Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water_cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator. Plant growth processes and productivity of agro ecosystems depend highly on evapotranspiration from the land (soil-crop cover complex) surface. Through links between stomatal conductance, carbon exchange and water use efficiency in plant canopies, ET serves as a regulator of key ecosystem processes. Remote sensing has proven to be the best tool to estimate evapotranspiration at regional level. The remote sensing data provide representative parameters such as surface temperature, surface albedo, and vegetation indices. Multi-band sensors that measure reflected solar and emitted thermal radiation within discrete wavelength intervals could obtain detailed data over an extended area. This paper reviews significance and applications of estimation of evapotranspiration in plant system, water cycle and agriculture and role of remote sensing techniques in the estimation of evapotranspiration. It also evaluates future outcomes as well as remaining challenges associated with remote sensing techniques at present.
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