Abstraction from Precipitation
Transpiration
Transpiration is the process by which water leaves the
body of a living plant and reaches the atmosphere as water
vapor. The water is taken up by the plant-root system and
escapes through the leaves.
The important factors affecting transpiration are:
atmospheric vapor pressure, temperature, wind, light
intensity and characteristics of the plant, such as root and
leaf systems.
Evapotranspiration
The total amount of water loss in the form of
water vapors into the atmosphere from
surface of the soil, canopy interception,
water bodies as well as from the aerial parts
of the plants in a process known as
evapotranspiration.
The term consumptive use is also used to
denote this loss by evapotranspiration.
If sufficient moisture is always available to completely meet the needs of vegetation
full covering the area, the resulting evapotranspiration is called potential
evapotranspiration (PET).
The real evapotranspiration occurring in a specific situation is ca lled actual
evapotranspiration (AET).
Field capacity is the maximum quantity of water that the soil can retain against the
force of gravity.
Permanent wilting point is the moisture content of a soil at which the moisture is
no longer available in sufficient quantity to sustain the plants.
The field capacity and permanent wilting point depend upon the soil
characteristics. The difference between this two-moisture content is called
available water, the moisture available for plant growth.
For a catchment i n a given period of time, the hydrologic budget can be written as
𝑷 − 𝑹𝒔− 𝑮𝒐− 𝑬𝒂𝒄𝒕 = ∆ 𝑺
Where: P = Precipitation
Rs = surface runoff
Go = subsurface outflow
Eact = actual evapotranspiration (AET)
∆𝑺 = change in the moisture storage
Measurement of Evapotranspiration
Lysimeters
A lysimeters is a special watertight tank
containing a block of soil and set in a field of
growing plants
Evaporation is estimated in terms of the amount
of water required to maintain constant moisture
conditions within the tank measured either
volumetrically or gravimetrically through an
arrangement made in the lysimeters.
Field Plots
In special plots as the elements of the water budget in a known interval of time are
measured and the evapotranspiration determined as
Evapotranspiration = [precipitation + irrigation input – runoff – increase in
soil storage groundwater loss]
Measurements are usually confined to precipitation, irrigation input, surface runoff
and soil moisture.
