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. . . . lJD:2lJ4fl The abatement of saline return flows is accomplished by reducing irrigation system conveyance losses and on-farm losses. While the salinity control program is aimed at reducing the seepage that causes return flows, it also reduces the "non-productive" or "incidental" consumptive use that occurs during irrigation. The incidental consumptive use of water involves permanent, but unintentional, loss of water from the basin by evaporation from exposed water surfaces and. evapotranspiration by noncrop vegetation. These incidental losses are reduced by combining ditches, replacing open ditches with pipe, eliminating standing water, drying up water logged soils, and reducing wetland acreage. Based on climate data for the Grand Valley it is estimated that every mile of 2-foot wide lateral placed in pipe reduces evaporation losses by 1 AF per year. Every acre of wetland lost will yield approximately 2 AF per year of reduced incidental consumptive use. Data in the 1986 Grand Valley Stage II verification memorandum indicate that at full build-out Stage II would line or pipe 325 miles of canals or laterals and reduce wetland acreage by 300 acres. This scale of project would reduce historical incidental depletions and thereby produce 950 AF per year or less of "salvaged water" from the Grand Valley. With a construction cost of $37 million (excluding all overhead and design costs) this salvaged water would have an annual cost of approximately $3,700 per AF. The original Stage II program proposed by the Bureau was expected to reduce total seepage losses by 42,900 AF per year, 6,500 AF of which were from the GYIC system. Nearly all this seepage historically returned to the Colorado River system within. the Grand Valley. As more is learned about salinity in the Grand Valley, as construction costs increase, and as the voluntary participants opt in and out of the program, it is unlikely that all increments will remain cost effective and some will be deleted from the final implementation plan. Recent estimates indicate that the combined salinity program of USBR and SCS in the Grand Valley will reduce irrigation seepage by approximately 70,000 AF per year. As of December 1990, the USBR/SCS program in the Grand Valley had reduced irrigation seepage by approximately 27,000 AF per year. It is important to understand that these seepage reduction estimates are made for the purpose of determining salt loading. not quantifying water availability. As the hydrosalinity model data are revised, these seepage estimates may also change. The majority of the irrigation water potentially made available through improved efficiencies was not previously lost through consumption, but returned to the Colorado River below the confluence with the Gunnison. While these return flows are not lost to the river system, they historically have not been of benefit to users in Colorado because of the proximity of the Utah state line, the adequate supply of water that exists in the Colorado River below the Gunnison River, and lack of demand below Grand Junction. Those return flows support instream uses in the Colorado River between Grand Junction and Utah. Current demands for Colorado River water, and shortfalls in supply are in the headwaters areas, and the water that eventually becomes return flow has already been called past those demands. This water called past upstream head gates does provide significant instream values between the headwaters of the Colorado and the Cameo diversions. ? 5 J ;; ;t' ~ ,i, ,1 I ~: -:~ ,;- i~ f'_ _-~ " J, /!l >' ,,->' _ '-'j'l:' ..h~ . '" ..,~--, " , ". _ ...'"",,,,,,;",