Estimated amount of water needed to replace the water used by plants and evaporation from the past week. (Negative numbers mean amount of
water needed, positive numbers mean excess water is present.)
Evapotranspiration (ET) is a term describing the sum of evaporation and plant transpiration from the earth's land surface to atmosphere,
including soil (soil evaporation), and from vegetation (transpiration). The latter two are often the most important contributors to
evapotranspiration. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and
waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in
its leaves. Other contributors to evapotranspiration may include evaporation from wet canopy surface (wet-canopy evaporation), and
evaporation from vegetation-covered water surface in wetlands.
The evaporation component of ET is comprised of the return of water back to the atmosphere through direct evaporative loss from the soil
surface, standing water (depression storage), and water on surfaces (intercepted water) such as leaves and/or roofs. Transpired water is
that which is used by vegetation and subsequently lost to the atmosphere as vapor. The water generally enters the plant through the root
zone, is used for various biophysiological functions including photosynthesis, and then passes back to the atmosphere through the leaf
stomates. Transpiration will stop if the vegetation becomes stressed to the wilting point, which is the point in which there is
insufficient water left in the soil for a plant to transpire, or if the plant to atmosphere vapor concentration gradient becomes
prohibitive to plant physiological processes (e.g. photosynthesis).
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