Evaluating evapotranspiration at local and regional scales

Combining remotely sensed data with ground-based meteorological data allows the evaluation of evapotranspiration (the evaporation of water from soil and plant surfaces) at local and regional scales. Remote sensors can provide information on reflected solar radiation and surface temperatures. The remaining variables in the energy balance equations must be measured at ground level, estimated, modeled, or ignored. It is how these variables are evaluated that distinguish the several approaches to estimating evapotranspiration. In general, regional scale methods would apply to part or all of a satellite image, and use meteorological data from local weather stations. Local scale techniques would rely largely on airborne remote sensors and on-site measurements of the pertinent meteorological factors at the time of remote-data collection. In this paper, methods for estimating evapotranspiration on both local and regional scales are reviewed, and some factors that complicate its measurement are discussed.