Tuesday, 25 January 2011: 11:15 AM
605/610 (Washington State Convention Center)
Don A. Griffith, North American Weather Consultants, Sandy, UT; and M. E. Solak and D. P. Yorty

North American Weather Consultants (NAWC) has conducted operational winter cloud seeding programs in many of the mountainous areas of Utah since 1974. The goal of these programs has been to enhance winter snowpack accumulation in several mountainous target areas throughout the State. Studies have demonstrated that a large majority of the annual runoff in Utah streams and rivers is derived from melting snowpacks, which explains the focus on wintertime seeding. Augmented water supplies are typically used for irrigated agriculture or municipal water supplies. Programs are typically funded at the county level with cost sharing grants from the Utah Division of Water Resources.

Cloud seeding is accomplished using networks of ground-based, manually operated silver iodide generators located in valley or foothill locations upwind of the intended target mountain barriers. As such, these programs are classified as orographic winter cloud seeding programs. Orographic winter cloud seeding programs are typically categorized as those with the highest level of scientific support based upon capability statements of such organizations as the American Meteorological Society, the World Meteorological Organization, and the Weather Modification Association.

NAWC historical target/control evaluations of these Utah programs based upon high elevation precipitation and snow water content observations indicate a range of apparent increases in target area average precipitation or April 1st snow water content of 3-21%.

The Utah Division of Water Resources conducted an independent assessment of the seeding programs in 2000. That assessment confirmed the NAWC indicated increases in snow water content, and then took the additional step of estimating the increases in annual streamflow resulting from the estimated increases in snow water content. Average annual increases from four seeded areas were estimated to total 249,600 acre-feet. Factoring in the cost of conducting these programs resulted in an estimate of the average cost of the augmented runoff to be $1.02 per acre-foot.

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