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<br />18
<br />
<br />be made. ('lWe Bul. 6Gl4, Vol. I, p. 43.)
<br />
<br />When the ground-water level is above the elevation of water in the river
<br />
<br />channel, water will seep into the river from the ground-water aquifer. This is
<br />
<br />the normal source of river flow during periods when rain water is not running over-
<br />
<br />land to the stream. The aquifer supplying water to wells in the Valley is in turn
<br />
<br />supplied from the Rio Grande; therefore, increased well pumping will decrease the
<br />
<br />flow of the river. As there is no regulation of well withdrawals in Texas, the
<br />
<br />effect of continued, and perhaps future increased heavy well pumping should be
<br />
<br />considered in appraising the water supply that can be obtained in the future from
<br />
<br />the Rio Grande.
<br />
<br />Summary
<br />
<br />The average annual flaw of the Rio Grande at site of Falcon Dam,
<br />
<br />for the period 1900-64, would have been 1,706,000 acre-feet, if today's physical
<br />
<br />development had been in effect. During the last 20 years the average annual flaw
<br />
<br />would have been about 1,395,000 acre-feet, but as this period includes a severe
<br />
<br />drought, reliance on a higher yield is justified.
<br />
<br />Whatever quantity of water is finally selected as a safe annual yield, there
<br />
<br />is a chance that for any year the flaw will be less than the selected amount. The
<br />
<br />question is: What is this chance?
<br /> As an average over a long number of years, the annual yield will be:
<br /> 300,000 acre-feet or less one year out of 100 ( l'f, chance)
<br /> 425,000 " " " " " " " " 50 ( 2'f, " )
<br /> 625,000 " " " " II " II II 20 ( 5'f, II )
<br /> 800,000 II II II II " II II " 10 (10'f, II )
<br /> 1,020,000 II II II II II " II " 5.0 (20'f, II )
<br /> 1,100,000 II " II II II " II II 4.0 (25'f, II )
<br /> 1,250,000 II " " " II II II II 3.0 (33'f, " )
<br /> 1,400,000 II II II II II II II II 2.3 (43'f, II )
<br /> 1,500,000 II " II II " II " II 2.0 (50% II )
<br /> 1, 600,000 II " II II " II II II 1.8 (56'f, II )
<br /> 1,700,000 II II II " " II II II 1.6 (61% II )
<br /> 2,000,000 II " " " II II II II 1.4 (73% " )
<br /> Thus if development expects an annual yield of 1,500,000 acre-feet, there is
<br />a 50 percent chance there will be more water than this available and the same
<br />chance there will be less; or to express it in another way, there is an even chance
<br /> - ~,~ r: r L: ,,",
<br /> l. J'...", LJ:
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