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<br />..,,.....,('1 <br />;;;;V';' iJ <br />5B M r. T II 0 P 0 I.I T ,\ N IV ,\ T E H IJ 1ST R leT <br /> <br />storage and water surface elevations for'the years 1934 to 1946, <br />covering the period of ope.ration of the reservoir. To supplement <br />this historical water supply record, Fig, 6 shows the annual inflow <br />to Lake Mead since 1897, with recent years measured and earlier <br />data estimated from the best available SOUrces, This inflow record <br />is then replotted on the same plate in the form of a differential <br />mass curve, in terms of per cent of the average inflow for 1905- <br />1946, This curve shows the cumulative deviation from the mean, <br />to simulate the operation of Lake Mead under conditions of present <br />upstream depletion and full lower basin consumption, Each suc. <br />cessive year's excess or detkiency is plotted Upvt'ard or downward, <br />respectively, so that a sustained upward trend of the curve to tbe <br />right, as from 1905 to 19~O, indicates an abnormally wet period, <br />while a sustained downward trend to the right shows a subnormal <br />dry period of years, The steepness of the trend line, or of each <br />segment of the curve, indicates the intensity or severity of the <br />wet or dry period by its variation from the horizontal. The latter <br />represents a year, or succession of years, of average runoff. <br />The differential mass curve of Fig, 6 emphasizes the importance <br />of the recent 10-year period of drought, 1931-1940, in all present <br />and future water supply studies and project planning within tbe <br />Colorado Rivel' basin. As the curve shows, this drought period <br />still continues, the 1930-1946 period being about 10 per cent more <br />severe, measured by the cumulative deficiency as an index of result- <br />ing reservoir storage draw-down, The worst 10.year period of the <br />longer drought is commonly referred to, however, because of the <br />Colorado River Compact requirement (Art. HId) that the flow of <br />the Colorado River shall not be depleted below an aggregate of <br />75,000,000 acre feet at Lee Ferry for any period of ten consecutive <br />years. Recently announced plans and proposals for new project <br />developments in the upper basin show a possible tota,l export of <br />Colorado River water from its watershed to other sections of the <br />upper basin states of 3,:380.000 aere feet annually, or approximately <br />3,000,000 acre feet more than was generally reported and believed <br />to be the ultimate maximum export when the Compact was signed <br />in 1922. This great expansion of planned consumptive uses in the <br />upper basin makes it evident that the Compact minimum is in effect <br />the maximum flow of the main Colorado River at Lee Ferry that <br />can be anticipated during any recurrence of precipitation and run- <br />off conditions similar to the years 19<11.1940, 01' possibly during <br />such a longer period as 19:30-Hl46, <br />