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<br />The frequency of irrigation, although significant, cannot be a recommen- <br />dation of the plan because of the wide variation in daily meteorological <br />conditions. Therefore, recommendations for frequency of irrigation are <br />left to the Bureau's Irrigation Management Services program as the <br />recommended scientific approach or to the experience and judgement of <br />the individual farm operator. <br /> <br />Table 22, developed for each field by computer analysis, shows the <br />typical elements of management and cost for installing the identified <br />improvement needs for the field. <br /> <br />Irrigation System Improvements - Four types of improvements have been <br />identified as needed for existing on-farm irrigation systems. First, <br />ditch lining or pipelines will reduce seepage from on-farm distribution <br />and field head ditches. One standard design for ditch lining will be <br />applicable for all locations. In cases where a preference for pipe was <br />expressed, the pipe sizes considered ranged from 4 inches to more than <br />24 inches in diameter. Pipe sizes are displayed by size groups in <br />Table 23. Necessary components of the ditch lining program will be the <br />installation of water measuring devices and inflow and outflow control <br />structures. The measuring devices will include small wiers or critical <br />depth flumes at key locations on each farm so that water delivery to <br />each field can be carefully controlled. Outflow control structures will <br />be needed to dissipate kinetic energy in the flowing water and prevent <br />erosion. <br /> <br />Second, land leveling is needed on half of the irrigated land in the <br />valley. Land leveling serves two major purposes; it adapts the field to <br />the use of lined ditches or gated pipes, and it allows a more uniform <br />application of water across the field during irrigation. Ditch lining <br />and land level ing are mutually compl imenting practices. <br /> <br />Third, subsurface drains are recommended where field sizes and irrigation <br />system layout can be enhanced by closing existing open drains. Although <br />there are 1,600 acres of land where cu1ti'la1Cion and crOll yields are <br />impaired by an existing high watertable, the plan does not include <br />accelerated effort to install new subsurface drains. As canal seepage <br />is reduced by lining and as field irrigation efficiency is improved the <br />watertable elevation will recede by an undetermined amount. Experience <br />has shown that ditch lining has caused local watertable elevations to <br />drop as much as four feet. Also, it has been reasoned that the effect <br />of new subsurface drains would be counter to objectives of the salinity <br />control programs by providing an outlet to the river for salt laden <br />bodies of ground water. Installation of new subsurface drains will <br />continue under the on-going program. <br /> <br />C7 <br />" <br /> <br />1652 <br />