Meeting Date:

January 24, 2008





David A. Berger,




General Manager

Line Item No.:


Prepared By:

Darby Fuerst

Cost Estimate:



General Counsel Reviewed:  N/A

Committee Recommendation:  N/A

CEQA Compliance:  N/A


SUMMARY:  At its November 19 and December 10, 2007 meetings, the Board received presentations regarding the feasibility and potential cost of cloud seeding in the Carmel River Watershed in Water Year (WY) 2008 by District staff and North American Weather Consultants (NAWC), respectively.  In brief, NAWC determined that cloud seeding in the Carmel River Watershed in WY 2008 was technically feasible and would cost approximately $150,000 for a “core” program utilizing two ground-based seeding systems for a four-month period, assuming average storm occurrence and seeding.  Based on their experience in Santa Barbara County, NAWC indicated that seeding could increase rainfall amounts by 20 to 30%, which would equate to approximately three inches of additional rainfall during normal periods.  The estimate includes all costs for set-up, operations, expendables, end-of-season demobilization, and a project report. Shorter duration operational periods would result in lower costs.  Note that this estimate does not include the cost for environmental review, site access, and other permits that may be necessary to implement a cloud seeding project in the Carmel River Watershed this year.  These costs and the time required to obtain all necessary permits may limit the possibility of conducting cloud-seeding operations this year.


At the December 10, 2007 meeting, questions were raised by Board members and the public regarding the scientific basis for cloud seeding.  Specifically, references were made to a report prepared by the National Research Council (NRC) in October 2003, Critical Issues in Weather Modification Research, that suggested the initiation of large-scale operational weather modification would be premature.  A response to this report prepared by the Weather Modification Association (WMA) is included as Exhibit 20-A.  In their response, WMA supported the NRC recommendation that a coordinated national program be developed to conduct a sustained research effort in the areas of cloud and precipitation physics, cloud dynamics, cloud modeling, laboratory studies, and field measurements designed to reduce the key uncertainties that impede progress and understanding of intentional and inadvertent weather modification.  In addition, WMA argued that the coordinated national program should also support exploratory and confirmatory field studies in weather modification and that the program should capitalize on operational cloud-seeding programs.


RECOMMENDATION:  The District Board should determine whether or not to pursue implementation of an operational cloud-seeding program in the Carmel River Watershed in the 2007-08 winter season or in future years.  If the Board decides to pursue implementation of an operational cloud-seeding program, the Board should direct staff to: (1) request a proposal from NAWC to prepare a project description, including potential site locations and an operations plan, and (2) prepare an Initial Study to determine the level of environmental review necessary for construction and operation of the cloud-seeding project.  In addition, if implementation of an operational cloud-seeding program cloud seeding is pursued, the Board should direct staff to contact the Monterey County Water Resources Agency to coordinate cloud-seeding activities.


BACKGROUND:  Cloud seeding, which is also known as weather modification, is the deliberate treatment of certain clouds or cloud systems with the intent of affecting the precipitation processes within those clouds. Information regarding cloud seeding in general is provided at and includes its effectiveness, risks, regulation, and costs.  Basically, in cold cloud seeding, an ice-forming nucleating agent such as silver iodide is introduced into appropriate cloud regions to cause supercooled liquid water droplets to freeze.  Once these droplets freeze, the initial ice embryos grow at the expense of the water droplets around them (sublimation) and through contact with these neighboring droplets (riming).  These droplets, if they remain in favorable cloud conditions, will grow into snowflakes, falling to the surface as snow if surface temperatures are below or near freezing, or as raindrops at warmer surface temperatures.  Information in this report regarding cloud seeding specific to the Central California coast region, and the Carmel River Watershed, is based on cloud seeding programs in Monterey County and Santa Barbara Counties. 


Monterey County: MCWRA initiated a cloud-seeding program in 1990 to reduce the effects of the extended drought that began in 1987.   The program was designed to increase rainfall and subsequent runoff in the Nacimiento, San Antonio, and Arroyo Seco River Watersheds, with the increased runoff from the Nacimiento and San Antonio Rivers captured and stored in Nacimiento and San Antonio Reservoirs, respectively.  The program included ground-based and airborne cloud seeding.  For the 1991-92 winter season, MCWRA estimated a rainfall increase of between 12 and 16% due to cloud seeding. This percent increase equates to between 2.0 and 2.7 inches of additional rainfall and between 17,000 and 22,600 acre-feet of additional runoff into Nacimiento and San Antonio Reservoirs.  The total cost for the 1991-92 program, including agency staff time and consultant support, was $165,070.  MCWRA currently budgets and accrues $75,000 each year for cloud seeding.  Based on current costs, MCWRA estimates that approximately $250,000 per year is needed to fund a five-month, airborne-based, cloud seeding program in the Nacimiento, San Antonio, and Arroyo Seco River Watersheds.


Santa Barbara County: Winter-season, operational cloud-seeding projects in Santa Barbara County began in 1950 to augment supplies for municipal and agricultural uses. During the 1950-1955 period, a ground-based program utilizing silver-iodide generators was implemented, which resulted in an average 16% increase in rainfall. A research program, Santa Barbara I, was conducted during the 1957-1960 period that also indicated an increase in precipitation.  A follow-up study during the 1960-1963 period by NAWC found that most of the precipitation, updrafts, and supercooled liquid water suitable for cloud seeding occurred in relatively narrow traveling “convection bands” embedded within the winter storm systems that affected Santa Barbara County.  A second research program, Santa Barbara II, was conducted during the 1967-1973 period and focused on these convection bands as the primary target for winter cloud seeding programs in the San Ynez River Watershed. This program utilized ground-based (phase I) and airborne-based (phase II) cloud seeding using silver iodide. 


Santa Barbara County, through its Flood Control and Water Agencies, resumed operational seeding projects during the 1977-78 winter season using high-output, ground-based, seeding of convection bands.  After three seasons without any seeding, Santa Barbara County resumed cloud-seeding operations in the 1981-82 winter season and has continued operations to the present, with the exception of the 1985-86 winter season due to concerns about fire-damaged areas in the County.  This operational project has utilized a combination of ground-based and airborne seeding to optimize seeding effects.  The design of the operational cloud-seeding projects in Santa Barbara County have been based primarily upon the design of the Santa Barbara II research program and has focused on seeding convection bands. Operation of the projects has evolved to incorporate changing technologies such as the use of NEXRAD radar data, high output pyrotechnics at the ground sites, and real-time telecommunication.


In 1988, NAWC assessed the potential of augmenting rainfall in Santa Barbara County through seeding of convection bands.  Based on an analysis of the 1920-1980 rainfall record at Juncal and Gibraltar Reservoirs in the San Ynez River Watershed, NAWC concluded that the October-April precipitation could optimally be increased by 21 to 22%.  This percent increase equates to an average of between 4.5 and 5.0 inches of additional rainfall and approximately 113,500 acre-feet of additional runoff into the County reservoirs during the October-April season.  The annual program cost is approximately $300,000, which is shared on a 50:50 basis between Santa Barbara County and the local water purveyors. 


DISCUSSION:  If the assumption that cloud seeding causes a 20% increase in rainfall is correct, then cloud seeding in the Carmel River Watershed during the four-month winter season from December through March, would produce an additional 3.2 inches under average rainfall conditions (16.0 inches) and approximately 2.5 inches under critically-dry conditions (12.4 inches). Further analysis is needed to verify the amount of increased rainfall that could result from cloud seeding under various rainfall conditions, and to estimate the amount of additional runoff in the Carmel River that would be available under these varying conditions due to cloud seeding. 


It should also be noted that, unlike the situations in the Nacimiento and San Antonio River Watersheds in Monterey County and the San Ynez River Watershed in Santa Barbara County where there are relatively large reservoirs that are downstream of the cloud-seeding target areas to capture and store the increased runoff, there are no large reservoirs on the Carmel River that could capture increased runoff from cloud seeding.  Specifically, for the upcoming winter season, there will be a maximum of 70 acre-feet of available storage capacity in San Clemente Reservoir and 1,300 acre-feet of available storage capacity in Los Padres Reservoir.  Given this extremely limited surface water storage capacity, it is almost certain that San Clemente and Los Padres Reservoirs will fill without cloud seeding.  It is not certain, however, if the storage capacity in the Carmel Valley Alluvial Aquifer that underlies the Carmel River will be filled this year, if critically-dry inflow conditions persist.  Under current inflow and demand conditions, it is projected that there will be approximately 7,500 acre-feet of available storage capacity in the Carmel Valley Alluvial Aquifer in December 2007 at the beginning of the winter season.  Cloud seeding, if effective, would increase runoff needed to recharge the Carmel Valley Alluvial Aquifer this year.   In addition, increased rainfall due to cloud seeding could increase daily streamflow in the Carmel River below Los Padres Dam and allow California American Water (CAW) and the District to divert water for injection and storage in the Seaside Groundwater Basin under the cooperative Phase 1 Aquifer Storage and Recovery (ASR) Project. Further analysis is needed to assess the potential benefits to groundwater storage in the Carmel Valley Alluvial Aquifer and Seaside Groundwater Basin from cloud seeding in the 2007-08 winter season.


IMPACT TO STAFF/RESOURCES:  Presently, there are no funds in the District’s Fiscal Year (FY) 2007-08 budget for cloud seeding operations.  Given the critically-dry inflow conditions that occurred in Water Year (WY) 2006-07 and the possibility that these conditions could persist in WY 2007-08, there are available funds in the District’s Flood/Drought Reserve that could be used for cloud seeding in the 2007-08 winter season.  In addition, no staff time was allocated in the FY 2007-08 budget to analyze or implement a cloud seeding program in the 2007-08 winter season.  If the Board decides to pursue cloud seeding this year, Water Resources Division staff time will need to be allocated to the project, which would impact other planned activities. 



20-A    Weather Modification Association’s Response to the National Research Council’s Report titled “Critical Issues in Weather Modification Research”, dated March 18, 2004.