Current Rain Unrelated to Annual Cloud Seeding program
Precipitation enhancement, commonly called “cloud seeding,” artificially stimulates clouds to produce more rainfall or snow fall 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.
Precipitation Enhancement in California
The first serious cloud seeding program in California began in 1948 on Bishop Creek in the Owens River basin for Califor nia Electric Power Co. Precipitation enhancement in the form of cloud seeding has been practiced continuously in several California river basins since the early 1950s. Most projects are along the central and southern Sierra Nevada with some in the coast ranges. The projects use silver iodide as the active cloudseeding agent, supplemented by dry ice if aerial seeding is done. The silver iodide can be applied from ground genera tors or from airplanes. Occasionally other agents, such as liquid propane, have been used. In recent years, some projects have also been applying hygroscopic materials (substances that take up water from the air) as supplemental seeding agents. Figure 141 shows rain and snow enhancement programs for the 2002-2003 season.
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.
Operators engaged in cloud seeding have found it beneficial to seed rain bands along the coast and orographic clouds over the mountains. The number of operating projects has tended to increase during droughts, up to 20 in 1991, but have leveled off to about 12 or 13 in recent years. The total area covered by these projects is about 13,000 square miles.
Policy statements by both the American Meteorological Soci ety and the World Meteorological Organization support the effectiveness of winter orographic cloud seeding projects. The American Society of Civil Engineers has also shown interest with its Policy Statement No. 275 on Atmospheric Water Manage ment in 2003 and a new report, ASCE/EWRI 4204, “Standard Practice for the Design and Operation of Precipitation Enhance ment Projects” in May 2004. This standards document will be a sequel to ASCE Manual No. 81, “Guidelines for Cloud Seeding to Augment Precipitation,” published in 1995.
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 esti mates range from a 2 to 15 percent increase in annual precipi tation or runoff. A National Research Council (NRC) report on weather modification (Box141) 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. A detailed study by the Utah Department of Natural Resources in 2000 showed an average increase in April 1 snowpack water content ranging from 7 to 20 percent from a group of projects which had been operating from 9 to 22 years. The overall estimated annual runoff increase was about 250,000 acrefeet, or 13 percent for the study area. Actual increases in annual runoff are probably significantly less in California than in Utah. One conservative estimate is that the combined California precipita tion enhancement projects generate 300,000 to 400,000 acre feet annually, which would be an average of about a 4 percent increase in runoff.
Another 300,000 to 400,000 acrefeet per year may be avail able. Many of the best prospects are in the Sacramento River basin, in watersheds that are not seeded now. The Lahontan regions are already well covered by cloud seeding projects, except for the Susan River. 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.
Precipitation enhancement should not be viewed as a remedy for drought. 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 acrefoot 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 opera tions. Realizing the additional 300,000 to 400,000 acrefeet of potential new supply could require about $7 million, which would be about $19 per acrefoot. An initial investment of an estimated $1.5 million to $2 million in planning and envi ronmental studies would also be required. Over the next 25 years, precipitation enhancement costs are expected to total about $177 million.
2 Major Issues for Precipitation Enhancement Reliable Data
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 con trol 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 tar geting practices.
Concern over Potential Impacts
Questions about potential unintended impacts from precipi ta tion 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 (Ref. 1981 SCPP EIS). 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.
All operating projects have suspension criteria designed to stop cloud seeding any time there is flood threat. Moreover, the type of storms that produce large floods are naturally quite efficient in processing moisture into rain anyway. In such conditions, seeding is unlikely to make a difference.
Utility Seeding Operations
Four of the existing cloud seeding projects in California are sponsored by hydroelectric utilities. These four projects prob ably account for about a third of the estimated statewide water production by cloud seeding. There is some concern that if these power plants are sold, either as part of deregulation or for other reasons, new owners may not be interested in continuing cloud seeding. This would result in some loss in water supply for downstream users who have been indirectly benefiting from the added water. The State Public Utilities Commission is aware of this possibility and has tried to ensure, as a condition of transfer, that weather modification would continue.
Little federal research funding for weather modification has been available in the past 15 years. The Bureau of Reclamation had some funding in 2002 and 2003 in the Weather Damage Mitigation program. Desert Research Institute of Nevada did obtain a grant of $318,000 from this source early in 2003 to evaluate its seeding in the eastern Sierra.
Inadvertent Weather Modification
There is evidence that human activities such as biomass burn ing, transportation, and agricultural and industrial activities modify local and sometimes regional weather. The effects of aerosols on clouds and precipitation are complex. Recent studies by Ramanathan and Rosenfeld suggest suppressed precipitation formation in affected clouds due to pollution and dust. Some aerosols can enhance precipitation and some, especially the very fine aerosols in diesel smoke, can reduce precipitation. Much more research is needed to evaluate the air pollution effects on precipitation processes and the amount of impact as well as possible effects on cloud seeding programs. It is pos sible that some of the California cloud seeding projects have offset a potential loss in precipitation from air pollution, which may have obscured a more positive signal from the weather modification projects.
Recommendations to Increase
1. The State should support the continuation of current proj ects as well as the development of new projects and help in seeking research funds for both old and new projects.
2. DWR should collect base data and project sponsor evaluations of existing California and other western states precipitation enhancement projects, independently analyze them, and perform research on the effectiveness of this technology to supplement water supplies while minimizing negative impacts.
3. DWR should investigate the potential to augment Colorado River supply by cloud seeding, in cooperation with the Colorado River Board, the other Colorado River Basin States, and the U.S. Bureau of Reclamation.
4. DWR should support research on cloud physics and cloud modeling being done by the National Oceanic and Atmospheric Administration labs and academic institutions. With improvement, these models may become tools to further verify and test the effectiveness of cloud seeding activities.
5 . DWR should support efforts by California weather mod ification project sponsors, such as that proposed in 2002 03 by Santa Barbara County Water Agency, to obtain federal research funds for local research experiments built upon their operating cloud seeding projects.
ASCE Manual No. 81 “Guidelines for Cloud Seeding to Augment Precipitation”, 1995
ASCE Policy Statement No. 275, “Atmospheric Water Resources Management”, 2003
ASCE/EWRI 4204 “Standard Practice for the Design and Operation of Precipitation Enhancement Projects”, 2004.
National Oceanic and Atmospheric Administration
Desert Research Institute, Reno, Nevada
American Meteorological Society
World Meteorological Organization
USBR Project Skywater publications, various, 19751987, including those of the Sierra Cooperative Pilot Project in California.
A. Givati and D. Rosenfeld, “Quantifying Precipitation Suppression Due to Air Pollution”. 2004, Journal of Applied Meteorology Vol 43, pp 10381056.
Sierra Cooperative Pilot Project, Environmental Assessment and Finding of No Significant Impact, USBR, Denver, 1981.
NRC report “Critical Issues in Weather Modification Research”, The National Academies Press, Washington, DC, 2003.
V. Ramanathan, P.J. Crutzen, J. T. Kiehl and D. Rosenfeld, “Aerosols, Climate, and the Hydrologic Cycle”, 2001, Science magazine, Dec. 7, 2001. wwwramanathan.ucsd.edu/abc.html
V. Ramanathan and M. V. Ramana, “Atmospheric Brown Clouds, Long Range Transport and Climate Impacts”, EM, Dec. 2003, pp 2833. wwwc4.ucsd.edu
UNEP and Center for Clouds, Chemistry and Climate, 2002, “The Asian Brown Cloud: Climate and Other Environmental Impacts”, UNEP, Nairobi.
The Weather Modification Association’s Response to the NRC Report “Critical Issues in Weather Modification Research”, report of a review panel, pp 5382, Journal of Weather Modification, April, 2004.
North American Interstate Weather Modification Council Response to the NRC Report, April 2004, 2 pp, on www.naiwmc.org
Snowy Hydro Limited, Cooma, NSW, Australia, “Assessment of the Environmental Toxicity of Silver Iodide and Indium Iodide”, by Dr. Brian Williams, Adelaide University, 2004.