Performance Evaluation of Irrigation Projects - Practices of Irrigation and On-farm Water Management - page 114

4.1.2 Storage Efficiency/Water Requirement Efficiency

Storage efficiency indicates how well the irrigation satisfies the requirement to
completely fill the target root zone soil moisture. Thus, storage Efficiency ( E S )is
represented as

E S = (change in root zone soil moisture)
×

(4.4)

100 / (target change in root zone soil moisture)

where the change in the root zone soil moisture content is not measured directly,
the storage efficiency can be approximated by relating the average depth of water
applied over the field to the target root zone deficit. The root zone deficit is calcu-
lated using soil type, crop root zone, and soil moisture content data. In this case, the
storage efficiency is calculated as

E S = (average depth applied) × 100 / (root zone deficit) (4.5)

The maximum storage efficiency is 100%. Calculations with a result above 100%
indicate losses due to runoff or deep drainage.

4.1.3 Irrigation Uniformity

Irrigation uniformity is a measure of how uniform the application of water is to the
surface of the field. That is, it is an expression that describes the evenness of water
application to a crop over a specified area, usually a field, a block, or an irrigation
district. The value of this parameter decreases as the variation increases. It applies
to all irrigation methods, as all irrigation systems incur some nonuniformity. An
irrigation uniformity of 100% would mean that every point within the irrigated area
received the same amount of water as every other point.
An important component of the evaluation of in-field irrigation performance is
the assessment of irrigation uniformity. If the volume of water applied to a field is
known, then the average applied depth over the whole field can be calculated. In
most cases, one half of the field receives less than the average depth and one half
more than the average depth applied. Hence, if the average volume applied is the
target application required to meet the crop requirements, one half of the field has
been over-irrigated (reducing the efficiency of application) while the other half of
the field has been under-irrigated (potentially reducing yield). Thus, a major aim
of irrigation management is to apply water with a high degree of uniformity while
keeping wastage to a minimum.
The uniformity of application is primarily a function of the irrigation system
design and maintenance. Low levels of uniformity limit the maximum efficiency
achievable. Numerous irrigation uniformity coefficients are used in performance
evaluation. Commonly used irrigation uniformities are as follows: Christiansen's