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How cloud seeding can help replenish reservoirs in the west

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Whenever there’s a big storm in the American West, pilots are likely to fly in the eye, seeding clouds with a substance called silver iodide. The goal is to increase precipitation.

Cloud seeding has been around since the 1940s. It has become widespread lately as the West grapples with a drought of historic proportions. States, utility companies and even ski resorts are footing the bill.

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While it has been considered effective for decades, recent studies have helped prove that cloud seeding works, and there is no evidence that silver iodide is harmful at current levels. Experts say that cloud seeding usually produces a 5% to 15% increase in precipitation.

It’s not a drought cure, but cloud seeding can be an important water management tool.

“We can’t make a storm happen and we can’t create ideal conditions in this storm. They happen naturally,” said Jason Carkeet, a utility analyst and hydrologist for the Turlock Irrigation District in central California. Turlock started its cloud seeding program in 1990.

“What we’re doing is just taking advantage of existing conditions, natural conditions, and trying to make the storm again more efficient from a water supply standpoint,” Carkeet said.

How cloud seeding works

When done by air, cloud seeding involves loading a plane with silver iodide. Flares are placed on the wings and fuselage.

The pilot reaches a certain altitude, where temperatures are ideal, and fires the flares into the cloud. Silver iodide causes individual water droplets within clouds to freeze together, forming snowflakes that eventually become so heavy that they fall.

Without the freezing process, the droplets would not stick together and become large enough to precipitate as rain or snow.

“The cloud initially is all water,” said Bruce Boe, vice president of meteorology for Weather Modification International, a private company that has provided cloud seeding services since 1961. “Eventually, as you approach the summit of the mountain, it may be 50% ice or maybe more than that. But even if it is, there will still be a lot of liquid water left.”

Boe said there is a “window of opportunity” for the precipitation to be large enough to fall “before it reaches the top of the mountain and starts to descend and therefore warm up”.

Pilot Joel Zimmer, who works for Weather Modification International, attaches silver iodide flares to the bottom of a cloud seeding plane.

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For cloud seeding pilots like Joel Zimmer, who works with Weather Modification International to seed clouds for the Turlock Irrigation District, flying into the storm can be an exhilarating but intense experience.

“By the time the wheels are up, you’re on cloud nine,” said Zimmer, whose route involves seeding in the Sierra Nevada Mountains. “And we’re in the clouds the entire mission until we’re filming an approach back to an airport and then we come out of the clouds and we get a view of the runway. It looks like you’re a deputy commander in the Navy. You don’t see anything.”

From a water supply perspective, it is more valuable to seed clouds over mountains, where water is essentially stored as snow until spring runoff.

“When it’s in the lowlands like North Dakota, it’s still a benefit because it helps recharge soil moisture,” Boe said. “But it cannot be stored and used for a later date.”

While Texas uses cloud seeding to help irrigate fields for farmers, it’s more common in the west, where states like Idaho, California, Colorado, Utah and Wyoming use it to help fill their rivers and reservoirs. Most programs use airplanes for cloud seeding, but some use ground flares.

“It’s a lot more common than people think,” Carkeet said. “More basins have a seeding program than do not have a seeding program.”

Costs and impact

Boe says the cost is almost always worth it.

“It makes a lot of sense for water managers to go ahead and do this, even if the increase is on the order of a few percentage points,” he said.

Idaho Power spends about $4 million a year on its cloud seeding program, which produces an 11% or 12% increase in snow cover in some areas, resulting in billions of gallons of additional water at a cost of about $3.50 per acre foot. This compares to about $20 per acre foot for other methods of accessing water, such as through a water supply bank.

And while Turlock only sees a 3% to 5% increase in runoff from his program – which has a maximum budget of $475,000 – California will consume all the extra water it can get.

“It’s one of the things that makes it so difficult to measure is you don’t see a doubling or tripling of precipitation,” Boe said. “You see an incremental increase, but add that up over the winter and it could be significant.”

Watch the video to learn more about what it takes to make it rain.

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