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Competency Area 3: Drainage and irrigation AEM

PO 23. Understand the relationships of hydrology, the soil water budget, and crop water requirements as these pertain to irrigation system water requirements and the potential benefits of irrigation.

The soil water budget and balance is similar to balancing one's checking account. In fact one of the simple methods for irrigation scheduling is called the "checkbook" method. Rain is a deposit to the soil water budget, and the actual evapotranspiration is a withdrawal. The field capacity water content sets the upper limit (balance) of what the soil can absorb, or for the checking account analogy it is the minimum balance one may need to sustain. The daily rainfall (deposits) relative to the actual daily evapotranspiration (withdrawals) determines whether the soil water budget is wetting or drying. If the rainfall exceeds the actual evapotranspiration, the soil water increases.

If the soil water is already at its field capacity water content when this occurs, then the excess is drainable (deep percolation or runoff) water. When the soil water content is less than field capacity, the soil absorbs the rain, and the soil water content increases. However, if the rain amount is more than the evapotranspiration and the amount needed to raise the soil to its field capacity, the extra must drain. When there is no rain, the evapotranspiration gradually depletes the soil water. This can go on for awhile, until the readily available water is gone, and then the plant starts to stress and stops evapotranspiring. Either rain or irrigation must occur at this time to replenish the soil water, preferably enough to raise the water content back to the field capacity.

In the Northeast, the annual or growing season irrigation system water requirements range from about 4 to 12 inches (or acre-inches volume) to offset the difference between the rainfall and actual evapotranspiration amounts.