MAPPING EVAPOTRANSPIRATION AT HIGH RESOLUTION WITH INTERNALIZED CALIBRATION (METRIC): MODEL OVERVIEW

Application of remote sensing algorithms solving the energy balance using high resolution satellite imagery has proven useful for establishing estimates of evapotranspiration (ET) for large populations of fields and water users (Bastiaanssen and others, 1998; Allen and others, 2007).

ET is generally estimated in energy balance processes as a residual of the energy balance as:

LE = R_n - G - H (1)

where LE is the latent energy consumed by ET, R_n is net radiation flux density at the surface, G is heat flux density into the ground and H is sensible heat flux density into the air.

Models that solve the energy balance to estimate ET include Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC) (Allen and others, 2007). In METRIC, R_n is estimated by solving the radiation balance. G is estimated as a function of surface temperature, net radiation and a vegetation index. METRIC utilizes an innovative Calibration using Inverse Modeling at Extreme Conditions (CIMEC) method as pioneered by Bastiaanssen 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. ET_r is calculated outside of METRIC using hourly (or shorter) weather data from a weather station preferably located toward the center of the study area. The use of ET_r is generally effective in tying down the energy balance calibration, especially in arid and semiarid climates having advection. The use of ET_r for calibration and extrapolation of ET to longer time periods makes the METRIC process congruent with traditional ET_r based estimation methods. One of the outputs from METRIC is a map of the ET from each pixel stated as a fraction of ET_r, ET_rF. ET_rF is synonymous with the well known K_c (for an alfalfa reference basis). Daily ET_a maps are calculated by multiplying the instantaneous ET_rF calculated for each pixel with the 24-h summed ET_r. The resulting high resolution maps of ET cover regions typically up to 150 km in scale. When used properly, METRIC provides a rapid and cost effective method to determine ET for focused regions.

References

Allen R.G., Tasumi M., and Trezza R., 2007, Satellite-based Energy Balance for Mapping Evapotranspiration with Internalized Calibration (METRIC) – Model: 0 J. Irrig. Drain. Engrg., v 133(4), p. 380-394.

Bastiaanssen W.G.M., Menenti M., Feddes R.A., and Holtslag A.M.M., 1998, A remote sensing surface energy balance algorithm for land (SEBAL): 1. Formulation: J. Hydrology 212-213, 198-212.