Crop evapotranspiration is a physical process in which water passes from the liquid to the gaseous state while moving from the soil to the atmosphere. It refers to both evaporation from soil and vegetative surfaces and transpiration from plants. These two processes are considered together because they occur simultaneously in nature, and it is difficult to separate them when measured or estimated.

Crop evapotranspiration refers to the water losses from a cropped field, and this term is frequently used in irrigation scheduling and irrigation water management.

Crop evapotranspiration is affected by weather factors (radiation, air temperature, humidity, and wind speed), crop parameters (crop type, variety, and development stage), and local environmental and management conditions (soil salinity, application of fertilizers, tillage practices, planting density, presence of diseases and pests, windbreaks, irrigation practices, presence of groundwater and impermeable soil horizons, etc.).

Accordingly, it is possible to distinguish between crop evapotranspiration under standard (optimal) conditions and crop evapotranspiration under nonstandard conditions. Standard conditions refer to disease-free, well-fertilized crops, grown in large fields, under optimal water supply, and achieving full production for the given climatic conditions. Nonstandard conditions are defined as those that differ from standard conditions.

How does evapotranspiration affect the crops ?

Under normal circumstances, the plant roots absorbs moisture from the soil through osmotic pressure created at the surface of the leaves . Water in a plant acts as a medium of nutrients transportation from roots to the leaves. As the solution moves upwards , nutrients are absorbed at different level while the water is emitted at the leaf surface in form of tiny microscopic droplets.
These droplets evaporates at the leaf surface and hence creates a temporary water deficits within the cells below the leaf surface . Water from the adjacent cell then move into these cells to fill up through osmosis and the process is repeated downwards to the root hair which replenishes by absorbing more solution.
Under normal circumstances ie a bit of sunshine and slight wind the ET is almost at equilibrium ie water absorbed is equal to that which evaporates and the plant is happy .
However , when the temperatures are high , its windy and the relative humidity is low , evapotranspiration rate is speeded up which results into loss of more water than it is absorbed by the roots hence creating an imbalance.
The plant hence demands more water in order to cope and if the moisture in the soil isn’t sufficient to cope with the demand , the plant intelligence wilts the leaves in what is called temporary wilting point as a way of trying to reduce the leaf surface area of evaporation.
In another the scenario, the soil may be having adequate moisture but the roots can only absorb upto their maximum capacity which might be lower than the ET rate and the plant will show wilting symptoms.
Any time that the plant is subjected to high ET which is stress in plants , apart from wilting , reacts by increasing the leaf wax , leaf hardening plus other physiological adaptation which in turn affects photosynthesis negatively hence reducing the plant potential. ( lowering the yields )
How is a farmer supposed to proactively deal with such a situation?
In open field farming , a farmer may have no control on relative humidity but he can control the wind speed by erecting windbreaks or shading the crop by use of shade nets .
Erecting wind break ie both natural eg Napier or planting hedge crops and artificial eg shadenets at intervals is more feasible than shading the crop because it might be too expensive.
While Erecting windbreaks it’s important to note that the effect against wind is approximately a factor of 5 ie if your wind break is 2m , it will offer protection of upto 10m both ways..
By reducing the wind speed, the farmer will maintain yields in hash weather ie January, February and March and will also save on the water use by reducing the ET.

CREDIT: David Ndegwa

READ ALSO:   Planting season is here, but have you done a soil test before you prepare for planting?

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