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<br />lJ;)J753 <br /> <br />Technical Elements of Agricultural Water Use in Colorado <br /> <br />Consumptive Use <br />The consumptive use portion of a water right is specifically defined as water that is no longer available <br />within a stream or aquifer system because it has been evaporated, transpired by plants, incOIporated into <br />products or crops, consumed by people or livestock, or otherwise removed from the water supply, For crop <br />production systems, plant transpiration and evaporation from soil and crop surfaces accounts for almost all <br />the water that is conswnptively used, These combined evaporative and transpiration losses are referred to <br />collectively as evapotranspiration and abbreviated as ET, Evapotranspiration losses generally are asswned to <br />be equivalent to beneficial consumptive water use in inigated cropping systems, The consumptive use <br />portion of a water right is rarely as much as the amount of water that a water right is allowed to divert for a <br />given beneficial use, This is a significant source of confusion and contention in quantifying a water right. <br />With most inigation water distribution and application systems, some water is applied in excess of the <br />soil water holding capacity and actual use, The excess water either percolates through the soil and becomes <br />ground water that reenters the stream or river or flows overland through surface channels or drainages to the <br />stream or river, These returning waters, which are in excess of water actually used or held in storage within <br />the erop root zone, are called return flows, <br />Retwn flows are typically available for appropriation by downstream water users, When a water right is <br />transferred to another use, water laws in most western states, including Colorado, dictate that the change in <br />conditions resulting from a transfer (i.e" point of diversion and quantity of water diverted, for example) must <br />not and shall not injure other vested or decreed water rights not directly involved in the transaction, This is <br />generally interpreted to mean that only the amount of water actually consumed -- the consumptive use _ <br />under a historical set of -conditions and praetices, can be sold or transferred and that return flows are <br />Dl.int.ined, It is difficult, costly, and time consuming to detennine conswnptive use with complete accuracy; <br />therefore, engineering estimates are generally used to establish the terms and conditions to be included in the <br />change decree resulting from a transfer of use, Such estimates are based on historical experience with long- <br />used crop varieties and inigation technologies under historic hydrologic conditions and mayor may not be <br />good estimates for a specific operation. <br /> <br />Irrigation Efficiency <br />Many definitions of inigation efficiency have been developed and used as measures of inigation <br />performance, The common factor in all these definitions is that they involve the ratio of water quantity <br /> <br />10 <br />