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<br />DEC
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<br />OCT
<br />1899
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<br />1909
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<br />1 9 1 9
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<br />1929 1939
<br />YEARS OF
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<br />1949 1959
<br />RECORD
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<br />1969
<br />
<br />1 979
<br />
<br />Figure 4.- Variation in date of precipitation accumulation. Dashed line represents dates
<br />when 25 percent of the total fell; solid line represents 50 percent accumulation, and dotted
<br />line indicates 75 percent accumulation.
<br />
<br />February 13. Mean date for 50 percent accumulation is January 27, but has varied from December
<br />8 in 1899 to March 16 in 1904, indicating that heavy storms occurring early in the water
<br />year in the American River Basin can account for half of the year's total by mid-December. Con-
<br />versely, 50 percent accumulation may not occur until mid..March. A 3-month variation or greater
<br />should be expected if decisions to augment precipitation are based on the expected arrival of 50
<br />percent of the year's total precipitation.
<br />Seventy-five percent of the annual precipitation falls by March 10 in average years. This
<br />percentage has been reached as early as January 6 in 1965 and as late as April 20 in 1924, Again,
<br />the wide range in precipitation timing will complicate decisions about starting and terminating
<br />seeding operations. Some years have received 25 percent of the annual precipitation by February,
<br />but other years have received 75 percent of the annual precipitation in January. All dates
<br />appearing in figure 4 are listed in table 3.
<br />
<br />Daily Precipitation.-Mean daily precipitation intensity for days with measurable precipitation
<br />has decreased somewhat since the first third of this century (fig. 5). Mean precipitation intensity
<br />was 1.68 cm (0.66 in) per day over the 76 years of record. However, the daily mean intensity was
<br />1.80 cm (0.71 in) from 1899 to 1929, and 1.57 cm (0.62 in) from 1930 to 1974. High precipitation
<br />
<br />1-7
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