Transpiration
It is the process by which water (taken up by the plant from the soil through its root system) leaves the body of a living plant through the stomatal openings in its leaves and reaches the atmosphere as water vapour.
Important factors affecting transpiration are
– Atmospheric vapour pressure
– Temperature
– Wind
– Characteristics of the plant
– Light intensity etc
• For a given plant, factors affecting free water evaporation also affect transpiration
• Transpiration occurs only during daylight hours and rate of transpiration depends on the growth period of the plant
• On the other hand, evaporation occurs both during the day and night, although the rate of evaporation may be different during these time periods
Evapotranspiration
• While transpiration takes place from the vegetation, the land area where these plants stand also lose moisture by evaporation of water from soil and water bodies
• As it is difficult to separate these two losses in cropped fields,in designating water use by crops evaporation and transpiration are combined into one term called Evapotranspiration (ET) or Consumptive Use (CU)
• The term consumptive use is actually used to designate the sum of water lost due to evapotranspiration and water used by the plants for its metabolic activities
• Since water used by vegetation in the metabolic processes is relatively insignificant (less than 1% of ET), the term consumptive use is treated as equivalent to ET
• ET includes all water consumed by the plants plus water evaporated from bare land and open water surfaces in the area occupied by the crops
• In general, factors affecting the evaporation and transpiration processes also influence ET
• Other factors being the same, the stage of growth of a crop has considerable influence on the ET rate
• For a given set of atmospheric conditions and for a given crop and a given growth stage of the crop, the consumptive use rate or ET depends on the availability of water
Potential Evapotranspiration (PET) – Suggested by Thornthwaite (1948), it is defined as the evapotranspiration from a large vegetation covered land surface with adequate moisture availability at all times. Since moisture supply is not restrictes, PET depended only on the available energy. It is the upper limit of ET that would occur in a well watered crop field or the upper limit of ET for a crop in a given climate.
Actual Evapotranspiration (AET)- The evapotranspiration actually occurring in a specific situation is called AET
• If water supply to the plant is adequate, soil moisture will be at field capacity and the ratio of AET to PET =1
• If water supply to the plant is inadequate, the ratio of AET to PET < 1
• In clayey soils, AET/PET ~ 1 up to about 50% drop in available moisture
• When the soil moisture approaches the permanent wilting point, AET ~ 0
Figure: Variation of Actual Evapotranspiration