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<br /> <br />., ."i . <br />J. .A. ,., <br /> <br />CHAPTER 1 <br /> <br />Need for the Colorado River Aqueduct <br /> <br />THE EXTREME aridity of the Southern California climate is not <br />immediately evident to the recently arrived resident or <br />tourist, with only the luxnriant snbtropieal vegetation of the <br />cnltivated areas as a basis for judgment. The sharp contrast with <br />the Sahara-like deserts that must be crossed by everyone approach- <br />ing Los Angeles overland increases the impression of abundant <br />rainfall, until the fact is finally realized that all this vegetation is <br />continuously dependent upon frequent irrigation. The region con- <br />stitutes in reality a great oasis, shal'ply bounded in every landward <br />direction by deserts of utter sterility. It derives a limited and ex- <br />tremely variable water supply from the 1,000 square miles of rocky, <br />steeply sloping mountain watersheds which drain into the basin <br />from the north and east. On these mountain slopes the rainfall <br />averages about 30 inches annually, but this rate decreases toward <br />the ocean to only 15 inches at Los Angeles and about 10 inches at <br />the beaches: <br />These figures for average rainfall, as is often the case with water <br />supply means, are themselves misleading, for one very wet year <br />(with most of its run-off lost as floods into the ocean) outweighs <br />several very dry years in computing the average. As nearly as may <br />be determined from all available data, there have been dnring the <br />157 years of Los Angeles history, approximately 94 years with <br />less than average rainfall and more than 100 with less than aver- <br />age stream run-off. The median precipitation during the period <br />has not exceeded 13.5 inches; that is, at least half of the years have <br />yielded less than this amount of rainfall. <br />Precipitation in Southern California varies widely from year to <br />year, half of the seasons being "unnsually" dry, about one in five <br />unusually wet; few are normal or average. Further, when pre- <br />cipitation does occnr, it is extremely intermittent; often several <br />months ill succession will yield none whatever. Conversely, a single <br />month will sometimes bring one-half the total annual rainfall and <br />occasionally one-half the annual total may fall in a single storm. <br />[7 ] <br />