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<br />.- <br /> <br />I. Backeround and Purpose <br /> <br />Three water agencies from the three states of the Colorado River's Lower Basin ("Lower Basin <br />States") entered an agreement with the Southwestern Water Conservation District (SWWCD) of <br />Colorado on March 7, 2006, to extend winter cloud seeding operations in the San Juan <br />Mountains of southwest Colorado. This "trial extension" allowed the operational seeding <br />company, Western Weather Consultants (WWC) of Durango, CO to seed those mountains <br />throughout March of 2006. As part of the agreement, funds were provided to the U.S. Bureau of <br />Reclamation, Technical Service Center, to evaluate the effectiveness of the seeding trial <br />extension. The funding was pursuant to a February 10, 2006 proposal from Reclamation that <br />was included as an attachment to the March 7 agreement. Reclamation's principal investigator <br />for this evaluation is Steven Hunter, research meteorologist. This is the final report and <br />deliverable for the evaluation. <br /> <br />II. Evaluation Method <br /> <br />a. Target - Control Area Method <br />The primary objective of operational cloud seeding programs is to augment precipitation. <br />Therefore their design generally does not emphasize evaluation of their effectiveness. Most <br />importantly, the programs usually seed all storm events, based on different criteria used by the <br />commercial operator. This precludes a randomized statistical analysis, which is considered to be <br />the "gold standard" of evaluation. A randomized program dictates that some events or periods <br />be seeded and others unseeded. Then a response variable (typically precipitation or streamflow) <br />from the two periods are compared to discern any seeding effect in a single target area or <br />watershed. <br /> <br />Faced with these limitations, the "second choice" and most common evaluation technique for <br />operational seeding programs is the historical target - control area regression. This statistical <br />technique compares precipitation or streamflow from an area assumed to be targeted by seeding <br />and from a nearby but similar area unaffected by seeding (similar in geography, altitude etc.). <br />The technique has serious drawbacks as well1,2, and it requires a long period of data collection <br />(ten years or more). Because of this long period, perhaps the biggest shortcoming is an <br />assumption that the relationship between natural precipitation in the target and control areas is <br />stable with time, therefore little climate change. While we have SNODAS data (Section lIb) for <br />only three winters, we cannot use a historical regression and these factors become moot. <br />Nevertheless, we will borrow one aspect of the regression technique, namely, the concept of <br />target and control area comparison. That is, we will compare snow precipitation (see Section II <br />b) in the seeding target area and two selected control areas. <br /> <br />The control area selection is critical, to ensure that the areas were not affected ("contaminated") <br />by seeding effects. A map of the target and control areas is in Fig. 1. WWC uses wind <br />directions near mean ridge-top in the San Juan target area (about 10,000 feet elevation) from a <br />numerical weather model to estimate transport and diffusion (T &D) of their generator seeding <br />materials and they decide which generators to operate in order to affect certain portions of the <br />target area. Because these model winds have errors and because actual T &D in complex terrain <br /> <br />3 <br />