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-{:? ,~~\ )f,{ _n' , /'1 I \ ., " ~~ '-. ,', '~I ~ ~..~ '.,<,. 'll (H"~~!':I5 -./ ) 'r ':.....::) GENERAL EFFECTS OF DROUGHT ON vrATER RESOURCES B51 ,f this way a compact may limit both the advantages OT topographic position of the upstream user and of pri- 01'ity of actual beneficial use, The Colorado River Compact (Thomas al)d others, 1963c) guarantees a practically constayt annual quanti- ty of water to downstream areas regar'less of variations in streamflow. It specifies that the outflow from the Upper Basin, as computed at the compact point of Lee Ferry, Ariz. (below the mouth of the Paria River), shall not be depleted below an average of 7.5 million acre-feet per year, plus an additional 1.0 million acre- feet granted t.o the Lower Basin, plus half t.he 1.5 mil- lion acre-feet. which was subsequent.ly allotted t.o Mexico by the Treaty of 1944. In the drought period 1943-56 t.he comput.ed average flow at. Lee Ferry was 11.1 million acre-feet. per year, and in the 4 years 1953-56 it. was only 6.6 million acre-feet per year. The developed require- ; ment.s for domestic and agricultural use below Lee Ferry have exceeded 7 million acre-feet. per year since 1952; these were fnlfilled throughout the drought, in part by holdover storage in Lake Mead which has a usable ca- pacity more than three times as large as the annual allotment of water to the Lower Basin and Mexico. But the compact as modified by the treaty wit.h Mexico also apportions "in perpetuity to the Upper Basin"the exclusive use" of 6.75 million acre-feet per year, ofw1l:ich an est.imated average of about 2.5 million acr~:feet is consumptively used in t.he existing patt.ern'of develop- ment, A simple subtraction of the quantity that must flow past Lee Ferry (9.25 million acre-feet) from the measured flow during the period 1943"-56 (11.15 million acre- feet) indicates t.hat the unappropriated wat.er available for use in t.he Upper Basin would have been less than 2 million acre-feet. per year during that 14-year drought. period, and therefore considerably less than is allotted by the compact for use in the Upper Basin, Thus the compact has offset the Upper Basin's natural advantage of topographic position. Actual measurement of the flow of t.he Colorado River at Lee Ferry (above the mout.h of the Paria River) began in 1921, just before the compact. was negot.iated, but a record of hist.oric flow has been extended back to 1897 on t.he basis of available records at various stations in the basin, This compiled record, plus the measure- ments beginning in 1921, indicate that the average an- nual outflow from the Upper Basin in t.he 33-year period 1897-1929 was 15.3 million acre-feet, It. has been est.i- mated that t.he depletion by consumptive use in the Upper Basin increased from about 0.7 million acre-feet in 1897 to about 2,5 million in the 1920's and that the average depletion in the 33-year period was about 1.7 million acre-feet. Thus t.he calculated virgin flow at /i . , If l f , ! Lee Ferry in the period 1897-1929 would have averaged about 17 million acre-feet. In the 27-year period 1930-56 the calculat.ed average annual flow at Lee Ferry was 11.2 million acre-feet., a reduction of 4.1 million acre-feet from the average dur- ing the period 1897-1929, A small part of this reduc- tion-less than a million acre-feet per year-is ac- count.ed for by increased consumptive use and stream deplet.ion within the Upper Basiu during the lat.er dry period, The record now available thus indicates a wet period of more t.han 30 years durat.ion and a dry period almost as long which included both the drought of the 1930's and the subsequent drought in the Southwest. The average virgin flow during the wet period exceeded that of the dry period by more than 3 million acre-feet. Theoretically, with this long dry period a matter of record, we now have the basis for a more accurate de- termination of the average water yield of the Colorado River, The comparisons of streamflow with dat.a avail- "ble from tree-ring studies (p. B34) indicateth"t runoff in the 50-year period 1904-53 corresponds to the average in the past 8 centuries, If this is true, t.he average nat- ural yield of the Upper Basin is slightly less than the 16 million "cre- feet that has now been allocated by com- pact and treaty. But this average yield is not a safe yield, in t.he sense th"t it can be guaranteed to water users every year, unless means can be found for storing wit.hout loss the surpluses of wet years for use in dry years. Storage sp"ce "lre"dy avail"ble, plus that au- thorized by the upper Colorado storage project, will be equivalent to more than five times the average natural flow of t.he river at Lee Ferry, and this may be sufficient for t.he accumulation of all the surpluses in a prolonged wet period, But if the wet and dry periodshave a du- ration exceeding 25 ye"rs, "s indicated by available data, the quantities "vailable during a dry period would be reduced in comparison with those "vailable in the wet period, bec"use of the progressive evaporation of the water that must be held over for m"ny years, Unless such losses c"n be prevented, the n"tural flow "t Lee Ferry in the dry period 1930-56 (11.2 million "cre-feet, plus c"lcul"ted deplet.ions of about. 2.5 million acre-feet) is perh"ps the best measure of the qu"ntity "v"ilable for use "t ,,11 times, including the most "dverse conditions. In view of the comp"ct gu"mntees to the Lower Basin, t.he 6,75 million acre-feet "llotted to the Upper Basin may be "vail"ble only in the wet cycles "nd m"y be sub- ject t.o " reduction of more th"n 2 million acre-feet in prolonged dry clim"tic cycles. More recent int.erstat.e compacts in the Southwest in- dicate greater aw"reness of the effect of major climatic fluctuations, for they do not. gu"r"ntee specific quanti- ties of water t.o "nyone but instead att.empt to apportion