WSP08775

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 1/1/1979
• Author: USGS
• Title: Hydrologic and Human aspects of the 1976-77 Drought
• Water Supply Pro - Doc Type: Report/Study

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DROUGHT AND ITS RAMIFICATIONS

The average deficiency for 1974-77 is close to
twice the rates of the previous droughts listed
above, and the total deficiency of 29.49 in. is
81 percent of that from 1927-34 and 70 per-
cent of that from 1958-66; yet it developed in
just 3 years rather than in 7 or 8 years.

DROUGHT AND ITS RAMIFICATIONS

Numerous definitions of drought have been
proposed and used by prominent individuals and
organizations, yet only the very generalized
definitions have much of a relation to the
drought of 1976-77. A precise definition is not
practical because a drought is the result of
many complex factors acting on and inter-
acting with the environment.
Among the natural factors are the climate
of an area; the antecedent conditions as exem-
plified by the amounts of soil moisture, rain,
and snow; the distribution of rain and snow in
time and space; water-table levels during the
drought; water quality; and soil types.
Human factors include the degree of devel-
opment of water storage and distribution sys-
tems; the number, locations, and depths of
wells; the patterns of water use and per capita
consumption; the legal aspects relating to
property rights, project operating rules, water-
quali ty standards, and service contracts; eco-
nomic considerations; and many more.
Therefore, a definition of a drought must
be ta ilored to the conditions in an area at a
given time. Even so, the result is a general
definition. A drought may be defined as a
condition where and when the water supply is
deficient enough and for a long enough time to
damage the growth of vegetation, industrial
production, or domestic activities (J. S. Crag-
wall, Jr., written commun., 1977). Nace and
Pluhowski (1965) mention the concept that a
drought occurs wherever there is less water
than the amount to which people have become
accustomed. [Emphasis by writer.]
The World Meteorological Organization has
defined six types of drought as follows (Subrah-
manyam, 1967):

1. Meteorologic drought-<:lefined only in
terms of precipitation deficiencies in abso-
lute amounts, for specific durations.
2. Cli matological drought-<:lefined in terms
of pl'ecipitation deficiencies, not in spe-

5

cific quantities but as a I'atio to mean or
normal values.
3. Atmospheric drought-<:lefinitions involve
not only precipitation, but possibly temper-
ature, humidity, or wind speed.
4. Agricultural drought-<:lefinitions involve
principally the soil moisture and plant be-
ha viol', perhaps for a specific crop.
5. Hydrologic drought-<:lefined in tel'ms of
reduction of stream flows, reduction in lake
or I'eservoir storage, and lowering of
ground-watel' levels.
6. Water-management drought-this classifi-
cation is included to characterize water
deficiencies that may exist because of the
failul'e of water-management practices or
facilities such as integrated water-supply
systems and surface or subsurface storage
to bridge over normal or abnormal dry
periods and equalize the water supply
through the year.

Each type is rather restrictive and by
definition describes only one part of a complex
whole.
Also, the kinds of drought enumerated by
the Wodd Meteorological Organization may
OCCUI' sequentially, or ovel'lap, or be combined
in several ways. For example: A small
amount of precipitation, a meteorological
drought, over a long enough period will become
a climatological drought as the precipitation
will be a small pel'centage of normal. In turn,
streamflow and storage in reservoirs and under-
ground will be reduced to create a hydrologic
drought which brings on problems of distribu-
tion and use which is a water-management
drought.
In humid regions where rainfall normally
occurs frequently during the growing season, a
drought effect on nonirrigated agriculture can
be achieved after only a few days of hot, dry
weather. J. C. Hoyt (1938, p. 2) stated:

In general, however, in humid and semi-
arid states there are no serious drought
effects unless the annual precipitation
is as low as 85 percent of the mean;
tha t is, unless there is an annual defi-
ciency of 15 percent or more.

Hoyt also mentioned that the above cri-
terion has shortcomings because it does not
considel' the effects of temperature and the