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<br />032488 <br /> <br />the augmentation level. However, MBA said that the year-to-year <br />variability, plus the variability of the percentage increase as a <br />function of the year's total precipitation, makes these averages <br />somewhat misleading. For example, the proportional gain from <br />augmenting the snowpack is significantly greater in dryer years. <br /> <br />Another consideration in the streamflow analysis was that a substantial <br />portion of the Sierra Nevada streamflow is composed of immediate <br />runoff. Because of the generally moderate cl imatic conditions, <br />the precipitation below 1830 m (6000 ft) is fed into the streamflow <br />within a few days; unlike the Rocky Mountains where most precipita- <br />tion falls as snow and remains in the snowpack until spring melt. <br />Again, however, a significant factor is the variability; in this case, <br />the variability in the elevation of the snowline from storm to storm. <br />Further, the maximum precipitation accumulation occurs very close <br />to the average snowline elevation. Thus, larger increases in the <br />streamflow, during the May to October period, than were indicated by <br />these analyses could be expected from only a moderate redistribution <br />of this maximum to a higher elevation. <br /> <br />Seedability Climatology <br /> <br />The climatologies of the basins are critical to the design of seeding <br />operations. The American River Basin is an essentially windward <br />slope, characterized by some convective activity imbedded in the <br />general orographic cloud structure. Conversely, a large part of <br />the Feather River is a carryover basin with most precipitation <br />caused by orographic clouds. Between the two, the Yuba River <br />is in a transition zone, being primarily a windward-type basin, with <br />some portions of the catchment being carryover. <br /> <br />, <br />~ <br />, <br />! <br />I <br />I <br /> <br />MBA's seedability analysis was based to a large extent on CENSARE <br />data and thus is generally basin dependent. Extensions of that <br />data to basins of dissimilar characteristics through physical <br />arguments and interpretation of operational program results were <br />included in the MBA report. These arguments, combined with precipi- <br />tation differences between orographically dissimilar basins, strongly <br />suggest that different physical mechanisms, if not in kind, at least <br />in degree, dominate. Since MBA's recommendation was to conduct the <br />SCPP in the American River Basin, seedability will be discussed for <br />that region only. <br /> <br />MBA found that two different storm types account for most (over <br />90 percent) of the precipitation in the study area. These are the <br />"southerly" storms which tend to be warm and the "westerly" storms <br />which tend to be cold (where "warm" and "cold" are used in a relative <br />sense). There are a number of storms which are difficult to cate- <br />gorize by the directional extremes, and these storms will have a <br /> <br />13 <br />