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I lCLOUD AND PRECIPITATION proc- esses are well enough understood that under controlled, responsible programs, eatastrophic effects from seeding simply Bhould not occur. No clouds are seeded when the effects might cause excessive rain, snowfall, floods, hailstorms, or other Bevere environmental conditions. If a natural threat materializes, seeding activi- ties can be halted immediately. Even if a forecast is wrong and fails to identify a potential threat, the probability of severe weather resulting from seeding is very low. ][i'loods resulting from spring snowmelt are related to massive accumulations of snow late in the season. There appears to be an increased probability of more than minor flooding when the water content of the seasonal snowpack approaches 200 percent of normal. Seeding, however, is strictly governed by criteria designed to suspend activities before they could con- tribute to a potential flood threat. For :instance, in the Colorado River Basin Pilot Project, seeding is suspended if the snow- 1.00 'U ,,,, ~ E ,5 .8) .~ ~ o Ii: (5 .60 c: ':E Ii 1= AD / APRIL I. / / // /MARCH I. / / / / / / ,., '5 ~ 0 '" ~ :;: o 20 ....FEBRUARY I /' / / -" 100 125 175 200 225 250 150 Snow Water Equivalent in Percent of Average Typical prob~bilityof more than minor flood dam- age for tributary streams to the San Juan during snow melt. 7 pack on February 1 is 200 percent of the long-term normal. This ceiling drops to 175 percent on March 1 and to 150 per- cent on April 1. One of the most persistent, popular con- cerns about precipitation manageIpent has been the notion that increases in precipi- tation in one area will necessarily result in compensating decreases in regions downwind. This is contradicted by both theory and experience. A study of several recent winter cloud-seeding programs con- ducted in the United States has shown that areas of substantial increases are common in downwind regions. An exten- sive field study in California found that precipitation increases extended up to 100 miles downwind from a seeding site, with only a few isolated pockets of slight de- crease. Tentative hypotheses to explain these increases are being developed and examined. Once the cause is better under- stood, more efficient methods for increas- ing precipitation over large areas may be revealed. J ,~ I .,~ Extensive studies, to date, indicate no catastrophic or irreversible effects on the ecology from precipitation increases in the amounts expected from cloud seeding. These studies are continuing, particularly in regard to long-range or cumulative ef- fects of additional precipitation and the use of various seeding materials. Experiments in the field and laboratory indicate the seeding materials are harm- less in the concentrations used in cloud s'eeding, and the amounts involved pose no pollution problems. Asan example, melted snow from seeded storms contains about 1 ounce of silver iodide in each 500 million gallons of melted moisture. Silver iodide, in turn, is the least soluble salt known. Most of the real risks in precipitation man- agement can be and are ~inimized in the planning stages of a responsible program. Therefore the operation of a well-conceived project would not create ecological surprises.