Estimating Seasonal ET from Multispectral Airborne Imagery: An Evaluation of Interpolation-Extrapolation Techniques

Abstract

Irrigated agriculture in the Texas High Plains uses over 90% of the ground water from the Ogallala Aquifer. Efficient water use through improved irrigation scheduling is expected to moderate the aquifer decline rate and improve sustainability. Thus, an accurate estimation of spatial actual daily and seasonal evapotranspiration (ET) is needed. Remote sensing may be used for monitoring distributed actual ET. Therefore, during 2007, the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX07) was conducted at the USDA-ARS Conservation and Production Research Laboratory (CPRL), Bushland, Texas. During BEAREX07, high resolution aircraft imagery (0.5 m pixel size in the visible and near-infrared bands and 1.8 m in the thermal band) were acquired during the cropping season using the Utah State University airborne multispectral digital system. Actual instantaneous ET was estimated using a two-source energy balance algorithm and extrapolation to daily values was performed using the evaporative fraction and the grass reference ET fraction (EToF). Cumulative/seasonal ET was estimated using EToF and cumulative grass reference ET. Data from four weighing lysimeters, in sorghum and corn fields, were used for evaluating ET predictions. Instantaneous ET was predicted with mean bias error (MBE) and root mean square error (RMSE) of 0.05 and 0.1 mm h-1 respectively. Daily ET was better extrapolated by the EToF method (error of 0.60.8 mm d-1, MBERMSE); while seasonal ET was slightly under-predicted for the short period of June and July by 8.930.4 mm. It appears that the aerodynamic resistance, in the soil sensible heat flux, has to be neglected under low biomass [leaf area index (LAI) 0.5 m2 m-2] and heterogeneous vegetation cover conditions. Furthermore, at LAI values around 5.0 m2 m-2, ET was over-predicted perhaps due to errors in estimating the fraction of LAI that is green, the clumping factor, the vegetation fraction, soil heat flux, and/or the soil resistance to heat flux term.