The Desert Research Institute operates several such cloud-seeders in California and has found that the process increases the precipitation output of a cloud by around 10 percent—though there is a lot of variability, cloud to cloud, and the effectiveness of the process has been debated. In 2003, a National Research Council study found little evidence in favor of cloud-seeding, but mostly because there weren't enough data. "This does not challenge the scientific basis of cloud-seeding concepts," the National Research Council assured in its conclusion, calling for more research. "The scientific community now has the opportunity, challenge, and responsibility to assess the potential efficacy and value of intentional weather-modification technologies," the council wrote. A study out of Wyoming is expected to be published in December to more precisely determine the benefit of cloud-seeding.
The California cloud-seeders are strategically placed in its northern mountainous region, where snowpack is an essential component of the yearly water supply. In the past year, the snowpack was depleted to one of its lowest recorded levels. As can be seen in the chart below, much of California's water comes from the snow-laden areas in the northern part of the state.
+ (U.S. Department of the Interior, Bureau of Reclamation)
Not only has the state's snowpack diminished, the ongoing drought in California has gotten so bad that the state is losing mass, as NASA has observed from space. That has wide-reaching implications for a state with a massive agricultural economy. "The water shortage could result in direct and indirect agricultural losses of at least $2.2 billion and lead to the loss of more than 17,000 seasonal and part-time jobs in 2014 alone," reports the National Science Foundation. By 2030, California is projected to have a water supply-to-demand deficit of 1.6 trillion gallons a year, the U.S. Interior Department has predicted.
"Even currently, the supply and demand are somewhat out of balance," Shawn Blosser, an economic consultant with the Blue Sky Group, a public policy consultancy, tells me. "There really is no single silver bullet that is going to solve the problem.
Part of the solution is combating the rising demand for water (Blosser, working with the California think tank Next 10 has developed a menu of policy ideas to reduce demand—the answers aren't easy, or cheap.) But part of the solution is also to increase supply, by cloud-seeding or perhaps a more scalable measure such as waste-water recycling. In all, Blosser and Next 10 project that an increased effort to seed clouds could reduce the looming water gap by 26 billion gallons in 2030, at a cost of $22 per acre-foot of water (325,851 gallons). That's markedly cheaper than other technologies to increase water supply. Water desalination, for instance, would cost $1,890 per acre-foot of water produced.