The Metric Model for Surface Energy Balance-Derived ET from Landsat Imagery and the Importance of Calibration and Accuracy for Water Management in Operational Agricultural Decision-Making.

The quantification of amount and spatial variability of evapotranspiration (ET) has now become a necessary and expected component of many water resources and hydrologic studies and management systems. ET maps produced at the 30 m scale of Landsat to enable the identification of ET at the field or parcel scale, which is the scale at which critical water management is generally conducted. The METRIC - Mapping evapotranspiration at high resolution with internalized calibration - model is a satellite-based image-processing procedure that calculates ET as a residual of the surface energy balance. The use of the energy balance that is driven by thermal and reflected imagery is useful for detecting actual rates of ET that may be lower than potential rates due to water shortage or low vegetation amount. METRIC is an ‘engineering model’ in that it provides for human review and tuning of images via the setting of energy balance and vegetation based parameters. Near-surface air temperature gradients are expressed as an indexed function of radiometric surface temperature. Calibration to reference ET reduces computational biases inherent to remote sensing-based energy balance and provides congruency with traditional methods for estimating ET. Slope and aspect functions and temperature lapsing are used in applications having mountainous terrain to account for terrain impacts on solar radiation and aerodynamic characteristics. METRIC algorithms are designed for relatively routine application by trained water professionals who possess a familiarity with energy balance and basic radiation physics. The use of reference ET for the extrapolation of instantaneous ET from periods of 24 h and longer accounts for regional advection effects. METRIC has some significant advantages over conventional methods of estimating ET from crop coefficient curves in that neither the crop development stage, nor the specific crop type need to be known.