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<br />to the field. Also sometimes used to indicate the ratio of total <br />evapotranspiration (ET) losses to total applied water (AE = ET/ <br />Applied) or even total transpiration (T) to total application. <br />Surface runoff (RO) and deep percolation (DP) are the principal <br />application losses (recoverable) while evaporation and transpiration <br />(ET) are the main nonrecoverable losses. Where excess water is <br />applied to leach salt accumulations below the root zone, the <br />"leaching requirement" (LR) is counted as a productive use (with ET) <br />and included in the numerator of the equation (AE = ET + LR/Applied). <br /> <br />4. Conveyance efficiency (CE) - the ratio of the amount of water <br />delivered to the end use (the field) to that dispatched from the <br />water source (CE = Delivered/Diverted) of wells, reservoir, or <br />gate. Most losses in route (spills, leaks, evaporation, seeps, <br />deep percolation) are recoverable and reduce conveyance efficiency <br />but not total system efficiencies. Generally less significant <br />nonrecoverable losses to evaporation from soil surfaces (Es), <br />evaporation from water surfaces (Ew), or ET (phreatophytes, <br />weeds, etc.) reduce both conveyance and system efficiencies. <br /> <br />5. Coefficient of (application) uniformity (or sometimes the <br />"equivalent water factor") is a common measure of application <br />efficiency and expresses the percent of excess application on parts <br />of a field in order to achieve adequate wetting depth on at least <br />90% percent of the field. This becomes a significant factor where <br />surface (gravity) systems are used on soils or slopes not well <br />adapted to surface applications. <br /> <br />6. Crop (physical) scientists commonly define water use efficiency <br />(WUE) as yield (V) per unit of evaportranspiration (ET) or WUE = <br />V/ET Unit. Economic (social) scientists may define WUE on a margi- <br />nal productivity basis where the last unit of water applied pro- <br />duces no additional incremental increase in yield. Another common <br />economic measure of agricultural water use efficiency is expressed <br />in terms of value of agricultural product per unit of water use. <br /> <br />vi ii <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />