Evapotranspiration (ET) overview
Evapotranspiration (ET) is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). It is an indicator of how much water your crops, lawn, garden, and trees need for healthy growth and productivity.
For ET to take place, the following conditions have to be met. First, water has to be present at the surface. Second, there must be some form of energy to convert the liquid water into a water vapour. Third, there must be a mechanism to transport the water vapor away from the evaporating surface.
Precipitation and irrigation are the two primary sources of water that plants use. Plant leaves and soil surfaces temporarily retain some part of the water applied to the field. This part is readily available for evaporation. The remaining part infiltrates into the soil. Plants extract the infiltrated water through their roots and transport it up to their leaves for photosynthesis, a process by which plants produce glucose (sugar). In addition to water, plants also need carbon dioxide (CO2) and light for photosynthesis. The light comes from the sun and CO2 comes from the atmosphere. In order to take in CO2 from the atmosphere, plants open their stomates, the microscopic pores on plant leaf surfaces. It is during this process that they loose their water to the atmosphere.
The conversion of liquid water into water vapour requires large amounts of energy (about 540 Calories per gram of water at a temperature of 100 °C). This energy is provided by the sun in the form of solar energy. The solar energy is absorbed by water molecules and converted to latent heat energy, the energy that is stored in vapour molecules. The water vapour thus produced escapes to the atmosphere because of a vapour pressure gradient between the surface and atmosphere. Once in the atmosphere, it is taken further away from the surface by wind (or other mechanisms), creating more gradient between the evaporating surface and the air above it. This process continues as long as the three conditions mentioned above are present.
Accurate estimates of ET are needed in many circumstances. In agricultural irrigation, for example, estimates of ET are necessary for system design, irrigation scheduling, water transfers, planning, and other water issues.
The above description was taken from "The California Irrigation Management Information System (CIMIS)", which offers the clearest explanation yet found, (8th April 2012).
No guarantee of accuracy or completeness of the above is expressed or implied.