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<br />800173 <br /> <br />. <br /> <br />(2) The opportunities for re-diversion of the return-flow water <br /> <br /> <br />(and therefore an increase in reach efficiency) depends somewhat upon <br /> <br />relative locations of water rights in the reach. For instance, if a <br /> <br /> <br />senior water right for a large amount of water is located at the upper <br /> <br /> <br />end of a particular reach, the downstream appropriators in the reach <br /> <br /> <br />have an opportunity to redivert return flow generated by the senior <br /> <br /> <br />right, even though their priorities are inferior, On the other hand, <br /> <br /> <br />if, in a particular reach, the large senior right is located at the <br /> <br /> <br />lower end of the reach, the water-use efficiency could be quite low. <br /> <br /> <br />During times of shortages the upstream junior rights would be required <br /> <br />to curtail diversions so as to allow water to flow to the senior right. <br /> <br />. <br /> <br />(3) Conveyance losses in the stream itself during low-flow condi- <br /> <br /> <br />tions can be significant. A broad streambed and a low flow results in <br /> <br /> <br />a large amount of surface area exposed to evaporation, <br /> <br /> <br />(4) Timing of return flows coming back into the stream is of impor- <br /> <br /> <br />tance, especially for direct-flow rights. If most of the return flow <br /> <br /> <br />generated from irrigation in June does not get back to the stream during <br /> <br /> <br />the irrigation season,it is not available to downstream direct-flow <br /> <br /> <br />rights. If the reach under study does not have facilities to store the <br /> <br /> <br />return flow accumulating during the nonirrigation season, that water is <br /> <br /> <br />lost to the reach. <br /> <br />River Basin Efficiency <br /> <br />As the size of the area under consider.ation incr.eases, so does the <br /> <br /> <br />opportunity for reuse of water, This is particularly true in a basin such <br /> <br /> <br />as the South Platte where the principal source of water is in the upper <br /> <br /> <br />reaches and the major uses occur in the lower reaches. The efficiency <br /> <br /> <br />of irrigation water use in the South Platte Basin as a whole is con- <br /> <br /> <br />siderably higher than the average field or farm irrigation efficiency <br /> <br />(or even the efficiencies within individual reaches) because of return <br /> <br /> <br />flow and reuse, <br /> <br />. <br /> <br />As discussed above for a river reach, the distribution of water- <br /> <br /> <br />right priorities can also have an influence on the overall basin water-use <br /> <br />-16- <br />