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<br />OOJ754 <br /> <br />consumptively used or stored for later use to the quantity delivered. <br />The general engineering defmition of efficiency is an output divided by an input, both of the same <br />character, If the difference between the output and input is not reusable, such as heat generated from an <br />engine, then this difference constitutes a loss to the process or system. The term irrigation efficiency <br />describes a ratio between water quantities, and inherently implies that water not consumed by the crop is lost <br />However, in irrigation, "nonconsumed" water (water not stored in the root zone) is not always lost to the <br />system, In fact, much of the water not directly consumed by the crop returns to the stream system as return <br />flows, Thus, irrigation efficiency values for individual operations can be misleading because they do not <br />account for water that eventually becomes available to other downstream users in a river basin. <br />In Colorado, the historical efficiencies of irrigation systems are taken into account by the legal system. A <br />reasonable amount of over irrigation due to unavoidable losses associated with a particular irrigation system <br />is considered a beneficial use, The water required to satisfy the consumptive use needs and also to account <br />for these losses is genera1ly referred to as the duty of water for a given irrigation system, <br />Return flows are also important because there is a time lag between an irrigation event and the time that <br />the return flow water reaches the stream and is available for subsequent diversion and beneficial use, This <br />time lag can range from a few hours to several months depending on whether it is surface or ground water <br />return flow and the characteristics of the river basin and it s soils, Early research by the Colorado State <br />University Agricultural Experiment Station documented that return flows resulting from irrigation greatly <br />increased late-season streamflows in the South Platte basin (parshall, 1922), The historical record prior to <br />irrigation indicates that streams in eastern Colorado were likely ephemeral in many years (Wemer, 1993), <br />In the Arkansas, South Platte, and Rio Grande basins, the apparent inefficiency of individual irrigation <br />systems creates water storage in a shallow aquifer within the river basin, The additional inflow to the stream <br />from this aquifer increases-the water supply for downstream users in the latter part of the irrigation season, <br />Withont this temporary ground water storage, natural high levels of river flows in the early part of the <br />growing season would flow downstream and be unavailable for later use, <br />Degradation of water quality resulting from repeated use of the same water is of concern also, In river <br />basins where return flows are used several times, concentration of suspended solids and dissolved salts <br />increase downstream, This problem is more severe in some basins than others because of natural salt-bearing <br />soil layers, <br />The traditional irrigation efficiency term describes the overall performance of an irrigation system in <br />delivering water to the crop root zone of an individual farm or field without considering the reuse of return <br />flows, For example, if a 60% efficiency is obtained by an irrigation system on a particular field, it doesn't <br /> <br />11 <br />