Em4885 irrigation management practices to protect ground water

4 CHAPTER
figures depicts the depth of the actual soil water deficit at irrigation. This is the amount of water
that the grower would be trying to soak into the soil to satisfy crop water use requirements.
The dotted-dashed line depicts the actual depth of water infiltrated during the irrigation. Deep
percolation is indicated whenever the actual depth of irrigation (the dotted-dashed line) is below
the soil water deficit line (the horizontal, dashed line). Conversely, under-irrigation is indicated
whenever the actual depth of irrigation line is above the soil water deficit line. The depths
multiplied by the area of a field indicate the volumes of water applied, stored, and percolated.
Figures 4-1 and 4-2 demonstrate that there must be good distribution uniformity before there can
be good application efficiency, if the crop is to be sufficiently watered.
In Figure 4-1, the farmer has irrigated to sufficiently water the entire field. The poor distribution
uniformity has resulted in excessive deep percolation. That is, the deep percolation was much more
than would be needed to maintain a salt balance.
Depth of soil moisture deplet
Area of deep percolation
Actual depth of water application
FIGURE 4-1. Depiction of irrigation resulting in poor distribution uniformity and excessive deep percolation
In Figure 4-2, the farmer has acted to prevent excessive deep percolation. Now part of the field
remains under-irrigated. Under-irrigation usually results in a high irrigation application efficiency
as most water applied is stored in the root zone, available for plant use. But it may not be an
effective way of growing as the resulting water stress on the crop in some parts of the field will
usually decrease yields. Also, there is the need for some deep percolation for leaching to maintain
a salt balance. Note that the leaching must be uniform over a number of years to prevent areas of
excessive salt accumulation.
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