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. t. ,I l ~, f 11., I ~ ! i , , .~ I'" .....!"':"' I~ ..J .. ...J HYDROGRAPHY 67 to Lake !\lead since 1897, wit.h years since 1921 measured and earlier data estimated from the best available upstream or downstream gaging records. This inflow record is then replotted on the same plate in the form of a differential mass curve, in terms of the average intlow for 1905-1949, This nUlSS curve shows the cumula- tive deviation from the mean, to simulate the operation of such a reservoir as Lake Mead (but assumed to be of unlimited capacity and with no allowance made for losses) if the same cycle of runoff now recurred under conditions of present upstream conditions 01' development and with full lower basin development or consumption approximated, the mean annual inflow being uniformly released from the reservoir. Each successive year"s excess or deficiency is plotted upward or downward, respectively, so that a sustained up- ward trend of the curve to the right, as from 1905 to 1930, incli- cates an alJl10rmally wet period. while a sustained dowl1\\'ard trend to the right shows a subnormally dry period of years. The steepness of the trend line. or of each segment of the curve, indicates the intensity or severity of the wet or dry period by its variation from the horizontal. The latter represents a year or succession of years of average runoff. As the differential mass curve of figure 5 indicates, the recent 10-year critical period of drouth, 1931-1940, is really only a part of the longer drouth of 1930-1948, which as of 1949 is obviously not nece~~arily ended yet. Conditions O'ier the longer period have been auout 10 per cent more severe than the 10-year critical period. as measured uy the cumulative deficiency of inflow as an index of resulting drawdown of reservoir Htorage. As a base for this mass curve the 1905-1949 mean runoff has been used as a more probable approximation to the long-time mean than is the longer 1897-1949 mean of record (and eady estimates, the probable accuracy of which diminishes considerably as their date recedes further into the pre- record past), because the longer number of years includes parts at least of two major drouths and the one intervening long wet period, The gradual extension of the recent (or present) drouth has of course made this distinction small in amount and of less significance than in eHrlier years. Owing to general interest in the lower Colorado River hydro- graphy at the present time and because the runoff records are not widely available immediately, due to the unavoidable publication lag in issuing Federal serial bulletins and water supply papers, the preliminary hydrographic data for the year ending September 30, 1949 are summarized in table 14.