WSP08073 - Laserfiche WebLink

Home Browse Search

, " SUMMARY Virgin Valley Subevaluation Unit, Nevada, is the second part of a USDA study of the Virgin River Unit in Arizona, Nevada and Utah. See Figure 1. The first part studied was Moapa Valley. The USDA studies identify alternative solutions for reducing salt loading of Colorado River from irrigation and other diffuse salt sources. Virgin River flows through Virgin Valley into Lake Mead formed by Hoover Dam, on the Colorado River. Upstream from Littlefield, Arizona, the river flows through a narrows. The irrigated area above the narrows, called the St. George agricultural area in Utah, is being studied and reported on separately. The total irrigated acreage in Virgin Valley is 4,625 with 3,525 acres irrigated by surface methods and 1,100 acres by sprinkler and drip systems. The surface irrigated acreage by agricultural community and state in the study area is Littlefield, Arizona, 435 acres; Mesquite, Nevada, 1,820 acres; Bunkerville, Nevada, 875 acres; and Riverside, Nevada 395 acres. The area being irrigated with sprinkler and drip systems has very little seepage or deep percolation and does not signi- ficantly contribute to the salinity of Virgin River. therefOre, irriga- tion improvements for this portion were not evaluated. The existing condition and three alternative levels of irrigation improvements to reduce salt loading were analyzed: Future Without Pro- gram, Alternative I, and Alternative 2. The benefits and costs associated with the alternatives are summarized in Table 2, page vi. Virgin River contributes an average of 426,000 tons of salt and nearly six million tons of sediment to Lake Mead each year. An annual reduction of 37,200 tons of salt could be accomplished in this subevalu- ation unit by: (1) improving the irrigation delivery system to reduce canal seepage (6,800 tons of salt); (2) improving water management by increasing the average onfarm irrigation efficiency from 44 to 62 percent (30,400 tons of salt) and (3) a minor reduction of sediment by irrigation management (7 tons of salt). These components are proposed in Alternative 2, the Recommended Plan. Implementation of the Recommended Plan (Alternative 2) would require semiautomated onfarm irrigation systems on about 3,500 acres (438 in Arizona) with a construction cost of $1,937,000. The annual operation and maintenance cost would increase from $5,000 to $30,000 because of additional maintenance and replacement costs needed for the automated systema. About one-fourth of these increaaed costs would be offset by increased efficiency of crop production. The existing canal and lateral distribution systems in Virgin Valley need improvement to reduce excesaive seepage and improve farm delivery. Total installation costs for improving 6.4 miles of off-farm distribution .~r,'r~o UU\)..;:J 11