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<br />60 <br /> <br />1 40 <br /> <br />Z 50 <br /> <br />~ <br />u <br /> <br />120 <br /> <br />80 <br /> <br />Z <br />o <br />f- <br /><[ <br />f- <br /> <br />a.. <br />u <br />w <br />0:: <br />a.. <br /> <br />Z <br />o 40 <br />f- <br /><[ <br />f- <br />a.. 30 <br />u <br />w <br />0:: <br />a.. 20 <br /> <br />l? <br />Z <br /> <br />0:: <br />a.. <br />(f) <br /> <br />100 <br /> <br />60 <br /> <br />40 <br /> <br />l? <br />Z <br /> <br />0:: <br />a.. <br />(/) <br /> <br /> <br />20 <br /> <br />o 0 <br />1899 1909 1919 1929 1939 1949 1959 1969 1979 <br />YEARS OF RECORD <br /> <br />Figure 10.- Variation in spring precipitation. <br /> <br />Figure 11 shows spring precipitation by 12.7 cm (5 in) class. Nearly all years (86 percent) fall <br />below the 25-cm (1 O-in) class. The mode, 19.1 cm (7.5 in), mean, 17,8 cm (7.0 in), and median, 16.5 <br />cm (6.5 in), are all in the 13- to 25-cm (5- to lO-in) class. Individual years in each class are listed in <br />table 7. <br /> <br />Fall and Winter.-Figure 12 shows the percentage of total annual precipitation contributed by <br />fall and winter. The distance from the 100-percent line to the plotted line represents the portion <br />of annual precipitation totals added by spring. Mean fall/ winter precipitation in the central <br />Sierra Nevada is 85.8 percent, but ranges from 66.6 percent of the annual total in 1947 to 98 <br />percent in 1918. Years such as 1909, 1918, and 1945 may have been good candidates for snowpack <br />augmentation, since less than 5 percent of the total precipitation occurred in spring. The yearly <br />distribution of fall and winter precipitation is shown in table 8. <br /> <br />Correlation of Seasonal Precipitation.-It is important to make cloud seeding decisions as early <br />in the water year as possible. Approximating annual precipitation, if correlations could be <br />detected between seasonal precipitation totals, would be helpful in achieving this goal. <br />Spring precipitation is normally about 18 cm (7.0 in). Even ifthis amount could be augmented <br />by 50 percent, total annual precipitation would be increased by only 8 to 18 cm (3 to 7 in)-Iess <br />than 10 percent of an average year's total. It would be desirable, therefore, to augment the annual <br />precipitation in fall or winter. To do this, it would be convenient to predict the winter and spring <br />precipitation totals from the precipitation occurring during the preceding season(s). <br /> <br />1-16 <br />