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<br />L\) <br /> <br />c,., <br />~ Off-farm Lateral and Onfarm Ditch Seepage Monitoring <br /> <br />r.~.l <br />'- <br /> <br />The reduction in seepage from off-farm laterals and onfarm earth field <br /> <br />delivery ditches by installing pipelines and ditch lining is being evaluated <br /> <br />for effec tiveness to reduce salt loading. During planning, several seepage <br /> <br />studies involving measurements were made in Grand Valley and Uinta Basin by <br /> <br />SCS, HR, ARS, and CSU. <br /> <br />Baseline unit factors for seepage rates were <br /> <br />determined for various size ditches and laterals occurring in different <br /> <br />soils. <br /> <br />Good accuracy can be obtained by using existing seepage data and applying <br /> <br />these values to the units of pipeline and ditch lining installed. Therefore, <br /> <br />no additional monitoring of seepage from earth ditches or laterals is <br /> <br />anticipated or needed. <br /> <br />USDA is primarily responsible for installing pipeline and ditch lining on <br /> <br />small laterals. In the Grand Valley, unit values of average seepage have <br /> <br />been estimated and are presented as an example of how seepage will be <br /> <br />calculated and the salt load reduction evaluated. Developing unit values <br /> <br />involves the following steps: <br /> <br />Example for the Grand Valley using available data. <br /> <br />Step 1. Estimate the average salinity concentration of inflow irrigation <br /> <br />water, subsurface outflow, and average salt load pickup. <br /> <br />Inflow irrigation water <br /> <br />500 ppm <br /> <br />Base outflow (in drains and washes) <br /> <br />~,-2_0~_l!.pm <br /> <br />Net change (salt concentration) <br /> <br />3,700 ppm <br /> <br />Net change (salt load pickup) <br /> <br />5.0 tons/ ac]~e- foot <br /> <br />35 <br />