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 WATER RESOURCES

B31

ing the 1930's was generally less than during either the
1892-1904 or the 1943-56 drought. Figure 8 also shows
that the trends in yearly runoff of Colorado River at
Lees Ferry, Ariz., have been similar to those of streams
in the San .r uan Mountains. This station was not in-
cluded in the statistical studies for this report., because
t.he basin extends beyond the Sout.hwest. drought area as
previously defined and because the runoff from the
1l0,000-square-mile basin has been considerably modi-
fied by man.
The stations in the Great Plains include one group
along t.he flanks of the southern Rocky Mountains in
cent.ral New Mexico and, several hundred miles to the
southeast., three contiguons groups deep in the heart. of
Texas. The'snbdivision int.o three groups in Texas has
been necessary, because there is a wider range in runoff
characteristics and lesser correlation between nearby
stations than is common in t.he other met.eorological
zones, The yearly runoff of four groups is shown on
figure 9, The Upper Pecos River basin group shows
effect.s of t.he droughts of 1909-18, 1930-40, and 1946-56,
which have been recorded in the Great Plains farther
eastward-in Kansas, Oklahoma, and the Texas Pan.
handle (Thomas, 1962). The 2., 3-,5-, and 10-year
periods of minimum runoff were all recorded in the last
of these three drought. periods. In central Texas the
three groups (west.-central, central, and south.cent.ral)
show several similarities and also seve-ral divergences
in the effect.s npon runoff of these several periods of
drought in the Great Plains. The effect of" the 1909-18
drought. upon all three gronps is shown to be marked.
By contrast, the decade 1930-40 was not a period of
marked reduction in runoff, although 1934 was general-
ly a year of very low flow, In all three groups the re-
cent drought has been more intense than the earlier
droughts and resulted in the 1-, 2-, 5-, and 10-year
periods of minimum runoff in the period of record. The
effect of this most recent drought was variable among
the three groups of stations, however; two groups re-
corded flow 'above the median in 1947-49, although the
third group registered less than median flow in each of
the years 1948-56,
Several of the streams in central Texas drain areas
that are underlain by the ground-water reservoir in the
Edwards limestone (Thomas and others, 1963b), which
effects the amount of runoff variously. Storage and
delayed release of water mll,y account for the trend of
annual runoff shown in the graphs representing the
three groups in central Texas. To an extent greater
than is indicated by the graphs in figures 6-9, the years
of high runoff are commonly followed by a second year
of relatively high runoff, and there are indications of a
long.continued declining trend in runoff after years of
maximum runoff, such as 1919 and 1935 (Thomas and

others; 1963b). Plate 1 shows maps of the Southwest
that delineate thea,reas where the annual runoff was
less than the median for each of the years 1942-57.
These areas are similar in major outline to the areas of
precipitation deflciency in corresponding years (Thom-
as, 1962). The inadequate coverage of the Southwest
by the 85 selected gaging stations is evident from in-
spection of the resulting maps, but more adequat<J data
concerning this period of drought are not likely to be
obtained.

STREAMFLOW AS INDICATED' BY TREE RINGS

The derivation of the median runoff in the base period
1904-53 and the conclusions about the effects of drought
in terms of deviation from this median lead logically
to the following question: Where does this 50-year base
period stand in relation to the period of occupancy of
the Southwest by man? This question is analogous to
that posed about climate generally in that we have a
century of record that can be projected back to earlier
centuries and millenniums, Studies of tree rings have
provided some of the most definitive conclusions con-
cerning the climat<J of the Southwest during these
earlier centuries. Studies of tree rings also enable us
to make intelligent deductions concerning streamflow
in bygone centuries,
Tree-ring studies for the Southwest drought area
covered by this report, except for Texas, indicate that
mean runoff during the base period 1904--53 was closely
representative of mean runoff for the 1M-year period
1800-1\153 and also for periods of 850 years or more.
Thus the 50-year period was representative, despite the
fact that runoff in the first part of the period was far
above the mean and in the last part far below the mean;
or perhaps it is more accurate to say that the 50-year
runoff was representative, because the period included
both high and low runoff periods, which happened to
occur at about the proper time to give a representative
average. The conclusion is that the figures presented
in this report showing runoff deficiencies during the re-
cent drought in terms of the 50-year (1904--53) median
runoff show reasonably well the runoff deficiencies in
terms of median runoff for much longer periods, In
other words, the recent drought is a real drought; it is
not, as has sometimes been conjectured, a period of nor-
mal runoff following a great excess of runoff in the early
part of this century.
Schulman (1956, p, 65) has drawn several conclusions
regarding specific areas of the Southwest from his tree-
ring studies. He regards the present dronght in the
Colorado River basin above Lees Ferry as having
started about 1930, and he says of that basin:

We may conclude that (1) the average departure during the
interval 1300-1300, the wettest during the pOust eight centuries