Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

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Rita Maguire, President and CEO — ThinkAZ uch has been made of recent agreements reached among the states of Arizona, California, and Nevada to address the strains that growth has placed on the limited supplies of Colorado River water in the Lower Basin. But even before the ink was dry on these agreements, a new threat arrived, one that undermines the states' ability to resolve water management issues. Drought conditions throughout the Colorado River watershed have changed the dynamics of what were once well thought -out, long -range water agreements and set the stage for new ones. Creating Sustainable Long -Term Agreements for Water Use �V� LV a ol Reprinted with permission of David Fitzsimmons. During the 1990s, the Lower Basin states of Arizona, California, and Nevada developed and began implementing a plan for long -term, sustainable use of the Colorado River. Significant agreements were reached between the Secretary of the Interior and the three states. The cumulative goal of the agreements was to ensure the integrity of the allocation system established under the Boulder Canyon Project Act of 1928. This required alleviating California's and Nevada's overdependence on the river. Irl jitsr fi�'c' t'CUi.c, nc( wSefvoir,ti c%chllcd hi Cornhined 24 Ind/1,0i7 t1Crt� feet, the equivalcill of olic' hill Luke Po well. Three agreements were reached. The first established an interstate banking program that allows Nevada and California to purchase Arizona's otherwise unused Colorado River water and store it in the state's aquifers for use later. The second 22 • March /April 2005 • Southwest Hydrology formally quantified the water rights of farmers in the Imperial Valley, facilitating the sale of up to 200,000 acre -feet to San Diego each year for the next 75 years. The third agreement allows surplus water to be drawn from Lake Mead until 2016 while California completes the infrastructure necessary to deliver the farmers' water to San Diego. These agreements were negotiated when Lake Powell and Lake Mead were nearly full and additional water could be released to meet their terms. Unfortunately, Mother Nature had a different plan for the future. Lower Basin Drought Impacts As the largest reservoirs on the Colorado River, Lake Powell and Lake Mead provide essential water storage to meet demand in the Lower Basin even when supplies fluctuate. In 1999, these reservoirs both were essentially full. At the close of 2004, U.S. Bureau of Reclamation (Bureau) data indicated the lakes were less than half full. In just five years, the reservoirs declined by a combined 24 million acre - feet, the equivalent of one full Lake Powell. Considering that 9 million acre -feet of water must be delivered from these reservoirs each year to end users in the Lower Basin states and Mexico, tough choices must soon be made if the drought continues. According to modeling runs by the Bureau, Lake Mead's elevation will continue to decline until 2008 and then rise slowly to no more than 1,120 feet over the next 20 years, assuming average runoff and normal operating conditions. Under runoff conditions similar to those experienced from 1953 to 1973, the Bureau projects that Lake Mead will drop below 1,050 feet by 2011, even with imposed shortages on the river. Under a worst -case scenario, if no shortages are declared, Lake Mead's elevation will fall below 1,050 feet before 2011. Since no one can forecast with certainty when the drought will end, prudent water managers in the Lower Basin states are making mid - course adjustments in their water budgets to compensate for the drought's impacts on system reliability, capacity, water quality, and ultimately, availability. Lake Mead Intake Lowered Declining lake levels in Lake Mead have caused Southern Nevada Water Authority (SNWA) to lower its first intake elevation from 1,050 to 1,000 feet as the base of the warm thermocline layer in the upper part of Lake Mead has fallen, approaching the intakes. Water quality at SNWA's intakes is expected to deteriorate due to the presence of low levels of perchlorate