Cloud seeding, no longer magical thinking, is poised for use this winter
11/11/2013 12:00 AM
10/06/2014 8:47 PM
As California concludes a second drought year and water managers hope eagerly to avoid a third, utilities across the state are poised for that first mass of pillowy gray clouds to drift ashore from the Pacific Ocean.
When it arrives, if conditions are right, they’ll be ready with cloud-seeding tools to squeeze out every extra snowflake, with the goal of boosting the snowpack that ultimately feeds the state’s water-storage reservoirs.
Once viewed by some as a fringe science, cloud seeding has entered the mainstream as a tool to pad the state’s crucial mountain snowpack. New technology to manage the practice, and research that points to reliable results, have cemented cloud seeding as a dependable and affordable water-supply practice.
“The message is starting to sink in that this is a cost-effective tool,” said Jeff Tilley, director of weather modification at the Desert Research Institute in Reno, which practices cloud seeding in the Lake Tahoe Basin and eastern Sierra Nevada. “The technology is better; we understand how to do cloud seeding much better. And because we know how to do it more effectively, it’s definitely taken more seriously.”
The Modesto and Turlock irrigation districts have seeded clouds over their Tuolumne River watershed, but they will skip it this winter because of the Rim fire, TID spokeswoman Michelle Reimers said. Parts of the 257,314-acre burn area have a moderate to severe risk of soil erosion into streams and reservoirs during major storms, federal officials said.
Cloud seeding is often misunderstood as a kind of magic that conjures rain from thin air. In reality, it is simple chemistry combined with careful weather monitoring.
As practiced in California and elsewhere in the West, cloud seeding involves spraying fine particles of silver iodide into a cloud system to increase snowfall that is already underway or about to begin. Silver iodide causes water droplets within the clouds to form ice crystals. As the crystals grow larger, they become snowflakes, which fall out to create more snow than the storm would have generated on its own.
Cloud seeding is done only when temperatures within the clouds are between 19 and minus-4 degrees Fahrenheit. This is the range at which silver iodide does its best work, as demonstrated by decades of research.
“It enhances precipitation that’s already occurring,” said Dudley McFadden, a civil engineer at the Sacramento Municipal Utility District who manages the utility’s cloud-seeding program. “Once you’ve got snow, you can make more with this approach.”
California has been seeding clouds for at least six decades. The first couple of decades were experimental, but since then it has been routine practice for a number of water agencies and hydroelectric utilities. In any given winter, there are cloud-seeding projects underway in 15 California watersheds, from Lake Almanor in the north to the San Gabriel River in the south. Most were up and running for the season as of last week, ready to take advantage of the first cold storms.
In a report this year, the California Department of Water Resources estimated these projects generate 400,000 acre-feet of additional water supply annually. That’s about equal to half the volume of Folsom Reservoir. They did so at an estimated cost of $2.27 per acre-foot, which is cheaper than almost any other water-supply option, including conservation projects.
SMUD has seeded clouds over its hydroelectric reservoirs in the upper American River watershed since the late 1960s. It does so exclusively to produce more water to generate electricity from its network of dams on the middle and south forks of the American River, McFadden said.
But that extra water benefits others, including the city of Sacramento and other water users downstream. Under the California Weather Resources Management Act of 1984, any water generated by cloud seeding must be considered “natural precipitation” when it comes to water rights. In other words, the water it produces does not belong to the entity that paid for the cloud seeding.
“It all flows downhill, as they say, and everybody benefits,” said Ed McCarthy, supervising meteorologist for Pacific Gas and Electric Co., which has been cloud seeding over the Sierra Nevada since the 1950s.
The silver iodide also flows downhill, and it stays in the environment. But those who practice cloud seeding say there’s no reason to fear the tiny particles, which are considered inert and not easily absorbed by other organisms. This differs from plain silver, which is released into the environment by mining activity and industrial and automotive emissions.
No California agencies require special permits, monitoring or testing before or after releasing silver iodide into the environment. The 1984 law requires only biannual reporting to the Department of Water Resources on the amount, method and location of silver iodide dispersal.
Tilley said he is “extremely confident” that silver iodide has no effect on water quality.
“You can go to any street in Sacramento and you can probably find a lot higher concentration of any type of silver compound than anything we would do,” he said.
The hunt for moisture
SMUD and PG&E approach cloud seeding differently. PG&E uses ground-based equipment to release silver iodide near Lake Almanor, in the Mokelumne River watershed, and in partnership with the Kings River Irrigation District. Silver iodide in a liquid solution is sprayed into a propane burner, which dissolves the silver iodide and shoots it upward into passing clouds.
All the equipment sits on a trailer that is rolled into position each winter and controlled remotely. The advancement of cellphone networks over the past decade means the equipment requires little tending in the field, which has helped slash costs. The Desert Research Institute pioneered these methods and uses similar equipment on trailers in the Tahoe Basin.
SMUD did the same in the Upper American River watershed until 2008, when it switched to cloud seeding by aircraft. It hires Weather Modification Inc., a contractor based on Fargo, N.D., to manage the program. This company also does the work for TID and MID.
The contractor stations a Cessna 340 twin-turboprop aircraft and two pilots at McClellan Airport in Sacramento for SMUD’s exclusive use during winter. Silver iodide is released by special flares – much like the roadside emergency flares many motorists carry in the trunk – attached to the trailing edge of the plane’s wings and set afire by a switch in the cockpit.
SMUD spent $137,360 on its contract with Weather Modification Inc. last year, McFadden said.
“It’s not that expensive when you consider the value of hydropower,” McFadden said. “Every additional megawatt we can generate from hydropower is one less megawatt we need to purchase. That’s where the cost-benefit pencils out very clearly.”
Weather Modification Inc. also provides the meteorological expertise to decide when to seed clouds. It can be dangerous, because the work calls for flying into storm clouds in conditions that cause the wings to ice over. The planes are equipped with de-icing systems but are often overwhelmed anyway.
“Probably 30 percent of the flights, they have to divert back towards the Valley and drop down to melt off the ice before they can go back, because they get more than the airplane can handle,” said Hans Ahlness, vice president of operations at Weather Modification Inc.
Drones on the horizon
In the early days, critics feared cloud seeding would suck all the moisture from a storm and rob downwind areas of snowfall. But research in recent decades has found no evidence for such claims. In fact, the opposite is often true: Cloud seeding can have a beneficial effect as much as 150 miles downwind from the seeding location.
This was demonstrated in a 2010 study by Bernard Silverman, a consulting meteorologist based in Colorado, published in the peer-reviewed journal Atmospheric Research. Silverman reviewed 11 Sierra Nevada cloud-seeding programs and found that six were successful in boosting streamflow. In the remaining five, the evidence was not conclusive whether they were successful.
One of those five was PG&E’s ground-based seeding program in the Mokelumne River watershed. Silverman found that the West Walker River watershed – on the other side of the Sierra Nevada crest – scored some additional snowpack thanks to silver iodide “most likely” transported on the wind from the Mokelumne watershed.
But how much extra water does cloud seeding generate, really? That is a subject of debate, and difficult to determine scientifically. The Desert Research Institute has studied the question for decades and determined that cloud seeding can produce an increase in snowpack from 8 percent to 15 percent, usually for a cost between $10 and $13 per acre-foot. In its report this year, the California Department of Water Resources estimated an average snowpack increase of 4 percent.
McFadden said SMUD needs only a 1.5 percent boost to make cloud seeding worthwhile, when compared to the cost to purchase electricity from any other source. But, McFadden conceded, “There’s no way to exactly, definitely measure how much more snowfall we get. That’s the pickle we’re put in.”
A coalition of agencies, led by the National Center for Atmospheric Research, is attempting to pin down that answer in a multiyear experiment in Wyoming. The experiment, expected to wrap up in 2014, aims to precisely measure the snowfall benefits of cloud seeding compared with unaided conditions. The results may lead to a broad new cloud-seeding project in the massive Colorado River watershed, which could ultimately benefit Southern California at the downstream end.
The next frontier in cloud seeding may involve the use of drone aircraft. The Desert Research Institute recently signed an agreement with a contractor to investigate dispersing silver iodide from drones. This could be cheaper than using manned aircraft, creating opportunities to attack reluctant clouds from multiple levels, with the hope of squeezing out even more moisture.
“Almost every year we come up with some new, small innovation to make the system better or more efficient,” Tilley said.
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