WSP10396

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 1/1/1964
• Author: USGS
• Title: General Effects of Drought on Water Resources on the Southwest
• Water Supply Pro - Doc Type: Report/Study

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GENERAL EFFECTS OF DROUGHT ON VVATER RESOURCES

B35

most certainly is the fact that this tree-ring record is
the only one weighted with regard to the percentage
of runoff from different parlJs of the draimtge basin.
The three lowest coefficients, ranging from 0.35 to 0.45,
pertained to southern Arizona, where a large part of
the yearly runoff is in the form of frequent flash floods
each summer, These Hoods cause the yearly runoff to
vary from year to yenr, but the variation is not fully re-
flected in the tree rings because of the short duration
of the Hoods and because they occur at a time of year
,vhen the trees normally gro\y but little. The low
coefficient of 0.56 in central Cali fOl'uia is attributed to
the insensitivity of the tree rings and to the fact that
most of the trees were located north and east of the
river basins used in this study.
Two other significant measures of the relation be-
tween runoff and tree-ring indices are shown in table
6. The standard error of estimate gives the percent-
age range (column 5) within which runoff for an incli.
vidual year may be estimated from tree rings. The
st.nndard errOr of the mean gives the percentage range
(column 7) within which the mean runoff for a period
covered by tree rings may be estimated. There are
serious statistica.1limitations upon the long-term mean
estimated from tree rings-for example, there is a high
degree of autocorrelation ,,"ithin anyone series of tree
rings, the percentage range is too high, the tree rings
way not be truly representative of the mean, tree rings
do not reflect slow secular trends in climate-but no
other method is known by which a better estimate can
be made,
Table 7 shows the relation of the mean tree-ring
indices for 1904-53 to those for longer periods. The
first eight columns show how estimates for 1904-53

were made for the tree-ring indices: colnmn 3 gives the
runoff, in percent of median runoff, for 1904-53; col-
umn 5 gives the runoff for the shorter period 1904 to
the last water year for which the indices are available;
column 6 gives the ratio of the two runoffs, which is
the figure by which the short-term runoff should be
multiplied to calculate the 50-year runoff; column 7
.gives the mean of the short-term tree-ring index; and
column 8 gives the mean of the tree-ring index for the
50-year period 1904-53, computed on the assumption
that the ratio between short-term and 50-year means of
runoff is the same for the short-term and 50-year means
of the tree-ring indices. All the tree-ring indices ex-
tend back at least to 1800 (column 12) and, therefore,
the relation of tree-rings for the 50-year base period
1904-53 to those for the 154-year period 1800-1953 was
compnted for all regions (column 11) on the basis of
data given in columns 8 to 10.
The range of the ratios given in column 11 is seen
to be rather limited, from 0.95 for the upper Gila E-iver
basin to 1.08 for southwestern Utah. There is a pos-
sibility that the real difference between the ratio 0,95
and the ratio 1.08 is negligible and that either may
apply equally well to the Gila Basin and southwestern
Utah. However, the ratios given in column 11 are the
most probable ratios of mean runoff for the period 1904-
53 to that for the period 1800-1953, despite the large
possible range in standard error, These ratios ind.icate
that the mean runoff for the base period 1904-53 was
close to the mean runoff in the 154-year period 1800-
1953. The average ratio for the 9 indices in table 7 is
102 percent, and if the Colorado River basin above Lees
Ferry is excluded, it is 100 percent, It is, therefore,
conclnded that the deviations of runoff below the 50-

TABLE 7.~Relation of means of tree-ring indices for 1904-53 to those for 1800--1958 and for longer periods for several hydrologio regions
of the Southwest

Mean 01
tree-ring
Mean of index for
Mean 01 Mean 01 tree.ring period from
Runoff, in Ratio Mean 01 two-ring tree.ring Index lor Ratio first year Ratio
Runoff, J~ast year perecntol column 3 tree-ring index index for 1800-1953, column 8 First in which column 8
'l'able In in per- lorwhieh 1904-53 colmnn5 index for for base period adjusted column 10 year for index is column 13
~~?t~;l~~ cent of tree-ring median, period period 1800 to since 1904 whlel] used to
HydrologIc region median, index is for period 1904 to 1904-53 year on basis tree-ring 1953,
(1956) for base available 1904 to year (column given ill of mean index adjusted
period year given In 6X column given in Is used sinceHI04
1904-53 given in column column 4 column 8 on basis
column 4 4 7) of mean
given in
column 8
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
-- ---
Contral Callfornla~_.. 17 130.3 1941 134.7 0.97 101.1 98.1 101.4 100.4 0.98 1353 99.7 0,98
Southern Californla__ 18 144,3 1950 148.0 .98 106,3 104.2 100,2 ;99.7 1.05 1414 98.8 1.05
Southwestern Utah... P) 105,8 1950 107.2 .99 241.7 241,3 238.5 238.4 1.08 1800 n.nn~hn nUUh_n.
Mogollon Rim_mnn 67 119,2 1953 119.2 1.00 9S,6 9S,6 ".9 911.9 .99 1800 ------iori)' ~--n--n:gi
Uppor Gila River. n. 65 125.2 1939 134.0 .98 98,6 91.7 9S.5 96,3 .95 1603
San Juan Mountains. 70 IDS. 1 1951 108.S .99 96.5 95.5 100.1 99,8 .96 1375 99,4 .96
Colorado RIver basin 49 103.9 1950 104.6 .99 107,8 106,7 99.11 99.7 1.07 1099 9S.8 1. OS
above Lfes Ferry, 51 103.9 1950 104.6 .99 107,6 106,5 99,4 99.2 1.07 1800 -------T03
(0) 103.9 1945 105.0 .99 103,9 102,9 99.7 99,6 1.03 1288 ------ioo:i-

1 Yoarly mean ring widths from Schulman (1950, table 4-D, p. 14).
2 Figures represent mean ring widths; Indices are not available.

3 Yearly m~n ring widths from Schulman (1945, table 5, p. 38).