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April 2009 Silverman 7 <br /> AN INDEPENDENT STATISTICAL EVALUATION OF THE <br /> VAIL OPERATIONAL CLOUD SEEDING PROGRAM <br /> Bernard A.Silverman,PhD <br /> Consulting Meteor loOst <br /> Centennial,CO.USA <br /> Ag Amt.An independent target-contrd statistical evaluation of the Vail operational cloud seeding pro- <br /> gram over its period of operations from 1977 to 2005 was conducted using ratio statistics and,in particu- <br /> lar,the bias-adjusted regression ratio.The water year(October-September)streamtlow expressed in . <br /> Acre-Feet(AF)served as the response variable in the evaluations.The effect of seeding on eight(8), <br /> closely-spaced sub-basins in the Vail watershed was evaluated using the controls that give the most pre- <br /> cise evaluation results possible with the available data.Evidence for statistically significant seeding et- <br /> fads ranging from+6.3%to+28.8%was found for 5 of the 8 seeding targets.The maximum seeding <br /> effect Is centered on Bighorn Creek(GBH)and decreases for targets both northwest and southeast of <br /> GBH.An analysis of the time evolution of the seeding effect suggests that the percent change in stream- <br /> flow at each of the target sub-basins was about the same from water year to water year. <br /> 1. INTRODUCTION gating wind direction envelope.Over the next few <br /> years an additional three seeding sites were <br /> The Vail operational cloud seeding program be- added to the total generator network(for a total <br /> gan during the last half of the 1976-77 winter of 17)to further improve the total seeding plume <br /> season. The ski areas and water conservancy coverage of cloud nuclei that would feed into the <br /> • districts contracted Western Weather Consult- cloud systems forecast to move over the two ski <br /> ants to mount a wintertime doud seeding pro- areas. In the seeding methodology of Western <br /> gram in the Vail basin in an attempt to counter Weather Consultants, the ground generators are <br /> the effects of drought conditions and poor winter • located at sites that utilize upwind ridges and <br /> snowpadc. The Vail Program has operated con- channeilng valleys to push the seeding material <br /> tinuously since then with a regularly scheduled into the lower cloud region with the most favor- <br /> three-month program that starts on November able seeded regions being downwind of the initial <br /> first and continue& to the end of January each barriers. <br /> winter season. A few of the operational seeding <br /> seasons have been extended into February and In 2001, Western Weather Consultants corn- <br /> occasionally into March during winters of below pleted a ten-year evaluation(Hjermstad,2001)of <br /> normal precipitation. Only once has the seeding the more recent operational years with the most <br /> season been terminated early(in late December) complete sets of available precipitation data. A <br /> during a winter with exceptionally above normal non-statistical analysis of the Snotel data sug- <br /> snowfall amounts. Brief periods of suspensions Bested a seeded precipitation increase of 15%on <br /> of seeding operations have occurred during pert- all of the seeded days and an increase of 7%for <br /> ods of avalanche concerns or warnings. the average seasonal precipitation. A non- <br /> statistical analysis of the Ski area data suggested <br /> The initial program at Vail had 8 ground-based an average 31% increase of the precipitation on <br /> silver iodide nuclei generators that could provide seeded days in the ski areas and a 15%increase <br /> continuous seeding plume coverage over and of the average seasonal precipitation. Western <br /> around the Veil ski slopes with targeting wind Weather Consultants(Hjermstad, Personal Com- <br /> directions varying from 240 degrees to about 350 municatlon) obtained its estimates of the non- <br /> degrees. In 1981, the target area was expanded seeded precipitation for each of the Vail targets <br /> to include the Beaver Creek ski area and the net- by extrapolating the average precipitation value <br /> work of seeding generators was expanded to 14 of 2 nearby non-seeded sites (one on each side <br /> seeding sites *thin the same approximate tar- of the target)to the Vail target by adjusting them <br /> for the differences in nature/precipitation due to <br /> Corresponding author address:Bernard A. elevation and distance. The Vail operational <br /> • Silverman,7038 E.Peakview Place cloud seeding program has never been subjected <br /> Englewood, CO 80111;e-mail: cloud <br /> a statistical evaluation. Until now, statistical <br /> silvermanb @aol.com <br /> -Scientific Papers- <br />