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<br /> <br />crops grown and the climatic <br />conditions experienced. <br /> <br />Flows Not Consumptively Used <br /> <br />In order to deliver a given amount <br />of irrigation water to the crop it <br />is necessary to divert from the <br />supply source amounts greater than <br />those to be consumptively used. <br />Diverted water not consumed by the <br />crop may take the form of seepage <br />from the canals, laterals, and <br />onfarm ditches; operational spills <br />when water in the canal and <br />lateral system is not delivered to <br />the farm but cannot be retained <br />for use within the system; <br />deep percolation beyond the <br />root zone after application; <br />farm tailwater runoff; and <br />evapotranspiration in non cropped <br />areas. <br /> <br />Seepage wi 11 vary dependi ng upon <br />the type, capacity, length, and <br />condition of the canals, laterals, <br />and onfarm ditches. Pipe and <br />lined systems generally have lower <br />seepage losses. The length of <br />time water is in the system also <br />influences loss amounts. <br /> <br />Operatlonal spills usually result <br />from a reduction in demand for <br />water within the system after the <br />water has been withdrawn from the <br />supply source. These spills may <br />return to the natural stream <br />channels via wasteways, where they <br />often become avail abl e for other <br />instream or downstream uses. <br /> <br />Some deep percolation is desirable <br />du ri ng i rri gati on to remove <br />excess salts that woul d otherwi se <br />accumul ate withi n the root zone. <br />Excessive percolation of water <br />past the root zone often results <br />in excessive leaching of <br /> <br />nutrients, waterlogging, and <br />reduced crop yield or quality. <br /> <br />Seepage from canals, laterals, <br />and onfarm ditches and deep <br />percolation may return to the <br />natural stream channel seither <br />directly via drains or indirectly <br />to ground-water aquifers <br />tributary to natural stream <br />channels. Again, such water <br />reaching natural stream channel s <br />often becomes avai lable for <br />instream or downstream uses. <br /> <br />, <br /> <br />" <br /> <br />Tailwater runoff occurs at the <br />lower end of a farm field and may <br />be a result either of careless <br />application or the necessity to <br />keep water on the field long <br />enough for adequate infiltration <br />of the soil. Some of th i s wa ter <br />may evaporate, be consumed <br />by phreatophytes, percolate to the <br />ground water, or reach the natural <br />stream channels. In some cases <br />this runoff is pumped back into <br />the farm di stribution system for <br />further use. Rate and method of <br />application, field shape and size, <br />and soils and topography are <br />factors influencing the amount of <br />tailwater runoff. <br /> <br />Phreatophyte consumption is <br />noncrop vegetative use of water <br />that may occur adjacent to natural <br />stream channels, canals, laterals, <br />and drains. This use generally <br />causes extra expense for farmers <br />and water user organizations in <br />their weed control programs. In <br />many irrigation service areas, <br />the existence of such noncrop <br />vegetation provides excellent <br />wildlife habitat. <br /> <br />. <br /> <br />, <br /> <br />Through seepage or deep <br />percolation, diverted irrigation <br />water that recharges a utilized <br /> <br />10 <br />