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<br />" <br /> <br />ON-6 PROGRAM :E))R SALINITY CONTROL. <br />-~D VALLEY SALINITY CONTROL <br />SOIL CONSERVATION SERVICE <br /> <br />m <br />0") <br />I.'" <br />u <br />'~) <br />,=) Irrigation System Improvements - Five types of improvements have been iden- <br /> <br />tified as needed for existing on-farm irrigation systems. First, ditch <br /> <br />lining or pipelines will reduce seepage from on-farm distribution and field <br /> <br />head ditches. Necessary components of the ditch lining program will be the <br /> <br />installation of water measuring devices and inflow and outflow cQntrol <br /> <br />structures. The measuring devices will include small weir:3 or critical <br /> <br />depth flumes at key locations on each farm so that water d,~livery to each <br /> <br />field can be carefully controlled. Outflow control structures will be <br /> <br />needed to dissipate kinetic energy in the flowing water and prevent erosion. <br /> <br />Second, land leveling is needed on half of the irrigated l1~d in the valley. <br /> <br />Land leveling serves two major purposes; it adapts the fie:,d to the use of <br /> <br />lined ditches or gated pipes, and it allows a more uniform application of <br /> <br />water across the field during irrigation. Di tch lining and land leveling <br /> <br />are mutually complimenting practices. <br /> <br />Third, subsurface drains are recommended where field sizes and irrigation <br /> <br />system layout can be enhanced by closing existing open drains. Although <br /> <br />there are 1,600 acres of land where cultivation and crop yields are impaired <br /> <br />by an existing high watertable, the plan does not include accelerated effort <br /> <br />to install new subsurface drains. As canal seepage is reduced by lining and <br /> <br />as field irrigation efficiency is improved the watertable elevation will <br /> <br />recede by an undetermined amount. Experience has shown the,t ditch lining <br /> <br />has caused local watertable elevations to drop as much as four feet. Also, <br /> <br />it has been reasoned that the effect of new subsurface drains would be <br /> <br />counter to objectives of the salinity control programs by providing an <br /> <br />outlet to the river for salt laden bodies of ground water. Installation of <br /> <br />new subsurface drains will continue under the on-going program. <br />