Use of Irrigation Efficiency Terminology
Irrigation efficiency is a term that has a specific meaning to engineers, but it alone cannot be used to evaluate the impacts of changes in irrigation systems and practices on agricultural water supplies. Irrigation efficiency considers only part of the water budget, i.e., crop water consumption as compared with farm delivery or gross diversions. Return flow is not part of this term.
Net irrigation efficiency compares crop water consumption with net water depletion. An increase in efficiency means either greater crop water consumption with the same depletion, or a reduction in net depletion, or both.
The following efficiency values are obtained when using the data in Figure 9.1:
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In this example, the return flow is 78 percent of the excess diversion (100/(81 + 48) = 0.78). If the project irrigation efficiency was increased from 41 to 82 percent and the same crops were grown, the gross diversion could be reduced to 110.5 km3 . If the return flow percentage remained the same (78 percent), the net depletion would be 90.6 + (1 – 0.78) (19.9) = 95 instead of 119 km3 . Doubling the irrigation efficiency in this example would decrease net depletion by only 20 percent.
The major ways to increase "net irrigation efficiency" and available water supplies are: (1) to reduce evaporation from soil and surface reservoirs; (2) to reduce evapotranspiration by lowor nonbeneficial vegetation of farms and flood plains; and (3) to reduce all system losses and reduce waste of usable drainage and operationally spilled water. Greater production per unit of water consumed can be achieved by increasing water use efficiency.
Figure 9.1
U.S. Irrigation Water Budget in Percent and in km3 /year
Source: SCS, 1980.