Em4885 irrigation management practices to protect ground water

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CHAPTER 4
Irrigation efficiency was defined by the American Society of Civil Engineers’ On-Farm
Irrigation Committee in 1978 as the ratio of the volume of water which is beneficially used to the
volume of irrigation water applied. Beneficial uses may include crop evapotranspiration, deep
percolation needed for leaching for salt control, crop cooling, frost control, and as an aid in certain
cultural operations.
There are many specific mathematical definitions of efficiency in use. Differences in definitions
are due primarily to:
1. Accounting for runoff and deep percolation.
2. Whether it is for an individual irrigation or an entire season.
3. Whether it is for an individual farm, irrigation project, or basin.
Many people will hold to a strict measure of efficiency considering only the beneficial use on the
individual field. Others will classify reuse of any surface or sub-surface drainage by other farms as
beneficial use. Some will ignore measurements of individual irrigations to focus on a seasonal
efficiency. For the Manual, application efficiency is defined for each irrigation as:
Applied water stored in the root zone or used for leaching
Application Efficiency = ............................................................................................ x 100
Total water applied
Application efficiencies are also expressed as a percentage between 0 and 100%. A 100%
application efficiency is not theoretically attainable due to immediate evaporation losses during
irrigations. However, there could easily be close to 95% application efficiency if a crop is underwatered.
In this case, assuming there was no deep percolation, all water applied and not immediately
evaporated would be used by the crop.
Underwatering a crop will theoretically result in a high application efficiency. However, it may
not be a very effective way of farming and could actually lead to an overall inefficient use of resources.
This could be because of an inefficient use of fertilizer, a weak crop that is more susceptible
to pest pressures, thus requiring additional chemical applications, or sub-standard yields that would
require additional cropped acreage.
Note that the terms “irrigation efficiency” or “application efficiency” should not be confused with
the term “water use efficiency” (WUE). Water use efficiency is generally a measure of yield per unit
water applied.
Relationships Between Distribution Uniformity and Application Efficiency
Generally, the distribution uniformity of the irrigation system is the first concern. The reason for
this is explained by the following four graphs. They are a profile view of two adjacent sprinklers in a
field and the root zone under them. The spray patterns from the adjacent sprinklers must overlap to
result in the same amount of water falling in all parts of the field. The horizontal, dashed line in the