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EXECUTIVE SUMMARY The need for additional water in the Colorado River Basin has been recognized and studied for many years. The Secretary of the Interior is specifically charged with the responsibility for development of the water resources of the Colorado River Basin (Colorado River Basin Project Act, 1968; Public Law 90-537), the protection of water quality (Colorado River Basin Salinity Control Act, 1974; Public Law 93-320), and the transfer of precipitation management technology (Reclamation States Emergency Drought Relief Act, 1991; Public Law 102-250). A number of options have been considered to provide additional water supplies for the Colorado River Basin. These options include importation, desalination, evaporation suppression, vegetation management, and precipitation management (weather modification by cloud seeding). Of all the options, precipitation management appears to be one of the most cost effective and economical means of providing additional fresh water supplies. Existing cloud seeding technology, when properly applied, appears to have the potential to increase winter snowpack in the mountainous areas ofthe Colorado River Basin. Current policy statements on the status of cloud seeding issued during 1992 by the AMS (American Meteorological Society) and the WMO (World Meteorological Organization) indicate that statistical analyses of some cloud seeding programs have suggested mountain snowfall increases of 10 to 15 pct per winter. Unfortunately, statistical evaluations of most experimental and operational seeding programs have been inconclusive, and physical evidence has been limited. Incontrovertible scientific proof is still lacking that cloud seeding can produce significant snowfall increases over wide areas over prolonged time periods. Documentation is also lacking concerning optimum seeding methods for winter mountain clouds. Evidence exists that some current seeding approaches are usually ineffective. Many potential users are reluctant to apply cloud seeding because of these uncertainties. . In spite of remaining questions about effectiveness, both the AMS and WMO experts found good reasons for optimism concerning winter orographic (mountain-induced) cloud seeding potential. This optimism is based on recent investigations which used new technology to physically demonstrate that hypothesized changes follow seeding of some orographic clouds. Many of the details observed in actual clouds have been simulated by state-of-the-art numerical models of air motions and cloud processes, another encouraging development. Both the AMS and WMO recommended that the physical basis of cloud seeding be strengthened by more comprehensive field observations supported by numerical model simulations. This physical evidence is needed to fully define the conditions under which precipitation can be increased in an economically viable manner, and to transfer the technology within and beyond the Basin. Investigations have shown that the streamflow enhancement resulting from 10 to 15 pct seasonal snowfall increases can be very cost effective. The additional water produced by precipitation management can increase hydroelectric capacity and energy production, and water supplies for agriculture, industry, domestic, and recreation uses. Increased water supplies can reduce salinity and increase minimum streamflows to help fish and wildlife. The ability to enhance water supplies will be particularly important if climatic warming results in a drier West as some numerical models predict. Benefits from operational application of cloud seeding technology Basin-wide are estimated to exceed $150 million yr -1, resulting in a benefit/cost ratio of about 10:1. xn I ,I I ,II I I I I t I :1 ' I ,I II 1" I I ,I I