CHAPTER 6 - Page 3 of 4
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Utah's Water Resources: Planning for the Future CHAPTER 6 - Page 3 of 4 |
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Weather modification, or cloud seeding, has long been recognized as a means to enhance existing water supplies. Cloud seeding assists nature in the formation of precipitation, by providing droplet-forming nuclei at the proper times and places. In mountainous regions like Utah, clouds form as moist air rises and cools during its passage up and over mountain ranges. By nature, many of these clouds are highly inefficient at releasing precipitation, retaining more than 90 percent of their moisture. Cloud seeding can greatly improve this precipitation efficiency. A schematic drawing of the process for seeding mountain clouds is shown in Figure 14. Typically, silver iodide is released into the air from strategically placed ground generators to produce artificial ice nuclei. Aircraft seeding is also used, but is much more expensive. The nuclei form ice crystals that attract moisture from the surrounding air, forming droplets large enough to fall to the ground as snow. |
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The first cloud seeding project in Utah ran from 1951 to 1955 in the central and southern portion of the state. Cloud seeding began again in central and southern Utah in 1973 and has continued to the present. Also in 1973, the Legislature passed the Utah Cloud Seeding Act. This law provides for licensing cloud seeding operators and permitting cloud seeding projects by the Utah Division of Water Resources. The act states that, for water right purposes, all water derived from cloud seeding will be treated as though it fell naturally. Since 1976, the state, through the Board of Water Resources, has shared the cost of cloud seeding projects with local entities.
Utah cloud seeding project areas are shown in Figure 15. In water year 2000 there were four active project areas in Utah. These were the Central and Southern Utah, Tooele County, West Box Elder County, and the East Box Elder/Cache county areas. The West Uintas project became active again in water year 2001. A study conducted by the Division of Water Resources estimates that these areas have realized a 7-20 percent increase in April 1 snow water content. This translates into an increase in estimated average annual runoff of about 250,000 acre-feet, 13 percent above historical runoff in these areas. The division estimates the cost of water developed from cloud seeding these areas to be about one dollar per acre-foot. Precipitation data from a number of cloud seeding projects have been examined for evidence of downwind effects. Results from these analyses show a slight increase in precipitation in areas up to 90 miles downwind from the project area. No decrease in precipitation has been detectable farther downwind from any long-term project. Cloud seeding is most effective when it is continued over several years. This increases soil moisture, provides greater groundwater and spring flows, and sustains base flows in streams and rivers. Seeding only in dry periods will not be as effective because of the lack of seedable storm systems. |
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CHAPTER 6 - WATER DEVELOPMENT Weather Modification |
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