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Last modified
7/28/2009 2:40:35 PM
Creation date
4/24/2008 2:52:55 PM
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Weather Modification
Title
The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
Date
12/1/1991
Weather Modification - Doc Type
Report
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<br />Hill (1980a) reported on the results of measurements of the dispersion of aircraft-released AgI during <br />nonconvective winter stonos over the northern Wasatch Mountains. He concluded that both vertical and <br />horizontal dispersion were "much lower than that desired for effective seeding." <br /> <br />Thus, while the statistical analyses of the two northern Wasatch seeding experiments were inconclusive, <br />examination of them in light of current knowledge suggests possible physical reasons for that result. <br /> <br />Cloud seeding has been conducted in some portion of Utah each winter since the 1973-74 season with the <br />exception of 1983-84 (Griffith et al., 1991). The first analysis of this long-teno program published in the <br />open scientific literature was by Hill (1978) who considered only the first two winters. He developed a <br />predictor technique using rawinsonde upper air and precipitation observations for seven unseeded seasons. <br />Hill concluded that there was little evidence to support or reject any change in precipitation due to the <br />seeding, and that any effect, if present, was likely less than 10 percent over the whole area. Of course <br />the data set was quite small at the time, making evaluation difficult. <br /> <br />Thompson and Griffith (1981) presented a post hoc statistical evaluation of the first 7 years of the Utah <br />operational project for central and southern Utah, including the Sevier River basin. Both ground <br />generators and seeding aircraft were used until 1979. Thereafter only ground releases of AgI were made. <br />Thompson and Griffith compared precipitation measurements in the target with control areas, both during <br />the seeded winters and for several winters prior to seeding. This common evaluation approach for <br />operational projects must assume that the target-control relationship did not change from the nonseeded <br />period to the seeded period except as the result of any seeding effects. The authors concluded that <br />primary target precipitation increases of between 13 and 20 percent were indicated. <br /> <br />Hill (1982b) reviewed the operational seeding program for approximately the same period but included <br />the eighth winter as well. His evaluation used not only target-control relationships from precipitation <br />gauges (with two different control areas), but also other covariates such as upwind surface pressure and <br />upper airobselVations. Hill's analysis questioned the apparent seeding increases shown in the precipitation <br />data based on historical regression techniques. This was partially due to the apparent increases being <br />randomly distributed between seeded and nonseeded days when various control sites were used. <br />Moreover, Hill used control precipitation gauges along the California coast to show that, relative to the <br />available historical records, precipitation increased eastward fono California to Utah during the seeded <br />period, especially during the years 1978, 1979, and 1980. These departures in the historical precipitation <br />pattern appeared to be due to causes other than seeding. This finding raises doubts about the stability of <br />the target-control relationships, and, hence, evaluations based on historical precipitation data. <br /> <br />Griffith et al. (1991) recently reviewed 13 winters of operational seeding in central and southern Utah, ~ <br />including the Sevier River Basin. The authors work .for the company that has conducted the operational <br />program in Utah for many years and are very familiar with the project Their analysis included the <br />1973-74 through 1982-83 winters and the 1987-88through 1989-90 winters. No seeding was conducted <br />during the 1983-84 winter and only one. county was seeded the following three seasons. Actually, only' <br />two widely separated counties in central and southern Utah were seeded during the 1987-88 winter so it <br />is surprising that it was included in the "seeded winter" list. The basis of this analysis was the use of a <br />historical regression relationship between the target and a control area for several winters prior to th~ <br />seeded period. Results indicated that for the primary target area, which includes a large portion of the <br />Sevier drainage, seeding produced an II-percent increase in seasonal precipitation. The ratio of observed <br />to predicted target area precipitation (predicted calculated from the target-control regression equation for <br />nonseeded winters) was greater than 1.0 for 12 of the 13 winters. Values ranged from 0.96 to 1.29, the <br />latter suggesting a 29-percent increase. <br /> <br />j <br /> <br />8 <br />
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