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

B5

effect of precipitation upon ground-water reservoirs
are based chiefly on data concerning changes with
t.ime in ground-water discharge and ground-water stor-
age and on the comparison of these changes with the
fluctuations shown by records of precipitation.
By terms of the hydrologic equation, fluctuations in
t.he rat.e of inflow may be reflected in bot.h t.he rate of
discharge and in changes of st.orage in a ground-water
reservoir, and st.udies have shown t.hat. t.his is usually
t.rue. However, t.here may be a wide range in t.he degree
to which changing recharge affect.s the rate of discharge
and the storage in the reservoir, At one extreme, per-
haps best. represented by reservoirs that discharge
through springs at virtually uniform rates, changes in
t.he rate of inflow are reflected almost. entirely by the
changes in storage. At the other extreme, the storage
in a ground-water reservoir may remain practically
constant despit.e changing rates of rechange, which in-
stead cause variations in the rate of discharge. The
great majority of ground-water reservoirs have suffi-
cient st.orage volume so that. t.hey can absorb recharge
at greatly varying daily, seasonal, and annual rates lUld
discharge wat.er at far more uniform rates. Thus the
nat.ural discharge from a reservoir t.hat. is not. affect.ed
by development. may approximat.e the long-t.erm aver-
age rat.e of recharge.

GROUND-WATER DISCHARGE
Ground water is discharged nat.urally by springs, by
seepage into streams, and by evapot.ranspiration where
the wat.er t.able is near enough t.o t.he land surface, This
ground-wat.er discharge is generally at. a more constant.
rat.e than st.ream discharge, and some springs flow at.
remarkably uniform rat.es. Nevertheless, t.here is some
variation in rat.e of flow at. pract.ically all point.s where
ground-wat.er discharge has been measured syst.emat.i-
cally over a period of years. The f1uct.uat.ions may be
daily, seasonal, annual, or of longer period, and some
have been correlated wit.h climatic fluctuations.
Presumably, ephemeral or "wet.-weat.her" springs flow
in direct response to recharge, ,vhieh in turn may come
from rainfall, snowmelt., or st.reamflow. Such springs
are widely distributed-more numerous in mountains
!md foot.hills in the Sout.hwest t.han in the arid lowlands
and doubtless more numerous in humid regions of the
East. and North than in the Southwest.. Some ephem-
eral springs flow for several months each year, others
only after st.orms, still others after a season or a year
of exceptionally abundant. precipit.at.ion. Although
such springs are numerous, there is very little quantita-
tive informat.ion concerning fluct.uations in discharge of
individual springs. Presumably many of these ephem-
eral springs const.it.ute t.he point.s of natural discharge
from very small ground-water reservoirs; they would
690-2120-68-2

t.hus be close enough to the recharge area that their rate
of discharge reflect.s fluctuations in the rate of recharge.
Perennial springs have been sources of wat.er for
domest.ic use since the early days of settlement of the
Southwest, and several of t.he larger ones have been
developed for municipal, irrigation, and industrial sup-
ply. In recent years many springs that. reportedly have
gone "dry" are not known t.o have ceased flowing in any
earlier years. The cessation of flow has commonly been
ascribed to drought., and evidence is indeed abundant
t.hat the less than average precipitation in recent years
has been a factor in t.he reduction of discharge from
many springs. In order to ascert.ain the effect of
drought upon a specific spring, however, it is necessary
to know to what. extent the spring discharge has been
affected by withdrawal from wells,
Fluctuat.ions in the discharge of many springs, partic-
ularly the larger springs, are known from frequent
periodic measurements or continuous recording gages,
some of which span periods of 30 years or more.
Among the available records are some for springs whose
discharge f1uctuat.es markedly in response to f1uctua-
t.ions in monthly and even in daily precipitation
(Thomas and others, 1962; 1963a; 1963b) and other
springs in which the discharge reflect.s climatic fluctua-
t.ions only faintly or wit.h considerable timelag (Thomas
and others, 1963c). In still other springs, the effects of
climat.ic f1uct.uations are either insignificant (Thomas
and others, 1963c) or are masked by the effects of pump-
ing from wells (Thomas and others, 1962). Some
springs show marked effects of wet and dry weather,
as well as of pumping from wells (Thomas and others,
1962). The only generalization that can be made about.
the effect of drought upon springs throughout the
Southwest is that the effect is varied,
The ground-water discharge into streams const.it.ut.es
a part. of the st.reamflow whose relations to drought are
discussed on pages B7-B15. Although the rate of
ground-wat.er discharge to any specific stream varies
from time to time, the variation as a rule is far less
t.han t.hat in total discharge of the stream, After an
extended period of dry weather throughout the drain-
age basin, the entire flow of the stream may be derived
from ground water, which thus provides the base flow
of the stream and is a feature of perennial streams as
distinct from ephemeral streams, During rainy
weat.her, the ground-wat.er contribution is presumably
greater t.han in dry weather, but it constitutes a far
smaller proportion of the total streamflow. Thus
ground-wat.er discharge t.o streams, by remaining com-
paratively constant from season to season and from year
to year, is a highly variable proportion of the total
runoff,