Irrigation control principles

Optimal and easily obtainable answers to the basic questions of how much and when to irrigate are of critical importance both in terms of maximizing irrigation efficiency and in terms of minimizing the pollution of the water table by agrochemicals.
Traditionally, decisions concerning how much water to apply are based on an estimate of how much soil water was lost from the plant's root zone since last irrigation.
Such estimates could be obtained directly from extensive field sampling of the soil water contents, or indirectly by computations of the evaporative demand of the microclimate based on accessible climatic parameters and corrected for the water loss by a growing crop.
A popular way to estimate crop water use is to correlate water loss from the growing crop with evaporation from a standard evaporation pan.
None of these climatic methods can evaluate in real time whether the quantity of water applied is sufficient, in excess or deficient.
The answer to the question of when to irrigate is more complex. It is generally dependent on the sensitivity of various plant processes to the level of water stress that develop in the plant as a result of water deficits.
Water deficits are the result of the lag in soil water uptake by the root system behind water loss from plant leaves. Plant sensitivity to water stress is a complex function of developmental stage, dynamics of water deficit and the level of other parameters influencing plant growth.

 

 
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