An experimental cloud seeding program conducted near Maricopa last summer generated an estimated 34,000 acre-feet of additional rainfall, according to a new Pinal County Water Augmentation Authority report.

About 30 test flights were conducted between July and September within the county’s Active Management Area in western Pinal County, Executive Director Joe Singleton told KTAR News 92.3 FM on Wednesday.

“Cloud seeding is taking artificial means to try to induce rainfall that you otherwise may have not naturally got out of a cloud,” Singleton said. “Rain clouds aren’t always as efficient as they could be, and cloud seeding encourages them to give up more moisture than they might have otherwise.”

Cloud seeding works by dispersing a seeding agent from flares mounted on a small aircraft directly into suitable storm clouds.

Unlike traditional glaciogenic cloud seeding, which uses silver iodide particles, the Pinal County authority opted for a hygroscopic seeding agent after objections from the Tohono O’odham Tribal Nation. Hygroscopic agents attract and hold moisture within clouds, helping water droplets grow larger and heavy enough to fall as rain. The material used is similar to potassium chloride, a common fertilizer component.

“There’s no credible studies anywhere that show that these seeding agents that we use, or that have been being used over the last decades, do cause a health hazard,” Singleton told the Phoenix radio station.

Cloud seeding is a documented weather-modification practice used in parts of the Western U.S. to enhance precipitation under specific atmospheric conditions. Since 2007, the Central Arizona Project has helped fund cloud seeding programs in California, Colorado, Nevada, Utah and Wyoming. Salt River Project has also studied the feasibility of cloud seeding in Arizona’s White Mountains.

The Pinal County program cost nearly $220,000, according to Singleton. Based on the estimated rainfall increase, he said the water cost works out to roughly $6.50 per acre-foot, significantly lower than the cost of many alternative municipal water supplies.

Questions about the program surfaced during a December meeting of the Arizona House Committee on Natural Resources, where some lawmakers asked whether cloud seeding may have contributed to flooding in Maricopa.

Singleton told KTAR that flooding in Maricopa occurred two days after a seeding flight in southeastern Pinal County and that no operations were conducted near Maricopa at the time. He said the location and timing made it unlikely the seeding caused the flooding.

Pinal County was the first to obtain a cloud seeding license from the Arizona Department of Water Resources, Singleton said, adding that other municipalities have expressed interest in similar programs.

If the program continues, Singleton said the authority plans to improve public outreach, so residents better understand how and when cloud seeding operations are conducted.