A cloud of confusion
Denver Water officials will have plenty to ponder as they decide whether to continue a widespread cloud seeding program intended to boost water levels in city reservoirs.Two recently released reports on last winter’s effort offer conflicting information. One study shows that snowpack levels at test sites in the targeted areas were about 14 percent above what could be expected, based on a statistical analysis of snowfall records.But the results of a second study suggest that the silver iodide used for seeding did not end up in the intended target areas, casting doubt on the efficiency of the program.One of the places researchers looked for traces of silver was at a Vail Mountain SNOTEL site, where concentrations of silver were measured at 6.7 parts per trillion (ppt), barely above the expected background level, according to the report. That concentration is "too low to expect meaningful snowfall augmentation," though it shows that some limited amounts of the seeding agent might be present, the researchers conclude."The lack of silver enhancement in the snowpack means the (silver iodide) plumes were not routinely transported over the sampling location. Such a finding indicates a failure to routinely seed the intended cloud regions," the report concludes."We will be trying to figure this out," says Steve Schmitzer, chief of water resources analysis for Denver Water. “Before proceeding with additional cloud seeding, the agency will seek input from additional experts.”Cloud seeding is an effort to provide seed material for snowflakes. Introducing tiny particles of silver iodide into cold clouds helps wring out more of the available moisture in the form of snow, operators claim.Denver Water’s $700,000 program, covered about 6,000 to 7,000 square miles of watersheds that deliver runoff to the city’s reservoirs. The agency was supported financially by several other Front Range water suppliers and several local ski resorts also hoping to cash in on a snowfall bonus. Officials estimated a gain of 35,000 to 50,000 acre-feet of water, at a cost of $12-$23 per acre-foot.The statistical study was done by Utah-based North American Weather Consultants, a company that operates several weather modification programs around the country. Their evaluation involves comparing many years worth of snowfall and snowpack from both inside and outside the intended target area and doing a statistical comparison, Schmitzer explains.That methodology is frequently used to gauge the success of cloud seeding programs, but experts also acknowledge that the system has some weaknesses primarily uncontrolled variables where snowpack measurements are taken. Small variations in regional climate could skew the result. Logging, insect infestation or forest fires may also influence target and control snowpack sites, researchers say.The second study follows guidelines set by a 1998 policy statement on weather modification by the American Meteorological Society, calling for a "physical evaluation to confirm that the statistically observed change was due to the seeding."The physical evaluation was done by Woodley Weather Consultants, whose field researchers took snow samples to look for traces of silver iodide. They found that only one of 10 target stations showed silver concentrations in the expected range for effective seeding.Schmitzer says that doesn’t necessarily mean the seeding didn’t work, says Schmitzer. It could mean that the silver just didn’t end up where it was intended to. The sampling locations were selected by Larry Hjermstad, of Western Weather Consultants, the Durango-based company that operated Denver Water’s cloud seeding program.The report suggests that a more high-tech seeding effort could show more reliable results. Manually operated ground-based silver iodide generators on the valley floors may not be the best way to disperse the silver iodide into the passing clouds. Instead, the generators should be higher up on the windward mountain slopes, where automated operation would be required.To figure where the silver iodide is going could require a more extensive study aimed at tracing the outlines of the plumes left by the chemical as it falls back to earth in the winter snows.This past winters widespread cloud-seeding was permitted by the state without any serious up-front effort to analyze potential environmental impacts, critics say. Of potential concern are trace amounts of silver iodide, although most experts say the concentrations are undetectably low. Once the snow is gone, the chemical "seeds" remain as inert, microscopic particles, according to Hjermstad."There has been a question about some of the substances used in cloud seeding. But, while toxic, they’re in such small quantities as to seem relatively benign, given the other chemicals we encounter daily from more clearly attributable sources," says Art Goodtimes, a Green Party member and San Miguel county commissioner from Telluride, where cloud seeding has been used to enhance snowpack for years.Other environmental advocates say the state should at least require a comprehensive monitoring plan. If cloud seeding is to be conducted extensively for years to come, there needs to be an effort to establish some baseline conditions, says Crested Butte resident Steve Glazer, a water activist with the Sierra Club."I totally agree," says Goodtimes. "It needs good science to demonstrate that the money spent on cloud seeding isn’t just another industrial cargo cult."Goodtimes also expresses a more fundamental question about the efficacy of snowmaking.In an e-mail interview, Goodtimes says: "We ought to be doing ecological footprint studies of bioregional areas to show the true carrying capacity of the land and its resources (primarily water in the semi-arid West), rather than throwing good money after bad, hoping our chemical intervention in the chaotic world of wild nature might just get us 3 or 4 percent more precipitation than a storm would normally deliver."