Page 19 - soil-plant-water relationship and water requirement

SOIL-PLANT-WATER RELATIONSHIP AND WATER REQUIREMENT

i. Water –budget method.
ii. Energy –balance method, and
iii. Mass –transfer method

2.2 EVAPOTRANSPIRATION PROCESS

In a crop field, evaporation from the soil and transpiration through plants occur
simultaneously.

Evapotranspiration is the combined evaporation from the agricultural field's soil surface
and transpiration via the crop plant (ET). So, the amount of water transpired by plants
throughout their growth or maintained in plant tissue, plus moisture evaporated from
the soil and vegetation's surface. Evapotranspiration (ET), also known as Consumptive
Use (Cu) .

The knowledge of ET is an important practical consideration in the planning, designing,
and operation of irrigation and the water management systems.

2.2.1 Evapotranspiration Terminology

Potential Evapotranspiration (PET):

The PET is the amount of evapotranspiration that occurs when the ground is
entirely covered by actively growing plants and the soil moisture is not limited,
i.e. there is enough water to meet evaporation requirements. It's possible to
think of it as a crop's evapotranspiration upper limit in a given climate. (Under
no-stress conditions, the maximum rate at which water can be extracted from
soil and plant surfaces if it is available)

Actual Evapotranspiration (AET)

The ET under existing atmospheric and vegitation condition (under stress
condition)

Reference Crop Evapotranspiration (ETo)

The rate of evapotranspiration from an extensive surface of 8 to 15 cm tall,
green grass cover of uniform height, actively growing, completely shading the
ground and not short of water.

2.2.2 Factors Affecting Evapotranspiration

 Crop factor - Ground cover, root density, plant height
 Climatic or weather factors - Solar radiation, temperature, humidity,
wind speed, day length
 Soil Factor- Soil moisture, salinity, presence of impermeable layer
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