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and Streamflow Snowpack, Runoff, THE Upper Colorado River Basin covers about 109,500 square miles, but only some of the area is suitable for WOSA. In gen- eral, suitable areas are those with heavy natural snowfall. We have seen that the effectiveness of WOSA can be expressed as a constant proportion of the natural snowfall-about 20-25 % . The natural snowfall increases with elevation. Areas Suitable for WOSA As a practical matter the areas where WOSA is effective are the elevations above 9,000 feet. In an average winter, precipitation will exceed 10-15 inches water equivalent at 9,000 feet or above, and WOSA would add at least 2 inches water equivalent at or above that level. The most suitable areas for WOSA are the mountain ranges that form the western and eastern bound- aries of the Upper Basin. The western ranges are generally lower, less rugged, and not as extensive as the eastern ranges that form the Continental Divide. These mountain boundaries outline the river basin watershed, where water from snowmelt or rain flows through a system of tributary streams to one cen- tral river (see Figure 5) . The mountain ranges that outline the region are not con- tinuous. They are oriented in different directions and they have (jifferent elevations and other surface features. They are spread out over such a great distance that each can experience unique weather conditions as a storm moves through the region. The 21 SNOWPACK, CLOUD-SEEDING, AND THE COLORADO RIVER the storms pass over the high mountains on their eastward journey. Once the air is lifted to a high elevation and has lost considerable moisture, it will not produce further precipitation by the same processes unless it is again lifted to the same eleva- tion or higher. The Colorado Rockies are among the highest ranges encountered by the air in its eastward passage. After passing over the Rockies, the air descends over the plains and dries out as it warms up. It is highly unlikely that this dried air could produce precipitation over the plains, whether seeding had occurred over the mountains or not. The primary moisture source for winter precipitation over the prairie states is the rela- tively warm air that flows in from the Gulf of Mexico under favorable conditions. Because of the different natural sources of moisture for snow over the Colorado River Basin and over the prairies, cloud-seeding over the mountains will not measurably reduce the snowfall over the plains. Weather modification investigators have found that silver iodide and the ice crystals that it produces can travel several hundred miles downwind on the air currents. These materials could seed storms that happened to be over the plains, and cause these storms to drop much more precipitation than they would under natural conditions. If this happened to a storm over Denver, the resulting heavy snowfall would cause serious and widespread inconvenience. Undesirable downwind cloud-seed- ing could be minimized if the silver iodide generators were turned off when favorable conditions in the mountains ended. During most of the time that seeding is effective over the moun- tains, conditions over the plains are unfavorable for seeding, with good weather conditions there. , Mountainous areas that are up to 150 miles downwind from a target area will also be affected by seeding. For example, seeding in the San Juan Mountains during southwest winds probably affects the snowfall over the Sawatch range. With west- southwest winds, seeding the San Juans will produce additional snow over the Sangre de Cristo Mountains (see Figure 4). ~: , ~: , 20 i \; .\'.\(1\' _Ih . " "'i {.i,! I I !