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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A5 <br />slightly and a higher proportion came from surface-water <br />sources. <br />If precipitation and runoff were distributed uniformly <br />in space and time, then average values could be relied <br />upon by water users and managers. However, both <br />precipitation and runoff in the valley vary widely dur- <br />ing each year and from year to year (figs. 3, 4). The <br />cumulative departure graphs (fig. 4) show wetter than <br />normal periods (cumulative departure increases) and <br />drier than normal periods (cumulative departure de- <br />creases) since the middle 1800's. Because annual pre- <br />cipitation at Fresno and Bakersfield is much less than <br />at Red Bluff and Sacramento, the magnitude of depar- <br />ture is also much less. <br />A fairly stable measure of the variability of runoff in <br />the Central Valley has been the sum of the annual flow of <br />the 15 largest streams because one end of the Central <br />Valley may have less-than-normal precipitation while <br />the other end may have above-normal precipitation. Only <br />twice between 1961 and 1977 was the total annual flow <br />of the 15 largest streams within 10 percent of the mean <br />annual flow of these streams, and only in 7 years of the <br />44-year-period record was the total annual flow within 10 <br />percent of the mean annual flow. These records indicate <br />I I I I I I i ill I <br />JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC <br />B <br />n L-I -I n. [1. <br />JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC <br />EXPLANATION <br />[ | PRECIPITATION ^ RUNOFF <br />FIGURE 3. Precipitation in the Sierra Nevada and runoff in the <br />Central Valley (modified from Williamson and others, 1989). A, <br />Sacramento Valley. B, San Joaquin Valley. <br />FIGURE 2. Sutter Buttes. View northeastward from southwest edge of buttes.