It is the process during which a liquid changes into a gas.
The factors effecting evaporation are air and water temperature relative humidity, wind velocity, and surface area exposed.
The evaporation can be measured by the standards U.S Weather Bureau Class A pan.
It is the process by which water vapour leaves the living plan body and enters the atmosphere.
The transpiration is measured by “photometer” .
3. Evapotranspiration: (ETP)
It is also called consumptive use. It does notes the quality of water transpired by plants during their growth or retained in the plant tissue plus the moisture evaporated from the surface of the soil and the vegetation.
4. Seasonal Consumptive Use:
The total amount of water use in evapotranspiration be cropped area during the entire growing season is called seasonal consumptive use and is expressed in mm or cm.
5. Peak Period Consumptive Use:
It is the highest consumptive use during the few days of the crop growing season.
This is design rate to be used in planning at irrigation system. The peak use rate for the various crop sin a given area may occur at different times in the crop season.
6. Reference Crop Evapotranspiration ( ETo):
It is defined as the rate of evapotranspiration from an extended surface of 8 to 15 cm tall green grass cover of uniform height, actively growing , completely shading the ground and not short of water.
7. Water Requirement:
It may be defined as the quantity of water required by a crop or diversified pattern of cop in a given period of time for its normal growth under field condition at a place.
The accuracy of the determination of crop water requirement will largely dependent on the type of climatic data available.
i. e. WR= ET + application, losses + special need