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<br />con~67 <br /> <br />. <br /> <br />Limits to field irrigation efficiency <br /> <br />It is important to recognize that the upper limit of irrigation <br /> <br /> <br />application efficiency, even under the most ideal field conditions, is <br /> <br /> <br />limited by the natural variability found in soil conditions. Soil intake <br /> <br /> <br />rates vary from one place to another so it is impossible to apply the <br /> <br /> <br />desired amount at one location without applying more than is needed at <br /> <br /> <br />another location. Some drainage below the root zone necessarily results. <br /> <br /> <br />Under furrow irrigation, tractor or implement wheels compact some furrows <br /> <br /> <br />resulting in greatly decreased intake rates. It is practically impossible <br /> <br /> <br />to have the intake time the same at all points along the length of the <br /> <br /> <br />field. Thus, it cannot be expected that irrigation efficiencies should <br /> <br /> <br />be above 70 to 75 percent if the entire irrigation requirement is met. <br /> <br />. <br /> <br />Soil salinity control under irrigated agriculture is an important <br /> <br /> <br />management factor. The only satisfactory control measure is to periodi- <br /> <br /> <br />cally leach the soil salts below the crop root zone by the application <br /> <br />of excessive irrigation water. By the above definition this decreases <br /> <br /> <br />the field irrigation efficiency, but it is a necessary requirement for <br /> <br />obtaining profitable yields. <br /> <br />Under many geological conditions in irrigated areas the "loss" of <br /> <br /> <br />water to deep percolation is not a total loss. The ground water may <br /> <br /> <br />flow to streams where it is again available for public use, or it may <br /> <br /> <br />be pumped for reuse at the surface, In fact, the storage of ground water <br /> <br /> <br />resulting from over-irrigation, and its rate of controlled release for <br /> <br /> <br />reuse, may be an important factor in water management of an irrigated <br /> <br />basin. <br /> <br />Farm Irrigation System Efficiency <br /> <br />Farm irrigation system efficiency includes in its definition the <br /> <br /> <br />losses in the delivery system from the farm turnout on the canal system <br /> <br />to the irrigated fields. All other components are the same as for field <br /> <br /> <br />irrigation efficiency. Thus, <br /> <br />. <br /> <br />Farm irrigation <br />system efficiency <br /> <br />= <br /> <br />Actual crOD evapot nmHoi T;::!. t i'on <br />Volume of water delivered to farm turnout <br /> <br />-10- <br />