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B32 DROUGHT IN THE SOUTHWEST, 1942-56 or more, equalled, and in SOIne decades greatly exceeded the much shorter major recent maximum in rainfall and runoff of the Colorado River basin, 1905-1930; '" II< '" (2) the current growth deficit, fJinl'~ 1930, does not as yet seem as severe in the upper bnsin as that:during the interval 1871-1904, and especially after 1892: (3) th~ average annual growth and, perhaps to a ~reat approximation, rainfall and runoff, during the 85-year drought of 1215-1299 seems to have been about half that during the drought since 1930 in this basin. The general cOliclusion would be, then, that the current drought is severe but has been exceeded in the not.too-distant past, and wet years and periods far exceeding those noted since gage measurements began llre a not unreasonable possibility. Schulman's conclusion that t.he current drought is not as severe in the Qolorado River basin above Lees Ferry as was the drought. after 1892 is in agreement wit.h the relative severity 'of the two droughts as computed from runoff records. 'The statement t.hat runoff during the drought of 1215-99 seems t.o have been about. half that. during the drought since 1930 emphasizes the facts that the current drought in t.he Colorado River basin above Lees Ferry is not record breaking over the centuries and that, although the amount of rnnoff in the fnt.nre is ex- pected t.o average about the same as that in 1904-53, there is always t.he possibility of a drought much worse than any known' since the coming of the white man, Schulman (1956, p, 66-67) has also drawn conclu- sions regarding the drought in southern Arizona and the Gila River basin, as follows: A major difference [from the upper Colo.rado. River basin] in recent decades is the relatively more pronounced nature of the current drought. Beginning in 1921 in the southernmost areas of the State [of Arizoua] and particularly after 1933 in the entire Gila Basin, wet years have been very rare. It is clear that the deficiency 'begInning in 1934 has been the most severe: since at least 1800 II< '" *. Reference to the 350 year series _for southern Arizona and for the GUa headwaters area, suggests that no. dro.ught during that entire interval is as seri- ous as: the current one '" oil '" It appears highly likely. in view of the general parallelism with the chronologies in Colorado and Utah, that this is the most severe drought since the l'ate 1200's'" '" "', It sho.uld be of significance in statistical forecasting that the total deficiency during the current drought is now (1955) greater than the total excess during the interval 1905-1920 in southern Arizo.na, a period which was probably one of the wettest in many centuries. PROBLEMS OF CORRELATION Many of the problems in correlating the fluctuations in streamflow with those in tree-ring widths are the same as the problems in the studies of tree rings as indi- cat.ors of climatic fluctuations. Among t.hese problems, Schulman (1956, p, 29-81) mentions the "standardiza- tion" of the growth rat.e by eliminating the "age trend" t.hat is characteristic of tree growth, the inhomogeneity int.roduced in computation of regional tree-ring indices by reason of the various number of t.rees and localities for which records are available, and the growth "re- leases" in numerous trees as a result of occupancy of the region by whit.e man. The correlation of tree rings with streamflow dnring t.he period of contemporaneous records also brings to light special problems, limitations, and requirement.s, some of which have been summarized by Schnlman (1945, p. 36-37) : if) t: z :) z 2 0 i= <( '> w Cl , Cl '" <( Cl Z <( t; " 0 :;; I I- 0' <( '" 0 ~ ~ -1 u: ~ 0 z ::> '" ~ -2 '" <( w >- - South-central Texas (stations 77-79) ^ ,\ t - - ^ J\ \ V " \\\f' "...., .. V "::':':'::::::::::::,1 @ ., .. ..... .. . .. tl .. .. . . V EXPLANATION \@ rBTIill] ..........", .. Deviation below median runoff I I I 1S90 1895 1910 1915 1900 1905 1920 1935 1940 1950 1955 1945 1925 1930 FIGlTRE D.-Graphs showing