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

B6

DROUGHT IN THE, SOUTHWEST, 1942-56

In several hydrologic studies in the Southwest, the
runoff as shown in available records has been segre-
gat.ed in the base flow and other components, and t.he
base-flow hydrograph has been analyzed separately.
From st.udy or the base-flow hydrograph ror a st.ream
draining a small mount.ainous area in sout.hern Cali-
rornia, Troxell and ot.hers (1954, pI. 10) have concluded
that.: t.he base, flow responds closely t.o changes in
g'round-wat.er storage wit.hin the drainage basin; in
many or the years or available record, t.here has been
practically no ground-wat.er recharg'e; and in other
years, t.he tot.al volume or recharge within t.he drainage
basin has been comput.ed rrom the base streamflow,
In st.udies or the San Ant.onio area in Texas, Petitt. and
George (1956, 1" 21-41) also separat.ed t.he base flow
rrom t.he f1oodflow or st.reams t.hat. recharge t.he princi-
pal ground-water reservoir in the area (Thomas and
ot.hers, 1963b).
Changes in rate or ground-wat.er discharge by evapo-
transpirat.ion are not measured quant.it.at.ively ror any
part or the Sout.hwest., except. in small experiment.al
plots or t.anks in connect.ion with special st.udies. Qual-
it.ative indicat.ioos or t.he effect.s or drought. upon such
discharge are reported, as rollows, rrom numerous 10'
calities: Alfalfa or other crops decline in yield where
t.hey are dependent upon nat.ural subirrig'ation; salt..
cedar, saltgrass, mesquit.e, and other phreatophyt.es
grow less luxuriantly than in rormer years, But. such
evidence is most. convincing when it is corroborat.ed by
evidence or lowering or t.he water t.able, which is con-
sidered in the rollowing section,

GROUND. WATER STORAGE
The term "ground-water reservoir" implies accumu-
lation or wat.er underground, which is analogous in
many respects t.o the storage or water in surrace reser-
voirs. l{eservoir operations have some similarity to
business or banking operations where t.here are daily,
seasonal, and annual f1uctuat.ions in income and similar
hut noncoincident f1uctuat.ions in outgo and where t.here
is some reserve t.o draw upon in periods or peak outgo,
:For efficient reservoir management, whether surface
water or ground water, one should have a cont.inuing
invent.ory or t.he st.orage in the reservoir and or th~
inflow and outflow. And he should know t.hese ractors
well enough to enable him t.o anticipat.e t.he rut.ure and
project his operations accordingly, The basic dat.a
needed ror efficient reservoir management, t.hererore,
are t.hose t.hat. permit computations or the reservoir
cmpacit.y, the usable storage in t.he reservoir, inflow or
recharge t.o t.he reservoir, and t.he outflow or discharge
from the reservoir,
At present, our knowledge or most ground-wat.er
reservoirs is so meager t.hat. t.he best we can do is t.o

make a rough guess or the usable storage. For many
of t.he developed reservoirs, available data are sufficient
only to indicate the changes in storage, which are usual-
ly measured annually but. somet.imes at. longer intervals.
The basic dat.a ror est.imat.ing t.hese changes are peri-
odic ,measurements or water levels in a net.work of ob-
servat.ion wells.
An important ract.or in ground-wat.er reservoirs, and
one that does not apply t.o the open wat.er ip. surrace
reservoirs, is the variable but. prevailingly slow rates
of movement. of wat.er t.hrough t.he rock mat.erials of
the reservoir, Some wells can be pumped so heavily
t.hat t.hey go dry and cause neighboring wells t.o do t.he
same, not. because the supply in t.he reservoir is ex-
haust.ed but because of t.he t.ime required for wat.er from
the rest. of t.he reservoir t.o move in and replace t.he
water pumped, Some ground-water reservoirs are con-
fined under beds of clay or ot.her impermeable mat.erial,
so t.hat. there is no possibility or recharge from above,
and t.he recharge occurs inst.ead in some dist.ant. area
where t.he reservoir is not confined, Wells in the con-
fined part or t.he reservoir may be limit.ed t.o a dischargc
far less t.han the recharge t.o t.he reservoir, because of
t.he slow rat.e of movement from the recharge area t.o
t.he wells.
Measurements of wat.er levels are made annually,
seasonally, moot.hly, or oftener in thousands or wells,
and records of fluctuations are obtained rrom cont.inu-
ous recording gages in hundreds of wells in t.he seven
Southwest.ern States, For numerous ground-wat.er
reservoirs, t.he available data are sufficient. t.o permit.
computat.ion or the volume of rock materials that have
been sat.urated or unwat.ered in a designat.ed year,
seaso/l, or ot.her period; and for several or t.hese reser-
voirs, the amount. of water required t.o saturat.e t.his,
volume of material can be estimat.ed with a rail' degree
of reliability. For many other I-tround-wat.erreservoirs,
however, the only indicat.ions or f1uct.uat.ions in st.orage
are those t.hat. can be gleaned rrom observat.ions in a
single well or in a rew widely spaced wells, Such
records are not an adequate basis for evaluat.ing the
effect.s of drought upon ground-water st.orage, but they
represent. the only available dat.a ror many part.s of t.he
Sout.hwest.
A major handicap in st.udying t.he effect.s of drought.
upon ground-water storage since 1942 is t.he shortness
or records of wat.er-Ievel f1uctuat.ions in many or t.he
existing observation wells, Records begun subsequent
t.o W41 provide no basis for comparing condit.ions dur-
ing t.he drought. wit.h predrought. condit.ions. Only t.he
rew records t.hat have been continued ror more t.han
30 years afford any basis for comparing t.he effects of
t.he latest. drought. and or earlier droughts,