California Water Plan Update 2005
Most projects are along the central and southern Sierra Nevada with some in the Coast Ranges. (image right of southern Sierra Nevada from NASA earth observatory website)
Precipitation enhancement, commonly called “cloud seeding,” artificially stimulates clouds to produce more rainfall or snowfall than they would naturally.
Cloud seeding injects special substances into the clouds that enable snowflakes and raindrops to form more easily.
Precipitation enhancement is the one form of weather modification done in California; hail suppression (reducing the formation of large, damaging hailstones) and fog dispersal (when fog is below freezing temperature) projects are conducted in other states.
Benefits from Precipitation Enhancement
In California, all precipitation enhancement projects are intended to increase water supply or hydroelectric power.
The amounts of water produced are difficult to determine, but estimates range from a 2 to 15 percent increase in annual precipitation or runoff.
A National Research Council (NRC) report on weather modification has limited material on winter orographic cloud seeding, such as practiced in California and other western states.
However, the report does seem to concur that there is considerable evidence that weather modification does work, possibly up to a 10 percent increase.
Many of the best prospects are in the Sacramento River basin, (image left) in watersheds that are not seeded now.
With the exception of the upper Trinity River watershed, and perhaps the Russian River, there is little new potential in the North Coast region because not much extra rainfall could be captured due to limited storage capacity.
There is also potential to increase water production by more effective seeding operations in existing projects.
Cloud seeding opportunities are generally fewer in dry years.
It works better in combination with surface or groundwater storage to increase average supplies.
In the very wet years, when sponsors already have enough water, cloud seeding operations are usually suspended.
Costs for cloud seeding generally would be less than $20 per acre-foot per year.
State law says that water gained from cloud seeding is treated the same as natural supply in regard to water rights.
It is estimated that about $3 million is being spent on operations.
Realizing the additional 300,000 to 400,000 acre-feet of potential new supply could require about $7 million, which would be about $19 per acre-foot.
An initial investment of an estimated $1.5 million to $2 million in planning and environmental studies would also be required.
Over the next 25 years, precipitation enhancement costs are expected to total about $177 million
Major Issues for Precipitation Enhancement
No complete and rigorous comprehensive study has been
made of all California precipitation enhancement projects.
Part of the reason is the difficulty in locating unaffected control basins for the standard target and nearby control area comparisons, since wind variations would cause spillover into adjoining basins.
Some studies of individual projects have been made in the past years on certain projects, such as the Kings River, which have shown increases in water.
It is difficult to target seeding materials to the right place in the clouds at the right time.
There is an incomplete understanding of how effective operators are in their targeting practices.
Chemical tracer experiments have provided support for targeting practices.
Concern over Potential Impacts
Questions about potential unintended impacts from precipitation enhancement have been raised and addressed over the years.
Common concerns relate to downwind effects (enhancing precipitation in one area at the expense of those downwind), long term toxic effects of silver, and added snow removal costs in mountain counties.
The U.S. Bureau of Reclamation did extensive studies on these issues.
The findings are reported in its Project Skywater programmatic environmental statement in 1977, and in its Sierra Cooperative Pilot Project EIS in 1981.
The available evidence does not show that seeding clouds with silver iodide causes a decrease in downwind precipitation; in fact, at times some of the increase of the target area may extend up to 100 miles downwind.
The potential for eventual toxic effects of silver has not been shown to be a problem. Silver and silver compounds have a rather low order of toxicity.
According to the Bureau of Reclamation, the small
amounts used in cloud seeding do not compare to industry emissions of 100 times as much into the atmosphere in many parts of the country or individual exposure from tooth fillings.
Watershed concentrations would be extremely low because only small amounts of seeding agent are used.
Accumulations in the soil, vegetation and surface runoff have not been large enough to measure above natural background.
A 2004 study done for Snowy Hydro Limited in Australia has confirmed the earlier findings cited above. In regard to snow removal, little direct relationship to increased costs was found for small incremental changes in storm size because the amount of equipment and manpower to maintain the roadway is essentially unchanged.
That is, the effort is practically the same to clear a road of 5.5 inches compared to 5 inches.