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

B27

runoff expressed in logarithmic standard-deviation
units is truly representative of the regional character-
ist.ics that affect runoff. The total area of all regions so
identified represents only a small proportion of the
Southwest drought area, because the 85 drainage basins
whose rnnoff records are suit.able for analysis represent
only abont a quarter of the total area affected by
drought. The remainder of the drought area is either
desert with little or no runoff or are.as where natural
runoff has been greatly modified by man's water devel-
opments. The 85 gaging stations include many that
measure rnnoff from high headwater areas, and the ag-
gregate urea embraces several of the principal water-
producing areas of the Southwest.
The principal regions delinated on the basis of these
correlative techniques are fonr hydrologic zones that
correspond t.o the meteorologic zones (Thomas, 1962)
already outlined independently on the basis of studies
of meteorologic factors. Thus the statistical studies of
streamflow records confirm the four meteorologic zones
as principal hydrologic subdivisions in the Southwest.
'\Vherevel' the streams in an extensive area have similar
runoff characterist.ics~that is, where the runoffs of all
streams in a region fluctuate from their median flows
with considerable consistency from year to year~these
fluctuations of runoff reflect., in large part, the meteoro-
logical forces acting over the region.
1Vithin each of the four met.eorological zones, further
subdivisions were indicated by the correlations. Table
5 shows the eventual groupings, the minimum and maxi-
mum correlation coefficients in each group, and the dis-
tribution of the ranges of the coefficients within each
group. A summary of the 390 correlations involving a
total of 85 gaging-station records shows that the cor-
relation coefficients for 6 were between 0.60 and 0.69, 29
were between 0.70 and 0.79, 152 between 0.80 and 0.89,
and 203 were more than 0.90. Thus only 1% percent of
the correlations resulted in coefficients less than the
adopted acceptable lower limit of 0.7.

REGIONAL RUNOFF OHARAOTERISTICS

Several illustrations show 1'.l1e yearly runoff, in log-
arithmic standard-deviation units, of each hydrologic
zone as comput.ed from the runoff of the streams com-
prising the zone. Figure 6 shows runoff for the two
groups of stations in the Pacific border zone; the dashed
line shows the mean for southern California, and the
solid line shows the mean for a group in Sahta Barbara
County and farther to the north. Both graphs indicate
a predominance of years with flow greater than median
in periods centering about 1910 and 1940, and a predom-
inance of years with less than median flow in periods
centering about 1900, 1930, and 1950, These major

trends reflect the fluctuations in precipitation noted by
Thomas (1962)-dry periods in 1892-1904, 1924-34, and
194&--56, and intervening wetter periods, There is a
distinction between the two groups in the magnitude of
deviation below the median during the three dry peri--
ods. In the southern group the annual flow was gen-
erally farther below the median during the 194&--56
drought and also during the 1892-1904 drought; in con-
trast, the flow at stations in the northern group was
farther below the median during the dry years 1924-34,
Thus the records of streamflow confirm the evidence
from precipitation records that the northern group of
stations is near the boundary of the area affected by the
recent Southwest drought.

TABLE S.-Summary of product-moment correlations of yearly
runoff for stations in each hydrologic region of the Southwest,
water yeaTS 1930-/j3

Stations comprising zones and regions
(see table 1)

Number of correlations giving coefficients

0.90-0.99 I 0.70-0.89 I 0.62-Q.69

Pacific border meteorological zone

1-6_____m
7-28_______

::::::::::::::::::::::::::::1

101
137

o
1

9!1

Sonoran horder meteorological lIone

lE!:::::::::::::::::::::::::::::1

11

~I

o
o
1

Great Basin~Colorado Plateau meteorological zone

It!!::::::::::::::::::::::::::::::::::1

311

.11
12

o
2
o

Great Plains meteorological zone

73-71L .. _________nun___nn__nnu 1 1 1
77-79_ no ___________u_ ____n_nn_n_ 0 3 0
80-85 ___u_ _____nn_n__nn___n_n_ 3 11 1
TotaL___ ------------------~---- 204 179 .

In the southern group of stations the 2-, 3-, and
5-year periods of minimum runoff occurred during the
1892-1904 drought, but the year and decade of minimum
runoff occurred during the 1946-56 drought, Thus, on
the whole, these two droughts were of about equal mag-
nitude in that region. On the other hand, among the
group of stations farther north, the runoff in 1-, 2-, 3-,
and 1O.year periods was least during the drought of
1924-34. Althoug~1 the driest 5 years of record in this
group were 1946-50, the effect of the drought of 1924-34
upon streamflow was generally more intense than the
recent drought in the central part of California,
The yearly runoff in the Sonoran border zone is
shown in figure 7. There are three groups of stations in
this zone that include stations along the headwaters of