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<br />i <br /> <br />2. SCIENTIFIC BASIS <br /> <br />2.1. Previous Studies <br /> <br />Initial cloud seeding testing began in the 1940's. Experimentation continued into the 1990's at increasing <br />levels of complexity and sophistication as knowledge and equipment improved. Operational field <br />programs of varying time duration have been conducted in the United States since the 1950's. However, <br />it is the winter cloud seeding and monitoring studies of the past couple of decades, conducted over <br />western mountains, that principally provide the scientific support and experience for pursuing a cloud <br />seeding project in the Headwaters Region. Recent studies have produced data and analyses indicating <br />precipitation increases in repeated seeding trials. Cloud treatment strategies have emerged that led to <br />observed precipitation increases in trials over mountain barriers. In the following discussion, a brief <br />summary is presented on the scientific basis for cloud seeding. Interested readers may consult appendix <br />A, chapter four, for more comprehensive coverage of this topic. <br /> <br />There is convincing evidence that winter orographic cloud seeding can increase snowfall under certain <br />cloud conditions. Scientists have worked at documenting key links in the chain of physical events from <br />release of seeding material, to increased snowfall accumulation on the mountain surface. Recent studies <br />that produced snowfall increases from seeding trials include the following (see appendix A, sections 7 <br />and 8). <br /> <br />~ Bridger Range, Montana (Super, 1974; Super and Heimbach, 1983; 1988) <br />~ Grand Mesa, Colorado (Holroyd et aI., 1988; Super and Boe, 1988) <br />~ Sierra Nevada, California (Reynolds, 1996) <br />~ Wasatch Plateau, Utah (Super and Holroyd, 1994; Super, 1995; Super, 1996; Super and <br />Holroyd, 1997; Holroyd and Super, 1998; Holroyd et aI., 1995) <br /> <br />The CaSE studies conducted in the Park Range emphasized study of the natural physical structure of <br />winter clouds. No cloud seeding was conducted in CaSE. However, considerable information was <br />collected on Park Range winter weather and clouds (Rauber et aI., 1986; Rauber and Grant, 1986; <br />Rauber, 1987). The COSE studies indicated cloud seeding potential exits for Park Range winter storms. <br /> <br />When liquid water was present in clouds prior to seeding on Grand Mesa, and Wasatch Plateau seeding <br />trials, snowfall increases were consistently observed either at aircraft level or by ground instrumentation, <br />or both. Targeting of seeding effects was possible in seeding trials. Statistical evaluation results are not <br />available because the studies were not designed strictly for physical measurements. In the proposed <br />. seeding program, targeting of seeding effects for the lee side of the western barrier will be necessary. <br />Lee side targeting must be tested in the design phase. Once a cloud model is properly configured to <br />handle targeting, different weather and cloud conditions should be simulated to test different targeting <br />strategies. Design phase field studies must assess the model's ability to simulate targeting for field test <br />sites. The model is expected to play an important role in assisting the determination of seeding device <br />installation sites and developing seeding strategies. <br /> <br />The Bridger Range Experiment (Super and Heimbach, 1983) was conducted during the early 1970s when <br />more capable field equipment was becoming available. The experimental design called for physical <br />measurements and statistical evaluation. Consequently, physical measurements occurred aimed at <br />determining the transport and diffusion of seeding agent and cloud response to seeding. Statistical <br />evaluation indicated a 25 percent increase in precipitation from the treatment sample versus the control <br />sample. <br /> <br />5 <br />