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Bypass Flow Replacement or Recovery Methods

Bypass Flow Index

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     YDP Operations
     Excess flows to Mexico
     Other proposals
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Listed below are several possible methods that could recover or replace water bypassed to Mexico's Cienega de Santa Clara over the 1.5 million acre-feet delivered under the Treaty of 1944. Most are conceptual in nature; no detailed analyses of operation, cost, or impacts have been completed. There may be other methods that will be identified during the public consultation and planning process. Each will be analyzed at an appropriate level and be considered during the evaluation process.

YDP Operations
There is renewed interest in operating the Yuma Desalting Plant (YDP) to recover a large portion of the water that is now being bypassed to Mexico (approximately 109,000 acre-feet annually, although this number is highly variable, based on actual farming and water management operations). It is currently estimated that it would take approximately 4 years and cost more than $26 million to bring the YDP to full plant capacity.

Currently, about 5 million acre-feet of water per year is delivered to agricultural users in the states of Arizona and California alone. Water conservation or land fallowing agreements to temporarily forbear irrigation on about 20,000 acres of land (less than 2 percent of the total irrigated acreage on the lower Colorado River) would provide adequate water to replace the full amount of water being bypassed into Mexico.

Under this approach, the Bureau of Reclamation would pay a district to voluntarily forbear a portion of its approved annual projected use of Colorado River water for a year, thereby leaving the water in the Colorado River system storage in Lake Mead. This water, then, would be considered a “replacement” for the water bypassed to Mexico.

Policy would be developed that considers local effects, addresses establishment of the quantity of water that had been forgone, identifies eligibility requirements, and protects holders of lower priority rights on the Colorado River in the United States.

Based on previous water transfer arrangements and a recent solicitation of water through a demonstration program, it is estimated that water obtained with forbearance agreements can be acquired at costs in the range of $60 to $150 an acre-foot, for an estimated total annual cost ranging from approximately $6.5 million to $16 million.

There are indications that an additional 90,000 acre-feet could be pumped every year from groundwater in the Yuma area without causing adverse impacts. In addition, groundwater in the vicinity of the Mexican border has a hydraulic gradient sloping toward Mexico due, in part, it is believed, to high rates of groundwater pumping in Mexico . This results in significant amounts of water flowing underground from the United States into Mexico that is not counted as part of Mexico’s treaty deliveries. Additional groundwater pumping in the Yuma area, then, would recover some of the water now going to Mexico and would contribute to the current efforts to keep groundwater elevations from rising too high.

Depending upon its quality and use, a portion of the recovered groundwater could be returned directly to the Colorado River to be used as replacement for bypass water, or the YDP could be used to treat the water so it could be provided to local (including Mexican) municipalities for domestic use. Use of this treated water would significantly improve the quality of water municipalities deliver to customers, providing a benefit that could result in repayment of some of the costs.

To pursue this possibility, additional technical work is required, particularly in the area of aquifer characterization and model verification. Costs, including wells/pumps/motors, energy, conveyance, and maintenance, would be relatively inexpensive on an acre-foot basis. However, cost would be significantly higher if groundwater was treated by the YDP and delivered to local municipalities for domestic use.

Excess Flows to Mexico
Flows to Mexico in excess of water orders occasionally occur when water delivery requests in the United States are reduced after releases have already been made past the last control point. Additional regulation on the lower end of the Colorado River would eliminate or greatly reduce these excess flows to Mexico.

While the volume of excess flows to Mexico varies greatly, depending upon hydrologic conditions, the long-term annual average is greater than 50,000 acre-feet. Construction of physical works necessary to control and salvage these flows could allow this water to be used as bypass flow replacement. Preliminary studies of the construction of regulating reservoirs for control of excess flow to Mexico indicate that costs of salvaged water could be less than $100 an acre-foot.

There may be opportunities to combine various methods, or portions of methods, for bypass recovery or replacement that will meet program objectives. One such combination has been suggested by a YDP/Cienega Workgroup organized by the Central Arizona Water Conservation District (read White Paper on proposal here). That suggestion includes development of storage on the Lower Colorado River to capture excess flow, forbearance, operation of the YDP using local groundwater as feedwater, and other provisions.

Other Possibilities
Other concepts that have been suggested include importing water into the Colorado River basin through water exchanges, implementing advanced irrigation techniques to conserve water, snowpack enhancement, vegetation management, and negotiations with Mexico.

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Updated:December 2005