Remote Sensing-Based Evapotranspiration for Water Management.

It is important to quantify the consumptive water use by the vegetation when managing regional water resources in areas with agricultural production. Suitable models and algorithms applied to high resolution (30 m) satellite imagery provide a cost effective and time efficient method to obtain evapotranspiration estimations from bare soil and vegetation. The METRIC image processing model calculates net radiation, soil heat flux and sensible heat flux through a number of steps before estimating evapotranspiration as the residual from the energy balance [Allen R.G., Tasumi M., and Trezza R., 2007, Satellite-based Energy Balance for Mapping Evapotranspiration with Internalized Calibration (METRIC) – Model: J. Irrig. Drain. Engrg., v 133, 380-394]. METRIC utilizes an innovative Calibration using Inverse Modeling at Extreme Conditions (CIMEC) method for estimating sensible heat flux by inverse modeling of the near surface temperature gradient (dT) for each image pixel based on a relationship between the dT and radiometric surface temperature at two “anchor” pixels. The advantage of the CIMIC approach to develop the dT vs. T_s relationship is that many biases in energy balance components are factored out, including those in T_s. The anchor pixels ideally represent the conditions of an agricultural field having full and actively transpiring vegetation cover and a bare agricultural field having no vegetation cover and little residual evaporation. The METRIC procedure utilizes the alfalfa based reference evapotranspiration ET_r to establish the energy balance at the cold pixel, thus establishing a ground reference for the satellite image based ET estimate. The complete energy balance obtained from the satellite images is calibrated using ground based reference evapotranspiration estimations. The paper describes an application of the METRIC model in South Dakota and how the selection of anchor pixels in a largely rainfed agricultural production system was selected.