Evapotranspiration is actually the combination of two terms – evaporation and transpiration.
The first of these, that is, evaporation is the process of liquid converting into vapour,
through wind action and solar radiation and returning to the atmosphere.
Evaporation is the cause of loss of water from open bodies of water, such as lakes, rivers,
the oceans and the land surface. It is interesting to note that ocean evaporation provides
approximately 90 percent of the earth’s precipitation.
Transpiration is the process by which water molecules leaves the body of a living plant and
escapes to the atmosphere. The water is drawn up by the plant root system and part of that is
lost through the tissues of plant leaf (through the stomata).
In areas of abundant rainfall, transpiration is fairly constant with variations occurring
primarily in the length of each plants growing season. However, transpiration in dry areas
varies greatly with the root depth.
Evapotranspiration, therefore, includes all evaporation from water and land surfaces, as well
as transpiration from plants.
WATER RESOURCES SURVEY
The term water resources refer to the exploitable surface water and groundwater in a defined water
management unit (e.g. catchment basin).
All Water Resources projects have to be cost evaluated. This is an essential part of planning.
Since, generally, such projects would be funded by the respective State Governments, in which the
project would be coming up it would be helpful for the State planners to collect the desired amount
of money, like by issuing bonds to the public, taking loans from a bank, etc.
Since a project involves money, it is essential that the minimum amount is spent, under the given
constraints of project construction.
Hence, a few feasible alternatives for a project are usually worked out. For example, a project
involving a storage dam has to be located on a map of the river valley at more than one possible
location, if the terrain permits.
In this instance, the dam would generally be located at the narrowest part of the river valley to
reduce cost of dam construction, but also a couple of more alternatives would be selected since
there would be other features of a dam whose cost would dictate the total cost of the project.
For example, the foundation could be weak for the first alternative and consequently require costly
found treatment, raising thereby the total project cost.
At times, an economically lucrative project site may be causing submergence of a costly property,
say an industry, whose relocation cost would offset the benefit of the alternative.