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<br />26 <br /> <br />ISSUES: <br /> <br />The 1983 and 1989 USDA Planning Reports: Various salt loading <br />factors from 2.0 to 5.3 tons per acre foot were used in the USDA <br />McElmo Creek Unit plan and EIS. As a minimum, the drainage <br />outflow concentration and the concentration of the drainage water <br />reduction have to be the same. A very basic assumption is that <br />onfarm deep percolation, ditch seepage, canal and lateral seepage <br />mix and return to the river at the same concentration. In an <br />area with an unlimited salt source, the basic assumption is that <br />the concentration of the drainage return flow will not change <br />with improvements, only the volume of seepage and deep <br />percolation changes. Therefore, new USDA salt budgets need to be <br />developed to correct this salt factor inconsistency between <br />components and alternatives and salt load reduction. <br /> <br />In the following analysis, 4.2 tons of salt pickup/acre foot/year <br />is used. This was the average salt load factor used in the 1983 <br />and 1989 USDA reports for computing salt load reduction (p. IV-7 <br />and V-J). <br /> <br />DEEP PERCOLATION REDUCTION: <br /> <br />Based on monitoring of surface and sprinkler irrigation systems <br />in Grand Valley, Lower Gunnison, McElmo Creek, and the Uintah <br />Basin, the reduction of deep percolation from 9 to 3 inches is <br />assumed due to the conversion of semi-controlled wild flooding <br />systems to sprinkler on 19,700 acres. A three inch reduction in <br />deep percolation is assumed on 1,850 acres of improved surface <br />irrigated land. If gated pipe and surge valves are installed on <br />surface systems the deep percolation reduction would be assumed <br />to be 6 inches, the same as for sprinkler. <br /> <br />The onfarm irrigation efficiency in Montezuma Valley determined <br />during USDA planning studies was 45 % for present conditions and <br />70 % for future with the salinity program. Based on monitoring <br />of irrigation water management in the Colorado Basin over the <br />last 11 years, these values are reasonable. <br /> <br />D~ep Percolation Reduction due to onfarm irrigation improvements: <br />Assume 2~ % of the potential subsurface return flow doesn't <br />return to the river, but is consumed by wetland vegetation. <br />Therefore, 80% (.8) returns to McElmo Creek. <br /> <br />19,700 ac sprinkler x 0.5 ac ft/ac (0.8) = 7,880 ac ft <br />1,850 ac surface x 0.25 ac ft/ac (0.8) = 370 ac ft <br /> Total reduction deep percolation = 8,250 ac ft <br />sprinkler - aeduction in deep percolation 0.4 ac ft/ac <br />Surface .eduction in deep percolation 0.2 ac ft/ac <br /> <br />salt Load Reduction due to installation of sprinkler systems and <br />improved surface systems and resultant reduction in deep <br />percolation: <br />