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How effective the system delivers water to a field from a water source is a major contributor to the performance of an irrigation system. Several terms, including water conveyance efficiency, water application efficiency, soil water storage efficiency, overall irrigation efficiency, and effective irrigation efficiency, have been used to assess the performance of a system (Irmak et al., 2011). Less-efficient systems usually require greater irrigation amounts to meet crop evapotranspiration (ET) demands due to water loss between the source and the crop as compared to more efficient systems. This results in increased energy use and operational cost for the producer. Several efficiency terms will be described; however, the reader is directed to the references below for further information regarding different efficiency terms.
Water application efficiency evaluates how well an irrigation system delivers water from the conveyance system to the crop and is calculated as:
where, Ea is application efficiency (%), Vs is volume of irrigation water stored in the crop root zone (acre-inch), and Vf is the volume of irrigation water delivered to the farm or field (acre-inch). In terms of irrigation efficiency “point-of-view,” Ea is used to evaluate crop yield response. Water application efficiency is always less than 100% due to water loss from various pathways. Figure 4 illustrates the water cycle for a center pivot-irrigated field. The contributing factors that reduce Ea are runoff, deep percolation below the crop root zone, wind drift, and evaporation from droplets, crop canopy, and the soil surface; however, if runoff is captured and reused, then Vf should be adjusted to account for the
Water application efficiency is affected, in part, by irrigation management. Table 6 provides typical Ea values for well-designed and managed irrigation systems. It is possible to have high Ea values yet unsatisfactory system performance if the irrigation system does not meet crop ET demands. A small amount of irrigation under low atmospheric evaporative demand can result in minimal water loss (i.e., high Ea), yet not meet crop ET demand, resulting in crop water stress. Table 6 provides a good estimate of the upper limit achievable for different system types under well-managed conditions in which crop ET demands are met. Other factors to consider when calculating and assessing Ea are the accuracy of measuring stored irrigation water, effective crop rooting depth, and spatial variability in Ea. Spatial variability in Ea can be, in part, attributed to poor water distribution of an irrigation system. Reporting both Ea and water distribution uniformity provides a better indication of overall irrigation system performance. Additional readings, listed below, will provide further information on factors impacting Ea.
Irrigation water may be applied to satisfy other objectives than meeting crop ET. Table 7 presents different uses of irrigation water between beneficial and non-beneficial as well as between consumptive and non-consumptive. Irrigation efficiency (Ei) is commonly used to assess the effectiveness of the irrigation system in delivering water for beneficial uses. It is defined as the ratio of the volume of water beneficially used (Vb, acre-inch) to the volume of irrigation applied (Vf, acreinch) and is expressed as:
The overall irrigation efficiency (Eo) represents the efficiency of the entire system to deliver water from a water source to a crop. It can be calculated by multiplying either water application efficiency (Ea, as decimal) or irrigation efficiency (Ei, as decimal) by the water conveyance efficiency (Ec, as decimal) calculated as:
where, Ec is conveyance efficiency (%), Vf is the volume of irrigation water that reaches the farm or field (acre-inch), and Vt is the total volume of water diverted from the water source (acre-inch). The conveyance efficiency will decrease as a result of water losses, including canal seepage, canal spills, evaporation from canals, and leaks in pipelines. For center pivot irrigation, Ec can be as high as 100% since there is minimal water loss in closed/pressurized conveyance systems. The selection between Ea and Ei to calculate Eo will depend on the purpose or objective of irrigation, and the equation to calculate Eo is:
Burt, C.M., A.J. Clemmens, T.S. Strelkoff, K.H. Solomon, R.D. Bliesner, L.A. Hardy, T.A. Howell, and D.E. Eisenhauer. 1997. Irrigation performance measures: Efficiency and Uniformity. Journal of Irrigation and Drainage Engineering. 123(6): 423-442.
Howell, T.A. 2003. Irrigation Efficiency. Encyclopedia of Water Science. DOI: 10.1081/E-EWS120010252.
Irmak, S. 2009. Estimating crop evapotranspiration from reference evapotranspiration and crop coefficients. University of Nebraska-Lincoln Extension NebGuide G1994.
Irmak, S., L.O. Odhiambo, W.L. Kranz, and D.E. Eisenhauer. 2011. Irrigation efficiency and uniformity, and crop water use efficiency. University of Nebraska-Lincoln Extension Circular EC732.
Irmak, S. and D.R. Rudnick. 2014. Corn soil-water extraction and effective rooting depth. University of Nebraska-Lincoln NebGuide G2245.
Rudnick, D.R., K. Djaman, and S. Irmak. 2015. Performance and hysteresis analyses of capacitance and electrical resistance sensors in a silt-loam soil. Transactions of the ASABE (in press).