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

B36

DROUGHT IN THE SOUTHWEST, 1942-56

year mean during the recent drought do represent devi-
ations of about the same order below the mean for the
154-year period.
Six of the nine tree-ring indices in table 7 cover
periods beginning prior to 1800 (column 12), Column
14 shows the ratio of the mean of tree-ring indices for
the bllBe period 1904-53 to the mean for periods that
range from 305 to 855 years. To the extent that tree
rings reflect runoff, this is the ratio of mean runoff :for
the period 1904-53 to that for the 305- to 855- year pe-
riods, For all the regions, except the headwaters of
the Gila River, the ratios are within 1 percent of those
given in colnmn 11 for a 154-year period; this small
difference suggests that it matters but little whether the
mean runoff for 1904-53 is compared to the mean for
154 years or for longer periods,

QUALITY OF WATER

By L. R. KISTER

Wherever "water" has been mentioned heretofore in
this report, the common usage of the word has been in-
tended: something wet and, in fact, the most abundant
of wet substances, In discussing the processes of the
hydrolog-ic cycle (1" Bl) "water" includes rainwater,
river water, soil water, lake water, wel! water, spring
water, and even sea water. Now, however, we want to
emphasize the great variety that is embraced by the all-
inclusive term "water," and to discriminate waters on
the basis of their physical and chemieal qualities,
To the chemist, water is the chemical compound H,O,
Long ago, when the chemistry of water was considered
to be relatively simple, it was recognized that pure H20
is not found in nature but must be obtained by such
processes as artificial distillation. However, as pointed
out recently by Buswell and Rodebush (1956), the for-
mula of water is not simply H20, and water is not a
single substance, The purest water that can be pre-
pared in the laboratory contains three isotopes of hy-
drogen and three of oxygen, which can be combined in
18 different ways, With the various kinds of ions that
can be formed from water's atoms, pure water contains
no fewer than 33 substances. Thus the formula H20
for pure water is a group designation,
Natural waters are solutions, suspensions, and mix-
tures of a great variety of chemical compounds and ele-
ments in H20, Water in each phase of the hydrologic
cycle is likely to contain measurable amounts of such
impurities. Even the water precipitated as rain, snow,
fog, frost, or dew commonly contains soluble and sus-
pended substances, Analyses show greater concentra-
tions of chloride in coastal than in inland areas; so at
least part of these soluble substances evidently come
from the oceans. In interior areas of the Southwest,

torrential storms wash significant quantities of dust
and soluble salts from the atmosphere-materials that
were picked up from the land by wind prior to the
storm.
Surface water and ground water may be only slightly
more mineralized, or they may be far more mineral-
ized, than the precipitation from which they were de-
rived, Wherever we find these waters, their quality is
a product of the environment through which the water
has passed since it fell as rain or snow, BecaUSe en-
vironmental changes over the years are relatively slight,
it is likely that the variations in quality of water at any
specific point will be less than the great variations that
are noted in waters from different geographic locations.
Generally we assume a fair degree of uniformity in the
quality of water from any individual well or spring,
Although the range in quality of surface waters is
greater, the water in some streams is characteristically
clear; in others it is muddy; in some it is relatively
pure; and in others it is charged with mineral matter,
Nevertheless, there is abundant evidence of signifi-
cant changes with time in the quality of water from
specific sources, The quality of water in a flowing
stream changes as the discharge chrnges, During pe-
riods of low discharge, most stream waters are more
mineralized than when flood flows occur, The quality
of water in lakes rnd reservoirs changes in response to
changes in quality of the inflow and also to the effect
of evaporation from the reservoir, The quality of wa-
ter from some wells and springs also has chrnged with
time; many of these changes have been traced to the
effects of increllBing development and use of ground
water, but some are clearly the result of fluctuations in
the rate of natural recharge.
If changes in quality result from changes in quantity
of surface water, as indicated previously, climatic fluc-
tuations must affect the quality as well as the quantity
of water, Thus drought affects directly the quality of
water in streams and lakes and also in some ground-
water reservoirs. Drought may also have indirect ef-
fects upon the quality of both surface and ground wa-
ters by changing the environment through which the
water moves, In any specific environment, when there
is less water to carry sediment or soluble mineral mat-
ter, the total amount of sediment or soluble matter car-
ried must be less; and as tributaries or springs cease
flowing, their contributions of mineral matter to major
streams must be nil. On the other hand, the concentra-
tion of mineral matter in streams may increase greatly
as the volume of water is reduced. Thus the general
effect of drought would be to increase the proportion
of impurities in water, and yet reduce the total qurntity
of those impurities, because of the reduced quantity of
water.