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<br />00. 8~' <br />l' J -;, <br /> <br />1922 estimates of the Colorado River's capacity have steadily <br />been revised downward. <br /> <br />"A shrinking supply accompanied by an increasin,::( demand <br />have thus conspired to generate a very bitter law suit indeed."' <br /> <br />After recounting the history of the river, the M a~ter gave <br />a detailed description of the major works in the Lower Basin <br />plus the irrigation projects and districts. This brought him then <br />to the subject of ~uppl.v. <br /> <br />"Accurate determination of future supply in a stream sys- <br />tem," he said, "is difficult in any case. but approaches the im- <br />possible in the case of the Colorado River. The reasons are not <br />hard to discover. Determination of future supply is at best a <br />prediction-.an estimate based on the past. The reliability of <br />the estimate is, of course, conditioned upon the accuracy of the <br />historical data and upon the probability that the past will to :l <br />substantial degree repeat itself. Neither of these two conditions <br />of reliability exists in the Lower Basin of the Colorado River. <br /> <br />"First, there is the difficulty of measuring stream flow, <br />which always involves a degree of inaccuracy_ One expert wit- <br />ness testified that there is always a percentage of error in stream <br />flow measurement. The reason for the inaccuracy beeomes <br />apparent when one eonsiders the mea'oU1'ement process. Stream <br />flow at any particular gauging point is determined by a series <br />of measurements and cnlculations which involve engineering <br />judgmellt. It is necessary to determine the cross-sectional area <br />of the stream at the gauging point and to obtai]) the velocity of <br />the flow for a given stage (iJ'.. sll!'face level of water) at that <br />point. Some gauging stations have a continuous stage recorder. <br />which give,~ a continuous measurement of the fluctuating- stream <br />level, but others do not. When stages are measured infrequently, <br />some error is necessarily introduced. The calculation of the <br />cross-sectional area depe'nds 011 the width, depth. and contour <br />of the stream bed, Determination of these data involves some <br />uncertainty, due in part to the fact that the cross-sectional area <br />is unstable, changing due to sedimentation and scolll'ing. Even <br />the determination of flow velocity bv current meters is inexact. <br />due to variations in readings depending upon the depth of the <br />meter below the water surface, The United States Geologica I <br />Survey, which maintains and publishes records of stl't'am flow <br />data, rates its records from excellent (error of 5 'C;, 01' less) to <br />poor (error in excess of 15 n;, ) , <br /> <br />"Determination of dimunition of supply resulting from res- <br />ervoir evaporation loss entails ~imihr inexactitude, Without <br />detailing the methods of measuring reservoir evaporation lo~s, <br />it is sufficient to say that the process is also one of estimate and <br />calcnlation, with ;lttendant inaecuracies. One witness chr.rad.er- <br />zied the often-used land pan method as reliable onl,'- to the <br />extent of 'general application on an annual b'lSis.' <br /> <br />"Second. t11el'e is the problem of incomplete stream flow <br />records, Historical flow records suffer not only from the ill- <br />firmities just described but al~o from the fact that th!>}' often <br />