OCR Text |
Show APPENDIX XII XII-63 about 730,000 acre-feet. In 1975, precipitation and runoff returned to nearly normal. However, cool temperatures during the growing season reduced irrigation demands. Reservoir storage recovered from the previous year's drawdown. Reservoir evaporation, also primarily affected by climatic conditions, demonstrated a pattern of variation similar to that of irrigation consumptive use. Transbasin exports, the second largest Upper Basin use, showed the greatest year-by-year variation and also the greatest net increase during the reporting period. In 1971, exports totaled 583,000 acre-feet. In 1975, exports had risen to 815,500 acre-feet primarily due to the opening of the Boustead Tunnel in Colorado and Azotea Tunnel, which outlets in New Mexico. Thermal power water uses in the Upper Basin more than doubled in the 5-year reporting period as four major powerplants went into operation: San Juan (New Mexico) in 1973; Navajo (Arizona) in 1974; Jim Bridger (Wyoming) in 1974; and Huntington (Utah) in 1975. During the 5-year reporting period, main stem regulating reservoirs recorded an increase of 9,906,000 acre-feet of surface storage. As storage increased, main stem reservoir evaporation rose from 458,000 acre-feet in 1971 to 607,000 acre-feet in 1975. LOWER COLORADO RIVER Water use within the Lower Colorado River system is increasing as a result of additional irrigated acreage and a fast-growing population. Irrigated land has increased from about 1,285,000 acres in 1971 to 1,440,000 acres in 1975. Population in 1970 was estimated to be.about 2.1 million, and 2.6 million in 1975. Mainstream Table LC-1 shows water-surface evaporation from mainstream reservoirs and channel losses; table LC-2, the change in surface-water contents of the reservoirs; and table LC-3, water uses along the Lower Colorado River mainstream and flood plain including water passing to Mexico. Water passing to Mexico is made up of deliveries in satisfaction of the Treaty, deliveries made pursuant to Minutes 218, 241, and 242, and regulatory waste. Mainstream reservoirs gained about 3.4 million acre-feet of surface storage during the 5-year reporting period. Water supplies necessary to meet the mainstream water use, including reservoir surface and bank storage, came principally from the regulated releases at Glen Canyon Dam. Annual reservoir evaporation and channel losses consumed about 1.5 million acre-feet. Table LC-9, a water budget below Davis Dam, results in an estimate of the overall channel losses in the reach to the International Boundary. Irrigated land has increased from about 331,000 acres in 1971 to 351,000 acres in 1975-most of the increase occurring in the Colorado River Indian Reservation. Municipal and industrial water use, including thermal powerplants in Nevada and Arizona, doubled during the 5-year period. Much of this demand is within southern Nevada. Pursuant to Minutes 218 and 242, saline return flows from the Wellton-Mohawk Irrigation and Drainage District near Yuma, Ariz., were bypassed around Morelos Dam at the International Boundary resulting in a substantial increase in the water passing to Mexico in excess of the Treaty requirements. Project plans to implement the United States measures required by Minute 242 call for reduction of bypassed water through improved irrigation efficiencies, reduced acreage to be irrigated on Wellton-Mohawk Project lands, and the construction of a desalting plant converting drainage water to an acceptable quality for de- Water Year Colorado River at Estimated Total Compact Point, near Tributary Lee Feny, Arizona Inflow to Mainstream (MAF) (MAF) (MAF) 1971 8.61 0.97 9.58 1972 9.33 0.78 10.11 1973 10.14 2.12 12.26 1974 8.28 0.85 9.13 1975 9.27 0.94 10.21 Average 1971- 1975 9.13 1.13 10.26 17 |