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<br />"'!>:.../. <br /> <br />CUOlG~ <br /> <br />. <br /> <br />IRRIGATION WATER-USE EFFICIENCY <br /> <br />Irrigation water-use efficiency can be discussed and defined from <br /> <br /> <br />several standpoints. From the standpoint of an individual farmer, the <br /> <br /> <br />efficiency of use once he has taken delivery of the water from the irri- <br /> <br /> <br />gation company canal (and/or a well) is the only part of the entire system <br /> <br /> <br />over which he has some direct control. These on-farm efficiency consider- <br /> <br /> <br />ations, field irrigation efficiency and farm irrigation efficiency, are <br /> <br /> <br />discussed in following sections, <br /> <br /> <br />From the standpoint of the canal and reservoir companies, the effi- <br /> <br /> <br />ciency with which the water they are entitled to can be diverted, conveyed <br /> <br /> <br />and delivered to the farm headgates is of interest. This is discussed <br /> <br />below under the heading "Canal and Reservoir System Efficiency." <br /> <br />From a broader standpoint, one needs to be also concerned about effi- <br /> <br /> <br />ciencies of water use on a larger scale, such as for an entire river reach <br /> <br /> <br />or river basin. These concepts are discussed below under the headings <br /> <br />"River Reach Efficiency" and "River Basin Efficiency." <br /> <br />. <br /> <br />Water Use in Crop Production <br /> <br />Water from the soil is absorbed by the plant root system and a very <br /> <br /> <br />high proportion of it is transferred through the plant to the leaf surfaces <br /> <br /> <br />where it is evaporated and lost to the atmosphere as water vapor. This <br /> <br /> <br />process is universally referred to as transpiration. Although most of <br /> <br />the absorbed water is ultimately transpired, a small but very important <br /> <br /> <br />amount is used in metabolism and growth by the plant. Plant growth is a <br /> <br /> <br />result of many complex processes within the various tissues, and the rate <br /> <br /> <br />of these processes, and thus the growth rate, is determined by genetic and <br /> <br />environmental factors. One of the most important environmental factors <br /> <br /> <br />is the internal water balance within the plant cells. This water balance <br /> <br /> <br />is a result of the relative rates of water uptake by the root system and <br /> <br /> <br />the loss of transpired water from the leaf surfaces. As soon as absorption <br /> <br /> <br />from the soil lags only slightly behind transpiration, a water deficit in <br /> <br /> <br />the plant occurs and a decrease in the quantity or quality of growth results. <br /> <br />I <br /> <br />. <br /> <br />-3- <br />