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<br />. <br /> <br />8 <br /> <br />HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT <br /> <br /> <br />about 0.02 and that for one lasting 6 years is <br />about 0.01. The probability of a 2-year <br />drought is in the range from 0.03 to 0.05. <br />A factor related to the probabilities of low <br />flows is the duration of low runoff. The num- <br />ber of months that streamflow was deficient is <br />shown in figures 3a, 3b, and 3c during 12-, 18-, <br />and 23-month periods starting in February 1976. <br />Monthly streamflow is deficient when it is <br />within the lowest 25 percent of record for the <br />month. See figure 10 as an example. <br /> <br />I <br />I <br />l_! <br /> <br />A month or two of deficient streamflow, or <br />even 3 or 4 months in some areas, will not be a <br />very serious hydrologic drought, particularly if <br />the months are not consecutive. <br />The number of months of deficient stream- <br />flow does not indicate how many of them were <br />consecuti ve nor when they occurred; but a <br />hydrologic drought is evident because when 8 <br />to 11 months out of 12 or 16 to 22 of 23 <br />months are deficient, many of the months are <br />consecutive and occur during the growing <br /> <br />\\ <br /> <br />= <br /> <br />EXPLANATION <br /> <br />MOr\th~ <br /> <br />.912 <br />.' 8 <br />014 <br />Do <br /> <br />Figure 3. Maps of the United States showing <br />