

Abstract

In this study, six extrapolation methods have been compared for their ability to estimate daily crop evapotranspiration (ETd) from instantaneous latent heat flux estimates derived from digital airborne multispectral remote sensing imagery. Data used in this study were collected during an experiment on corn and soybean fields, covering an area of approximately 12 9 22 km, near Ames, Iowa. ETd estimation errors for all six methods and both crops varied from -5.7 ± 4.8% (MBE ± RMSE) to 26.0 ± 15.8%. Extrapolated ETd values based on the evaporative fraction (EF) method better compared to eddy covariance measured ET values. This method reported an average corn ETd estimate error of -0.3 mm day -1 , with a corresponding error standard deviation of 0.2 mm day -1 , i.e., about 5.7 ± 4.8% average under prediction when compared to average ETd values derived from eddy covariance energy balance systems. A solar radiation-based ET extrapolation method performed relatively well with ETd estimation error of 2.2 ± 10.1% for both crops. An alfalfa reference ET-based extrapolation fraction method (ETrF) yielded an overall ETd overestimation of about 4.0 ± 10.0% for both crops. It is recommended that the average daily soil heat flux not be neglected in the calcu- lation of ETd when utilizing method EF. These results validate the use of the airborne multispectral RS-based ET methodology for the estimation of instantaneous ET and its extrapolation to ETd. In addition, all methods need to be further tested under a variety of vegetation surface homogeneity, crop growth stage, environmental and cli- matological conditions.