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<br />o [)\)4 7 8 <br /> <br />seasons as compared with the long-period annual <br />normals for the same set of stations. When this <br />same percentage is applied to the 46 -season (1912- <br />1957) average annual streamflow of 12,640,000 <br />acre-feet at Glen Canyon, we can relate this to the <br />actual flow which was measured in that water year <br />to get a rough approximation of the influence of <br />these particular major storrns--or multiple major <br />storms. <br /> <br />Table 5 shows the results without considering <br />any influence from other tributaries above Glen <br />Canyon and can, at best, only he considered as a <br />general guide. Several criticisms can be made of <br />this simple technique in determining major storm <br />influence I but it cannot he denied that these major <br />storms do exert a strong plus factor to increasing <br />streamflow. <br /> <br />The total extra runoff for the 15 storms <br />during the ten Sea.5OnS when they occurred amounted <br />to 22,068,000 acre-feet. This would be an average <br />per major storm of 1,400,000 acre-feet. This is <br />in addition to the direct fractional portion of the <br />total annual runoff attributable to the fractional <br />portion of the annual precipitation produced by each <br />single storm. <br /> <br />D. CONCLUSIONS <br />FROM STUDY OF MAJOR STORMS <br /> <br />Having reviewed the historical record of <br />major storms and, in a very general way, the <br />respective influence these storms have had on run- <br />off, the following conclusions have been reached: <br /> <br />1. A three-basin major storm is defined as <br />one which produces precipitation above 5 per cent <br /> <br />25 <br /> <br />of annual precipitation at one-half or more of the <br />stations in each of the three sub-basins and pro- <br />duces an 18-station total precipitation greater than <br />15 inches. This is to be collected in a period not <br />to exceed four days. <br /> <br />2. Snowpack totals can be used as a general <br />substitute for an annual "major storm." The <br />cumulative total of this "major storm" will differ <br />markedly from year to year, but will have a high <br />correlation with the total annual runoff figures at <br />Glen Canyon. <br /> <br />3. Major storms capable of producing within <br />four days an extra yield of 1, SOD, 000 acre-feet or <br />more of runoff are not a part of the annual recurring <br />weather phenomena. Therefore, long-term plan- <br />ning for the most probable one-year runoff values <br />should permit exclusion of the extra runoff yields <br />obtained from such major storms. A projected five- <br />year sample could logically contain one such storm. <br /> <br />4. Major storms can be identified [rOom the <br />current network of precipitation stations the day <br />following their occurrence. <br /> <br />5. The occurrence oLeven one major storm <br />adds a plus factor to the impending annual runoff <br />total. However, the one storm, in itself, does not <br />indicate an above normal water runoff year. This <br />will also depend on the precipitation occurring <br />during the other 361 days. <br /> <br />6. Since most major storms occur in the <br />four-month period, September through December, <br />a favorable lead time is gained to allow an upward <br />adjustment of the late winter and early spring <br />runoff estimates for the balance of the current <br />water year. <br />