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<br />were located at relatively high elevations, above 6600 ft. The results raise serious concerns about <br />generator siting, inadequate crosswind coverage by adjoining generator plumes and the reliability of <br />remote-controlled AgI generators. <br /> <br />Similar poor targeting results were reported by Warburton et al. (I 995b) for two large target <br />areas in the Sierra Nevada. They showed that on average no more than 20 percent of the <br />precipitation falling during seeding operations had "seeding silver" over several winters in the <br />Truckee- Tahoe region. Silver concentrations were above threshold in 42 percent of the westerly <br />wind seeded events in the Lake Almanor region, but only 8 percent with southerly flow, again <br />showing lack of proper targeting in about 80 percent of all cases. In fact, the results suggested <br />that intended upwind control areas were often seeded during southerly flow, again showing the <br />importance of documenting seeding plume trajectories rather than assuming where they will be <br />transported. <br /> <br />Other studies which found little indication of enhanced silver-in-snow, in these cases from Utah <br />valley seeding, include Long (1984) and Super and Huggins (1992a). A companion article to the <br />latter study, Super and Huggins (I 992b), found limited support for effective valley-based AgI <br />seeding using aircraft observations. A recent large operational seeding project in Colorado had <br />seasonal snowpack samples tested for enhancement of silver (Super et al. 2003). There was little <br />indication that the observed silver concentrations were high enough for effective seeding. <br /> <br />The following was stated by Holroyd et al. (1988) who reviewed two project reports, Hill <br />(1982b) and Long (1984), which are no longer readily available: "Hill (1982b) presented results <br />obtained during an evaluation of the Utah State operational seeding program. Ground and <br />airborne measurements were made in the vicinity of the Tushar Mountains to determine if the AgI <br />seeding material was being effectively transported from upwind valley generators to clouds over <br />the mountains. A high incidence of inversions was noted, in which the AgI was frequently <br />trapped. Significant IN (ice nuclei) concentrations were rarely transported to sufficient height <br />and in adequate concentrations to achieve significant seeding effects. It was concluded from <br />physical evidence that the ground-based generator network was ineffective. This finding was <br />reinforced by model studies described in Long (1984) that used a Tushar Mountain dataset. <br />Application of the GUIDE model, with local upwind soundings as input, resulted in the <br />conclusion that 'only 36% of the soundings considered showed seeding material was delivered to <br />the clouds.' In the majority of cases, intended transport was apparently not achieved, due to <br />easterly downslope flow, trapping inversions, or air flow parallel to the mountains." <br /> <br />The above cited evidence strongly suggests that use of valley or even foothill generators <br />generally does not provide a reliable seeding method, at least for the several projects cited. It is <br />recognized that this view is at variance with many internal historical target-control statistical <br />analyses of operational projects and even some peer-reviewed analyses of experimental research <br />projects, later questioned in the open literature. Reasons for regarding such statistical results with <br />caution, especially if they are not supported by physical evidence and reasoning, are discussed by <br />Dennis (1980) and Super and Heimbach (2003). <br /> <br />Not all long-term operational projects have failed to show routine enhancements in silver-in- <br />snow concentrations. McGurty (1999) reported on observations from the Southern California <br />Edison Big Creek program in the San Joaquin River drainage. Both ground-based and aircraft AgI <br />releases were routinely made, with inert trace chemicals also released from January to April 1994. <br />Cesium was used with the ground generators, located from 1,800 to almost 10,000 ft in elevation, <br />and indium was used with the aircraft generators. The snowpack was profiled at 2 cm (0.8 inch) <br />intervals at 11 different locations resulting in over 600 individual snow samples. On average, 73% <br /> <br />21 <br />