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<br />COj)172 <br /> <br />. <br /> <br />River Reach Efficiency <br /> <br />As discussed above, water losses from irrigated fields include water <br /> <br /> <br />which percolates below the crop root zone and water which runs off the <br /> <br /> <br />surface. Also, some of the "losses" in conveyance are made up of water <br /> <br /> <br />which seeps downward, and upon occasion there are also operational spills <br /> <br /> <br />into natural waterways. <br /> <br />In the cases of deep percolation and seepage, the water becomes part <br /> <br /> <br />of the ground-water system. Fortunately, in many areas of the South <br /> <br />Platte, the irrigated areas overlie permeable alluvium which serves as <br /> <br /> <br />a natural drainage facility. Water in the alluvium, mostly put there <br /> <br /> <br />from the irrigation activities, slowly moves back to the river to become <br /> <br />available for diversion again (either by wells or by downstream ditches). <br /> <br /> <br />This "return flow" is an important factor in the efficiency of water use <br /> <br /> <br />in a reach or an entire basin. <br /> <br />. <br /> <br />The over-land flow of tail water from irrigated fields, as well as <br /> <br /> <br />operational spills from ditches, also flow back towards the river, In <br /> <br /> <br />the case of these surface flows, however, the water is often intercepted <br /> <br />and used again by other irrigators either directly or through a lower <br /> <br /> <br />canal system. This reuse is also an important factor in the overall <br /> <br /> <br />water-use efficiency of a river reach or an entire basin. <br /> <br /> <br />Factors which influence reach efficiency include: <br /> <br /> <br />(1) The losses to nonbeneficia1 evaporation and transpiration which <br /> <br /> <br />deplete both the ground-water and the surface-water return flow between <br /> <br /> <br />the irrigation facilities and the river. Losses from the ground-water <br /> <br /> <br />system occur in areas where the water table is near the land surface, <br /> <br /> <br />resulting in direct evaporation as well as providing water for non-crop <br /> <br /> <br />vegetation. The most severe area of high water table generally occurs <br /> <br /> <br />in the immediate vicinity of the river. Typically, such an area supports <br /> <br /> <br />a growth of phreatophytic vegetation capable of drawing water directly <br /> <br /> <br />from the ground-water system. <br /> <br />. <br /> <br />-15- <br />