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IN THE UPPER COLORADO RIVER BASIN Snowfall at elevations above 9,000 feet can be increased ap- proximately 20-25 % by seeding. This percentage increase is the basin-wide average. It would vary from about 15 % in the drier northern-most part of the region to 25-30% in the wetter south. At elevations below 8,000 feet, the increase in snowfall would amount to less than one inch of water equivalent. The increases are produced mainly by changing hours in which little snowfall would have occurred naturally into hours of significant snowfall. Cloud-seeding makes it snow longer, not harder. r> I'..' ,. " i I!I' :1 :' II' ~ih.I.. 1 II The 20-25 % increase in snowfall produced by seeding will be the same whether the season is wet or dry. We can't turn a dry year into a wet one because we can't raise that 20-25 % . Cloud-seeding will give a seeding target area a snowfall similar to the natural snowfall at an elevation 500 to 1000 feet higher. With cloud-seeding, the target area of Silverton, Colorado at an altitude of 9,300 feet would experience a snowfall as heavy as a natural snowfall at around 10,000 feet. WOSA snowfall can be expected to vary from year to year and from one part of the region to another in exactly the same manner as natural snowfall. There may be reasons to limit the additional snowfall in a town or a drainage basin that would suffer undesirable consequences from too much snow. In that case, it might be desirable to limit the seasonal accumulation of snow in above-normal years to avoid spring flooding. However, weather patterns and stormi- ness can be predicted for only a few days in advance. If long- range predictions were possible, so that one could know in advance that a winter would have natural heavy snows, seeding operations could be shut down entirely. If little snow were ex- pected,all possible storms could be seeded. Since dependable long-range predictions are not available, the season to date is the main criterion. If snowfall were about average until late in --. 17 WOSA Limiting Additional Snowfall The Snowfall Expected from WOSA The research work that has been done and the experiments that have been carried out in the Upper Colorado River Basin tell us how much precipitation we can expect from different cloud conditions with properly conducted cloud-seeding. By applying this to the cloud conditions that occur during different parts of the winter, we can determine the increase in snowfall that can be expected. This kind of analysis provides the following results SNOWPACK, CLOUD-SEEDING, AND THE COLORADO RIVER These storm systems that move across the region bring both natural precipitation and the conditions that provide op- portunities for cloud-seeding. The amount of winter season- October through April-snowfall depends greatly upon the ele- vation. The average seasonal amounts vary from 5 inches of water equivalent or less in the river valleys to 30 inches of water equivalent over the highest mountain peaks. Ten to 15 inches of newly fallen snow is equivalent to one inch of water. The winter season snowfall varies greatly from year to year. Over a long period, one-third of the years can be expected to show a snowfall higher by at least a third or lower by at least a third than the long-term average. The snowfall varies greatly from month to month during a season as well as from year to year. Heavy snowfalls frequently occur in March, near the end of the season. In general over the area, a few days of heavy snowfall ac- count for a large portion of the seasonal total. Five out of every hundred snowfall days account for about 25 % of the total, \vhile fifteen out of every hundred account for 50%. On the average, about one out of five winter days in the south (about 40 days every winter) and one out of seven in the northern portion of the region (about 30 days every winter) will be suitable for seeding. Suitab!e seeding conditions will last typically for about 4 to 6 hours. I:' 'f 'I I I I 16 _L .