With the winter season fast approaching, many are looking skyward in the hope that this year rewards the state with more rain and snowfall than last.
According to the Department of Water Resources (DWR), last year ranked as the 25th driest year in terms of statewide runoff, based on a measured record of 112 years.
One agency giving nature a little push in the right direction is the Sacramento Metropolitan Utility District (SMUD), which annually seeds storm clouds with silver iodide to increase the yield of rain and snow.
SMUD, like a number of other water or power agencies in the state like PG&E, the Northern California Power Agency and Southern California Edison, operates its own weather modification program.
Carried out in the state since the early 1950s, cloud seeding has primarily been done along the central and southern Sierra Nevada, with a few projects in the Coastal Range.
SMUD began its own weather modification program in 1969, but its cloud seeding was initially done with ground equipment.
In 2008, the utility switched to aerial seeding, according to Dudley McFadden, Principal Civil Engineer for Power Generation with SMUD.
McFadden said SMUD will begin cloud seeding again this year around Nov. 15, depending upon when the first storm hits. “We will continue it until March 15 or into the next month if the snowpack continues to be below average and if the extra cost can be justified,” he said.
SMUD doesn’t actually do the seeding itself. Instead it contracts with a company called Weather Modification Inc. which has planes located at McClellan Airfield. A meteorologist with the company determines if the conditions are right before a plane is sent up.
“We seed clouds only during cold winter storms,” said McFadden, saying that once in the air, the pilots wait until there are icing conditions on the wing. “When that happens, we know the atmosphere has a surplus of liquid water waiting to turn into snow.”
The engineer said silver iodide acts as sites for snowflakes to form, creating particles smaller than aerosols. “The atmosphere is usually very clean at the elevations where they cloud seed,” he added. “In order to create precipitation, you need to create little dust particles or aerosols. That’s where the snowflake initially forms. And it has to be under conditions where it’s cold and there’s enough moisture in the air … water that’s below 32 degrees but hasn’t frozen yet. Those are conditions that billions of little particles dispersed into the air can cause snowflakes to form.”
McFadden said the silver iodide is dispersed from 30 flares mounted on the bottom of each wing as well as ejected from other devices aboard the plane. “But we’re only talking in the order of grams,” he added, saying that during the first storm of last year they released maybe a couple hundred grams of silver iodide with a total of 2,000 grams released during the entire year.
Two thousand grams is the equivalent of less than four and a half pounds of material.
“We can send flares into the air or down depending on conditions,” he said. “They ignite when launched and burn for about 80 seconds. We seed for 20 minutes or so once in position and do it until we either run out of fuel or out of flares.”
McFadden said they limit seeding to the west slope of El Dorado County. “The flight path is right over the watershed,” he noted. “We work hard to get the generated snow to actually fall into watershed and flow into Silver Creek and then down the South Fork of the American River.”
The cost benefit of seeding
Questions have been raised regarding whether cloud seeding actually works with the answer dependent on what study is referenced.
The most recently updated California Water Plan states it is difficult to determine the amount of water produced by cloud seeding. But it also noted a recent estimate by DWR staff that when taken together, California precipitation enhancement projects generate, on average, a four percent increase in runoff.
On the other hand, McFadden admits “there’s no direct evidence that it (cloud seeding) makes any difference. Many scientists have studied what happens when particulates like silver iodide are released under a variety of conditions and note the generation of snow and ice. In other experiments, one plane releases materials and another plane behind it measures the creation of ice. There’s lots of strong evidence that links efficacy to this method. More recent research is where they have one protected watershed where they don’t seed and another where they do, and they have come up with slight increase in precipitation. But we never have high enough evidence to conclusively determine using statistical analysis that it is significantly more.
“We believe it works, but I haven’t seen any direct proof myself. Talking to operators, it works in the laboratory, in the field and in controlled situations. And so based on the value of water to generate hydroelectricity, it is a positive cost benefit for us. The National Weather Service did one study that it increased precipitation up to 9 percent. I think that was optimistic. But even if we only increase precipitation by 2 percent, the program pays for itself.”
McFadden said the cost of cloud seeding varies from year to year. Last winter SMUD spent $137,360. The prior season it spent $166,290. Statewide, all weather modification programs cost $3 to $5 million annually.
SMUD’s program doesn’t get any financial help from the El Dorado Irrigation District, nor does EID have any kind of weather modification program of its own. But McFadden said that’s fine because SMUD is the primary beneficiary of its cloud seeding program.
The engineer added that at times concerns have been raised about the impact of cloud seeding with silver iodide.
“Caltrans worried about snow plowing expenses rising and conservation groups were concerned about the habitat of deer being affected by a larger snowpack. But there is no worry about the chemical impact,” he said. “It’s an inert chemical and the amount is so small it is below the detection limit. We would never release anything into the atmosphere unless there’s an active storm going on. And the amounts are so miniscule and actually are an inert salt product so they are not toxic even in concentrated forms.”
Asked if SMUD takes any other measures to promote more rain and snow, McFadden laughed and said no — other than when he and others in the industry get together and do a little rain dance.
Contact Dawn Hodson at 530-344-5071 or [email protected] Follow @DHodsonMtDemo on Twitter.