Is Scintillometer Measurement Accurate Enough for Evaluating Remote Sensing Based Energy Balance ET Models ?.

The three evapotranspiration (ET) measurement/retrieval techniques used in this study, lysimeter, scintillometer and remote sensing vary in their level of complexity, accuracy, resolution and applicability. The lysimeter with its point measurement is the most accurate and direct method to measure ET, however its application is limited to field scale. Remote sensing based algorithms is the only viable means to retrieve ET on a spatial scale, however its non-continuous nature is an hindrance to operational utilization. Scintillometer based on scintillation phenomenon is a relatively new technique with advantage of lager footprint area, but it has limited application on agricultural systems. The objective of this study is to inter-compare the three techniques and evaluate their accuracies and suitability. High resolution remote sensing images from multispectral sensors onboard aircraft were collected over irrigated and dryland cotton fields during 2008 summer cropping seasons at the USDA-ARS Conservation and Production Research Laboratory (CPRL) in Bushland, Texas. Surface Energy Balance System (SEBS) algorithm was employed to retrieve ET fluxes. Each of the four field (2 irrigated and 2 dryland) of approximately 5 ha was installed with large precision weighing lysimeters (3 m long x 3 m wide x 2.4 m deep), located in the middle of fields. Large aperture scintillometer (LAS) with the transmitter and receiver systems were installed at the edge of the fields. The source area (footprint) of the surface energy fluxes were computed using footprint model. Lysimeter measurements being point based may not be appropriate to validate the source area weighted value from scintillometer. Remote sensing based ET values was found to be more suitable for comparing the scintillometer derived ET.