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f. " J ') 5;) UU ..L removal of aspen timber in western Colorado. Studies show that the com- bined effects of weather modification and vegetation modification on the same area increase streamflow more than if the two practices are applied separ ate 1 y. E. Weather Modification The technology of modifying wintertime orographic cloud systems to increase mountain snowpack has been developing over the past 30 years. Both research and operational projects have contributed to a better understand- ing of weather modification. The accumulated evidence is that seeding under favorable conditions should increase seasonal snowpack 10 to 15 percent and this, in turn, should increase the flow of the Colorado River by at least 10 percent. Evidence about the potential for wintertime orographic snowpack augmentation in the high mountains of the Colorado River Basin is based largely on statistical data from past projects such as the Climax and Wolf Creek Pass experiments conducted by Colorado State University and Reclamation's Colorado River Basin Pilot Project. These results were consistent with the physical hypothesis that seeding under favorable conditions increases the precipitation efficiency of the cloud systems. In addition, we have gained considerable insight into the seedability of orographic cloud systems through Reclamation's Sierra Cooperative Pilot Project and Colorado State University's Colorado Oro- graphic Seeding Experiment. Recent advances in the technology of winter orographic cloud seeding have provided knowledge on which clouds should and should not be seeded and new instrumentation and techniques for recognizing opportunities in real-time and reacting to them in a timely manner with more effective seeding delivery systems. Several major resource studies have examined the augmentation options and concluded that the technology of cloud seeding for significantly increas- ing snowpack and runoff is scientifically possible. It appears to be the most cost-effective means for increasing water supplies, and is within reach. In 1974, a Department of the Interior report on Water for Energy in the Upper Colorado River Basin concluded augmentation by cloud seeding was feasible. In 1975, the Secretary released the comprehensive Westwide Study Report on Critical Water Problems facing the 11 Western States authorized by Title II of Public Law 90-537. The overall inadequacy of the natural supply was recognized. The report concluded: "Taking into consideration availability, quantity, quality, and cost of the augmentation alternatives, weather modification appears to be the most promising source of new water supply in the Wes tern Un i ted States." It recommended initiation of two comprehensive research projects and accompanyi ng soci al-env ironmenta 1 studi es with in the decade: (1) a demon- stration program in the Colorado River Basin, and (2) a cooperative pilot program in the Northern Sierra Nevada. 12