In the arid West, the perpetual search for new water supplies, like the mythical search for the Holy Grail, has given rise to numerous fantastical adventures and grand schemes.

Some of these have resulted in the impressive feats of engineering that have re-plumbed much of the Colorado River Basin, and some have remained fantasies, such as towing icebergs from the Arctic and building pipelines from the Great Lakes.

Cloud seeding, a practice of trying to wring more moisture from the sky, has long been hard to classify in those terms. Water agencies and ski resorts have been doing it for decades, mostly by sending plumes of silver iodide from ground-based burners into likely-looking clouds. Cloud seeding has been done in Colorado since the 1950s, and ramped up significantly after the 2002 drought. Still, until recently, no one could say with any certainty if cloud seeding really worked, or if so, how well.

Wyoming Study Conclusions

Draft conclusions from a much-anticipated, very rigorous nine-year study conducted in Wyoming indicate that cloud seeding can, in fact, increase the water yield from some storms, but the ultimate effect on the water supply is quite modest. The draft executive summary of the Wyoming Weather Modification Pilot Program study, conducted by numerous collaborators, was presented to the Wyoming Water Development Commission on Dec. 10. It is due to be finalized in March of 2015.

The study combined physical, modeling and statistical studies of the effects of cloud seeding in Wyoming’s Medicine Bow, Sierra Madre and Wind River Ranges and concluded that seeding could increase snow accumulations from some storms by 5 to 15 percent. This does not, however, translate to a 5 to 15 percent increase in the seasonal snowpack, because not all storms are seedable, and those that are cover only a limited area.

In order for the inserted silver iodide to stimulate significant additional snowfall, atmospheric temperatures must be below 17 degrees Fahrenheit, with sufficient moisture in the air and favorable winds. These conditions were met less than a third of the time during the winter in the study area. Using modeling, the study concluded that increased precipitation from seeded storms of 5 to 15 percent affecting 30 to 80 percent of the cloud seeding impact area could increase streamflow in Wyoming’s portion of the North Platte River Basin between 0.4 to 3.7 percent.

Is it Feasible?

Depending on numerous factors related to the operations of the cloud seeding program, the cost per acre-foot of water produced through cloud seeding ranges from $27-$427 per acre foot. This compares favorably with other options for producing “new water.” In a 2013 article in the Mountain Town News, Allen Best reports that the Bureau of Reclamation has estimated that desalting brackish groundwater in Arizona would cost $650 per acre-foot, and desalting ocean water near Los Angeles would cost $2,100 per acre-foot. Cost estimates for building new storage reservoirs range from a few hundred dollars/acre-foot to more than $1,000 per acre-foot.

In addition to assessing the potential impact of cloud seeding as a feasible strategy for increasing Wyoming water supplies, the study also assessed the environmental impacts of seeding and impacts on precipitation outside the seeded area (i.e. does it steal water from areas downwind?).

The environmental analysis found that concentrations of silver iodide in the affected snowpack was in the parts/trillion range, while concentrations already in the soil were much higher, in the parts/billion range, indicating a very minimal impact from cloud seeding. Modeling of impacts on precipitation outside the seeded area indicated effects of less than 0.5 percent, which is consistent with previous studies and undermines claims that cloud seeding injures those downwind of seeded areas.

This study indicates that cloud seeding likely deserves its relatively newfound respectability as a water supply strategy, but also that its impacts are far too small for it to be a panacea for the West’s water woes. To read the draft executive summary for yourself, go to the Wyoming Water Development Commission’s website at http://wwdc.state.wy.us/.

Hannah Holm is the coordinator for the Water Center at Colorado Mesa University. Holm is a friend of the Eagle River Watershed Council, which has a mission to advocate for the health and conservation of the Upper Colorado and Eagle River basins through research, education and projects. Contact the Watershed Council at 970-827-5406 or visit www.erwc.org.