How Evapotranspiration Was Estimated
While evapotranspiration (ET) can be directly “measured” at a particular location, the methods of doing so are expensive and only applicable for locations meeting certain strict criteria. For example, one can use the eddy covariance technique, which uses high frequency measurements of the variations in vertical air motion and humidity to determine ET. Eddy covariance instrumentation costs tens of thousands of dollars, analysis of the data is highly specialized, sites must be fairly flat, land cover must be uniform, and the results are representative of only a small area. To map ET over varied terrain with contrasting land cover types and steep gradients in climate, it is necessary to use models to estimate the important processes involved, and to make use of the spatially-extensive data provided by satellite-borne sensors.
In this work, we employed a widely-used model for estimating ET, developed by H.L. Penman and J.L. Monteith.
Evapotranspiration is the sum of the evaporation of water from wet vegetation, transpiration through the leaves of plants, and evaporation from soil. To better represent these different processes, we made separate estimates of wet-canopy evaporation (λEWC); (2) transpiration (λETransp); and (3) soil evaporation (λESoil), each using a modified form of the Penman-Monteith equation.