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<br />I' <br /> <br />The percent of annual rainfall that occurs each month on the average (from <br /> <br />TWC Bul. 6413) is about: <br /> <br />January.... .6. <br />February... .5. <br />March.......5. <br /> <br />April..... 7. <br />~.......12. <br />June..... .10. <br /> <br />Ju.ly'. . . . . . . .. 8. <br />August. . . . . .. ~9. <br />September....17 . <br /> <br />October.....IO. <br />November.... 5. <br />December.... 6. <br /> <br />Total percent ................ 100. <br /> <br />While the rain contributes a substantial amount of water to Valley farms, an <br /> <br />average of 2.02 feet per year, it does not alwa;ys come at the best time for growil};g <br /> <br />crops. <br /> <br />Evaporation <br /> <br />Part of the water suppJ;y- will be lost by evaporation. The <br /> <br />figures of river discharge used in this report considers evaporation in flowing <br /> <br />down river channel and through reservoirs, however an additional loss will be <br /> <br />suffered if water is held in storage for longer periods than contemplated in the <br /> <br />I. B. & W. C. studies. Additional storage time may well be needed if a fixed annual <br /> <br />suppJ;y- is to be withdrawn for use in the Valley and the surplus is to be held in <br /> <br />reserve for future use. <br /> <br />The average annual lake evaporation at Falcon Reservoir is about 64 inches <br /> <br />(TWC BUl. 6413); evaporation at Amistad Reservoir is greater. The quantity of <br /> <br />water lost by evaporation is a product of the evaporation and surface area of the <br /> <br />reservoir. An estimate of surface area vs storage capacity, for both Falcon and <br /> <br />Amistad reservoirs, may be made from curves of figure 8 (p. 45). <br /> <br />Evaporation has been deducted from annual flows computed by I. B. & W. C. <br /> <br />But if stored water is carried over into the next year there will be an additional <br /> <br />loss due to evaporation that must be considered in estimates of future suppJ;y-. <br /> <br />Ground-Water About 1,500 irrigation wells, numerous domestic wells, and <br /> <br />some industrial and public suppJ;y- wells tap the lower Rio Grande ground-water <br /> <br />reservoir. The total pump capacity is about 2,200 acre-feet per da;y. Because of <br /> <br />the variability of crops, rainfall, and river water, pumping from the ground-water <br /> <br />reservoir has been very irregular, and no quantitative estimates of pumpage can <br /> <br />.-. ~ r' (. l . <br />\J\...l~,..,v0 <br />