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<br />15 <br /> <br />The 1900-64 curve of figure 6 shows that the chance of the annual flow at <br /> <br />Falcon being 500,000 acre-feet or less in any one year is 3 percent, i.e. will <br /> <br />occur 3 times in 100 years. Likewise, a flow of 1,500,000 acre-feet or less has <br /> <br />a 50 percent chance of occurring in any one year, i.e. will occur every other year, <br /> <br />on the average. other values may be read from curve. <br /> <br />The 1945-64 curve of figure 6 shows that the chance of the annual flow at <br /> <br />Falcon being 500,000 acre-feet or less in any one year is about 10 percent, i.e. <br /> <br />will occur 10 times in 100 years. Likewise, a flow of 1,280,000 acre-feet or <br /> <br />less has a 50 percent chance of occurring in any one year, i.e. will occur every <br /> <br />other year, on the average. Other values may be read from curve. <br /> <br />Which one of the above curves to rely upon is anyone's choice, but the tendency <br /> <br />should be to lean towards the lower values. However, it should be remembered that <br /> <br />the 1945-64 period includes a period of unusually severe drought. <br /> <br />A probability study, such as this, assumes that the events occur in a random <br /> <br />manner. There is a good reason to believe that natural events, as rainfall and <br /> <br />droughts, persist from year to year, and thus are not entirely random. However, <br /> <br />probability data are of value and are necessary for making an economic evaluation <br /> <br />of the hazards of farming. <br /> <br />Persistence of low flow A year of deficient flow is bad, but several con- <br /> <br />secutive years of low flow is worse. The minimum average annual flows for con- <br /> <br />secutive years from 1 to 20 are listed in table 12, together with years comprising <br /> <br />each period. Data are plotted on figure 7 (p. 44). A bar graph of the 20 yearly <br /> <br />periods is presented on left side of figure 7. This graph shows that the years <br /> <br />of deficient flow were centered about the years 1951-53, and were all contained <br /> <br />in the period 1942-64. <br /> <br />A graph of the average annual flow in acre-feet for periods of 1, 2, 3, --20 <br /> <br />years is shown on right side of figure 7. The sharp break in direction of the <br /> <br />graph at the 6 year point indicates that during the years 1900-64, there was a <br /> <br />__' r' j- I'!, <br />\.; ',.,\ ~ ,'. ~ .L <br />