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OCR for page 24
2
CAUSES AND OCCURRENCE OF DROUGHT
John A. Dracup
Civil Engineering Department
University of California, Los Angeles
Droughts are indiscriminate in terms of geography,
climate, and political boundary. Each and every region
of the United States has experienced conditions of below
normal rainfall and streamflow runoff at some time. As
noted by Buchanan and Gilbert (1977), "Hydrologists can
pragmatically report on recorded data which show that
almost every year, some area of our country experiences
the conditions which constitute a drought."
Many climatologists have noted a significant increase
in the variability of the weather during the past two
decades (CIA, 1974; Fritz, 1977; Schneider, 1977;
Wallis, 1977~. It is now apparent that the years 1956
through 1971 constituted an abnormally stable period in
terms of temperature and precipitation fluctuations, and
that the disastrous worldwide weather conditions of 1972
heralded the end of that area. As Mitchell (1977)
states, "It appears that we're returning to normalcy,
and that means greater variability in the weather than
This trend toward increased variability
in the weather may be due either to a random fluctuation
in the complex weather-generating processes or to a
large-scale climate change. Unfortunately, the brevity
of available hydrologic records makes it virtually
impossible to distinguish between these two possible
causes (Lettenmaier and Burges, 1978~. Nonetheless, the
Present increase in meteorological variability should
we're used to.t' ~
cause a corresponding increase in the occurrence of
hydrologic droughts, at least in the immediate future.
Hence from these considerations it is apparent that the
study of droughts is an extremely relevant and essential
aspect of water resources analysis.
A surprisingly limited amount of attention has been
given to hydrologic drought events in the literature
-24-
OCR for page 25
(Whipple, 1966). A cursory survey of the Water
Resources Abstracts substantiates this claim, it is
found that the ratio of papers indexed under "Floods" to
those indexed under "Droughts" exceeds 6 to 1 in all
cases, and was nearly 10 to 1 in 1976. In addition,
many of the "drought" entries actually refer to
meteorologic droughts or low flows rather than to
hydrologic drought events.
DEFINITION OF DROUGHT
What is a drought? It brings to mind extreme pictures
of emaciated humans, hunger, and famine,- or simply empty
reservoirs, parched fields, and dusty roads. However, a
precise definition of drought is difficult to obtain
(Dracup, 1980a). The difficulty in devising an
objective definition of drought is fourfold. One source
of confusion is the unavoidable diversity in the ways in
which various fields of study view drought events. A
water resource engineer views drought as a problem in
supply and demand. He may state "there is no drought in
the ocean," meaning, of course, that without a specified
demand no drought can occur however severe the
precipitation shortage may be. Thus the engineer views
drought as a shortage in streamflow runoff and water
storage.
The geophysicist's view of drought will include
climatology, meteorology, hydrology, limnology, and oil
physics (Yevjevich, 196 7~. The farmer and
agriculturalist view drought as a function of the
specific crop under cultivation. Others may view
drought as a function of the impact the drought has on
institutional and human activity and on their response
during the drought.
A second factor militating against an objective
drought definition is the variety of connotations given
to the term "drought" in different parts of the world.
For instance, in Bali any period of 6 days or more
without rain is considered a drought, while in Libya
droughts are only recognized after 2 years without rain;
in Egypt before construction of the Aswan Dam, failure
of the Nile River to flood constituted a drought,
regardless of rainfall (Hudson and Hazen, 1964~. In
Britain an "absolute drought" has been defined as a
period of at least 15 consecutive days without 0.01 inch
of rain on any one day, whereas a "partial drought" is
OCR for page 26
-26-
taken to be a period of 20 consecutive days for which
the mean daily rainfall does not exceed 0.01 inch
(Rodda, 1965~. Other examples exist in the literature
(Tannehill, 1947) that serve to demonstrate that
definitions of drought events are strongly related to
the climatological and geological traits of a particular
locale.
The third problem in drought definition from the point
of view of the hydrologist is that a drought event is
manifested in terms of both a precipitation deficiency
and a streamflow deficiency. Hence a thorough and
complete definition of drought events requires
consideration of both rainfall and runoff. However, due
to constraints of time, economic resources, and
professional expertise, most drought studies have
focused on only one aspect of the drought event. Thus
two drought definitions are often specified, one based
on precipitation and the other based on runoff.
Finally, there is a curious lack of uniformity in the
conventional terminology relating to different
hydrologic events. For instance, since the term low
flow denotes an annually occurring minimum flow of short
duration, one would expect the annually occurring
maximum flow of short duration to be called a high flow;
however, this is usually termed a flood event. In
addition, because the term high flow may be used to
refer to extended periods of above mean discharge, one
would expect extended periods of below mean discharge to
be called low flows; however, these are usually termed
drought events. This lack of symmetry in hydrologic -
terminology is shown schematically in Figure 2-1.
It is in this context that general definitions of
drought events have evolved. These definitions have, of
necessity, been broad in scope so as to apply to as wide
a variety of particular drought manifestations as
possible. An extreme example of this is the drought
definition offered by Matalas (1963~: "A drought is
defined, in a broad sense, as an extended period of
dryness." In addition, most definitions describe
drought in relation to some locally determined water
requirement. For instance, the U.S. Weather Bureau
defines drought as a "lack of rainfall so great and long
continued as to affect injuriously the plant and animal
life of a place and to deplete water supplies both for
domestic purposes and for the operation of power plants,
especially in those regions where rainfall is normally
sufficient for such purposes" (Havens, 1954~. A typical
OCR for page 27
—2 7—
Di sc ha rge
mean
o
FLOOD ~ I GH FLOW
LOW FLOW DROUGHT
. .
day month year
FIGURE 2-1 Classification of hydrologic events.
Duration
OCR for page 28
-28-
textbook definition of drought is given by Linsley et
al. (1982~: ''a period during which streamflows are
inadequate to supply established uses under a given
management system.'' A less subjective definition is
offered by Whipple (1966~: "the term drought will refer
to prolonged periods of runoff, averaging less than the
long term mean." The generality of these definitions
clearly leaves them open to subjective interpretation by
individual researchers, a situation that has often
provided a barrier to the advancement of the state of
the art of drought analysis.
In the midst of this ambiguity and complexity, one
potentially satisfying drought definition has emerged in
the past decade as being objective and yet flexible
enough to be applicable to a wide variety of drought
concepts. In effect the proposed approach merely
systematizes the intuitive intentions of the various
definitions mentioned above.
The drought definition of interest has been proposed
by Yevjevich (1967), and is based on the branch of
statistical analysis known as the theory of runs. This
is a method of analyzing a sequential time series of
stochastic or deterministic variables, and hence is well
suited to the study of hydrologic event. The
fundamental parameters of the runs of an annual
hydrologic series are shown in Figure 2-2. The
parameters best suited to drought definition are tL
(drought duration) and YL (drought severity). In the
terminology of the theory of runs, t is referred to as a
run-length and y as a run-sum. A third parameter may be
identified by forming the ratio of y to t; this
represents the average magnitude of the drought event.
The selection of xo, the base value index by which
all other values of a hydrologic variable x are
described, is an important decision. Figure 2-2
indicates that the run-sum and run-length are determined
by the choice of xo. Some candidates values for xo
are the mean of the x series, the median of the x
series, or an expression such as
X0 = Xm + e Sx
where Xm is the series mean, SX is the series
standard deviation, and e is an elective scaling
factor. It is noted that xo need not be a constant;
it may be a stochastic variable, a dete'~,~inistic
function, or any synthesis of the two.
OCR for page 29
—29—
Annual
Average
Flow
X
o
Runs Parameters:
-H
'\
ITCH
.~N _ _ V _ _~!
. . . .
l\
v
.
.
1 2 3 4 5 6 7
Ye = Run-sum above XO
YL = Run-sum below XO (drought severity)
YL/tL =
Run-length above XO
Run-length below XO (drought duration)
Drought magnitude
8 9 10 11
Years
FIGURE 2-2 Parameters of the runs of a hydrologic series.
OCR for page 30
-30-
Once the desired value of xo has been stipulated,
the time series is divided into upper (above xo) and
lower (below xo) sections. A primary advantage of
using runs to define droughts is that the run-sum and
run-length series of the upper and lower sections are
amendable to statistical analysis in order to determine
properties such as time dependence, probability
distribution, or serial correlation. These properties
may be determined analytically or by a suitable data
generation approach (e.g., the Monte Carlo method). In
the case of drought events this implies that the
statistical properties of drought duration, magnitude,
and severity may be assessed (Dracup et al., 1980b).
The objectivity of this method of drought definition
is one of its strongest advantages; if a number of
analysts select the same value of JO to be applied to
a particular hydrologic record, then each should derive
identical statistical properties for duration, severity,
and magnitude. The method's flexibility is due to the
freedom allowed in the choice of xo; the hydrologist
may use mean annual flow, while the agriculturalist may
prefer seasonal soil moisture. The combination of these
two characteristics in an approach to drought definition
and description is particularly attractive because of
the wide variety of drought concepts held throughout
various scientific disciplines.
CAUSES OF DROUGHT IN TEMPERATE LATITUDES
The material covered in this section concerns the
statistical, synoptic, and physical aspects of drought
on time scales of a month to several years. The current
literature reveals that the causes of drought are
complex and are not yet completely understood by
meteorologists and climatologists. Namias (1985) states
. . . it should be made clear that there are many
unsolved "mysteries" of drought. While some
physical understanding has been achieved for
droughts that last for a month to a season, spells
of years characterized by drought are poorly
understood, and thus remain on the agenda for
research climatologists.
When studying the factors responsible for drought, one
is struck by the "chicken and the egg" analogy, that is,
OCR for page 31
-31-
which of the processes actually occurs first? Perhaps a
gradient of sea surface temperatures (SST) first is
formed between the warm eastern Pacific and the cold
central and western Pacific. This process may cause
high-pressure cells in the mid-troposphere to emerge and
persist. This in turn causes subsidence (sinking) or
warm, dry air in the middle troposphere, which then
causes adiabatic heating, low relative humidity, and a
reduction in the growth of cumulus clouds. The result
is a reduction of precipitation.
The reduced cloud
cover and precipitation in turn increases insolation,
drying out the soil and increasing its albedo, thus
further aggravating the process. Let us investigate
each of these phenomena in turn.
Recently, it has been suggested that there are
important teleconnections (relationships between two
phenomena that are physically hundreds or thousands of
kilometers apart) within the atmosphere-ocean system
(Namias, 1978a). As shown in Figure 2-3, there appears
.
to be a teleconnection between a warm SST anomaly and
the continental high-pressure cell.
It is well known that high-pressure cells exist over
drought-affected regions and that companion oceanic
high-pressure areas exist simultaneously. As shown in
Figure 2-4, strong high-pressure areas exist over the
Atlantic and the Pacific as well as the continental
United States. These high-pressure areas help shape the
upper-level long-wave westerlies. In turn, the
positioning of these long-waves determine to a large
extent climatic variations. During drought periods
there is a general northward shifting of pressure
zones. Therefore the westerlies are displaced northward
over the continental United States during droughts,
bringing stronger than normal winds to the high
latitudes as shown in Figure 2-5.
The result of these atmosphere anomalies is the
presence of (relatively) warm, dry air in the middle
troposphere. In drought regions, the warm air aloft
subsides at the rate of several hundred meters per day.
The sinking of the dry air and the attendant adiabatic
heating of it inhibits precipitation through the
suppression of the growth of cumulus clouds. The
subsidence of the warm, dry air aloft is caused by air
flowing out of the bottom boundary layers of the
high-pressure cells. This air is replaced by the
further sinking of air masses aloft. The warm air aloft
resulting from the subsidence is shown in Figure 2-6.
OCR for page 32
—32—
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OCR for page 33
—33—
AUGUST I 936
I,:
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700 mb
. .
CINear Normal
Abme Normal
TEMPDN
FIGURE 2-4 (Top) Average contours of the 700-mb surface for
August 1936, a drought month. (Bottom) Average temperature
departures from normal (OF) for August 1936. (DN: departures
from normal).
SOURCE: Beran and Rodier (1985). Permissions granted World
Meteorological Organization and UNESCO Press.
OCR for page 34
—34—
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OCR for page 38
—38—
700 rnb SUMMER MONTHS
TELECO~CTIONS ( CROSS -CORRELATIONS )
''I ~-
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V'~'-~
Mali
l
l .
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FIGURE 2-7 (Top) Teleconnections between 700-mb heights over
the field as correlated with a point in the North Pacific (label'
1.00~. (Bottom) Same with a point in the North Atlantic (labeled
1.00~. Note in each case the positive correlation with 700-mb
heights over the central United States.
SOURCE: Beran and Rodier (1985~. Permissions
Meteorological Organization and UNESCO Press.
granted World
OCR for page 39
-39-
at a point. The variable Y may take the form of an
index summarizing the total weather situation derived
using the principal components method. The forecasting
X variables must be observable at the time the forecast
is to be made and like the Y variable can include prior
values of pressure and temperature, summarizing indices
sea surface temperature, wind, ice extent, and even
sunspot number or some other cyclic variable. The
estimation of the coefficients be, bl, etc., is by
least squares and makes use of a run of back data,
typically 30 years for seasonal or annual forecasts.
In the case of streamflow, soil moisture and climatic
factors such as precipitation and temperature are used
as the independent variables.
V ~ 1 ~ ~ ~
CURRENT OPERATIONAL DROUGHT FORECASTING TECHNIQUES IN
CAL IFORNIA AND THE WESTERN UNITED STATES
The current operation drought forecasting techniques
as presented here are divided into meteorological
methods and hydrological methods.
Meteorological Forecasts
The Scripps Institute of Oceanography (SIO) provides
the California State Department of Water Resources
(CDWR) with seasonal temperature and precipitation
forecasts as shown in Figure 2-8. Namias (1984) states
that these forecasts are
physically based using statistical and synoptic
methods which employ large scale fields of Northern
Hemisphere atmospheric geopotential height and
Pacific Ocean sea surface temperatures.
The forecasts are made in equally likely tercile
classes, light (L), moderate (M), and heavy (H). The
forecasts are called "experimental" and are part of
ongoing research at SIO aimed at improving
long-range-forecasting techniques. A total of four
forecasts is made each year starting in September. The
fall forecast is for September, October, and November
In December, the winter forecast is for December,
January, and February. Similarly, three-month forecasts
are made for the spring and summer months.
In addition
OCR for page 40
-4~
PREDICTED
WINTER G82-83 tOEC. 82, JAN., FEB. 83 )
j.~
TEM,PERATURE ~` ~ ~.·~-.·.~ .~\
B—BELOW NORM i 4
~ ~ i
N—NORMAL ! rV./
A—ABCVE NORM i 1 ~1
~1
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_ ~ _-;~ · · a · ~ ~~ -~.~5 A
~ ;, _ · _ · ·~a · ~ . · · · ·t ~ . ~ . .] ~ . .~. )~ IV
/ ~ ~ ~ ~ ;i- ~ - ~ - e>. ~- a - ~; ~ i ~ H · ~ ~ . ~ ~ .-2`. ~ -e ~ ~ a ;~. ~ _. 3~\
r" _, ;~ '~~ ·-a!- ~ ~ · ~e~" '~ · '- · ·~< ~ \
, ~' · ~ ~ ~t · ~ ~ ~ 's ~ te ~ ~ ~ , ~ ~ a ~a a a ~ ,j - e . ~ \
_ ~ {~e ~ ~ i · - ~ -'~--~i~ ~-~ - ^~;'—;-~~ / \ ~
\/ _
, L—LIGHT ~ | ' 'ce
· · . . . . · ., . · ~ · .—.. . ~ - . - . -1. - . ,. -~ , ~
~L ~ [~~ a —~al a ·~a~-~a~~'~.-~ ' 1t A ~ \
-/~\ ~ ~-- - \~a-~-~;~;~;~;~;-\ -7~~ ~ 1Vl_ \
I PRECIPITATION l - ` · e~ · ~ ~ ~ ~ ~ - ~ ~ - ~ ~ [a · /~ ~ \
I M—MODERATE i /
UFA\/v I I
Completed Nov. 29,1982 from data ending Nov. 23,1982 J.lMamias
FIGURE 2-8 Forecasts of temperature and precipitation for
winter 1982-1983.
SOURCE: Namias (1984~.
OCR for page 41
-41-
to the precipitation forecasts, the movement of various
upper level winds and storm tracks also are predicted
shown in Figure 2-9. In addition to pictorial
forecasts, an annual report is furnished (Namias,
1984~. The report contains a discussion of results of
the project forecasts to date and an analysis using a
"skill score."
The CDWR translates these forecasts into California
Water Supply Outlook report (1985) and a State Water
Project Water Delivery Rule Curve and Criteria for 1985
report. These reports are mainly used to operate
reservoirs throughout California mainly for irrigation
supply deliveries (agricultural irrigation accounts for
87 percent of water supply deliveries in California).
In addition to DiC tonal
Hydrologic Forecasts
The National Weather Service (USDC) and the Soil
Conservation Service (USDA) jointly publish monthly
reports on Water Sunplv Outlook for the Western United
States for f
year. Similarly, the CDWR publishes a report entitled
Water Conditions in California. These reports contain
streamflow forecasts for the entire water year (October
through September), streamflow forecasts for specific
on
irrigation periods (April to September), and data
current reservoir storages. It is important to note
that these forecasts are for unimpaired flows. Similar
reports are published throughout the United States.
The meteorologic and hydrologic forecasts are
principally used to allocate irrigation water supplies
to senior and junior irrigation districts with
appropriative rights in California and throughout the
West.
The only "drought index" used in California is the
'iFour River Index," which comprises the sum flows of the
Sacramento River, the American River at Folsom, the Yuba
River at Smartville, and the Feather River inflow to
Oroville Dam. This index is published in Water
Conditions in California (Bulletin 120-85) and is
-
pr~mar~ly used as a means to meet salinity standards in
the Sacramento Delta. Other input to the Water
Conditions in California report include snowpack
measurements, precipitation, reservoir storages, and
streamflow runoff.
OCR for page 42
—42—
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OCR for page 43
—43—
FORECAST ACCURACY
In Thomas Heggens classic play Mister Roberts (1946),
he describes life aboard a cargo ship during WWII:
Now in the waning days of the second World War,
this ship lies at anchor in the glassy bay of one
of the back islands of the Pacific. It is a Navy
cargo ship. You know it is a cargo ship by the
five yawning hatches, by the house amidships, by
the booms that bristle from the masts like
mechanical arms. You know it is a Navy ship by the
color (dark, dull, blue), by the white numbers
painted on the bow, and unfailingly by the thin
ribbon of the commission pennant flying from the
mainmast.
Heggens goes on to state:
It has shot down no enemy planes, nor has it
fired upon any, nor has it seen any. It has sunk
with its guns no enemy subs, but there was this
once
that It rlreo. Tnls periscope, the lOOKOUt
sighted it way off on the port beam, and the
Captain, who was scared almost out of this mind,
gave the order: "Commence firing "' The five-inch
and the two port three-inch guns fired for perhaps
ten minutes, and the showing was really rather
embarrassing. The closest shell was three hundred
yards off . .
.
Hopefully, meteorologic and hydrologic forecasts are
currently doing better than the guns of the USS
Reluctant.
Namias evaluates his meteorological forecasts for
California and the western United States using the
following skill score equation:
Skill =
Correct Forecasts - Correct Forecasts Expected by Chance
Total Forecasts - Correct Forecasts Expected by Chance
For the time period 1975-1976 through 1981-1982, skill
scores of approximately 0.67 were experienced. However,
during the 1982-1983 water year a skill score of only
+0.25 (6 out of 12 correct forecasts) was achieved, and
during the 1983-1984 water year a skill score of 0.00 (4
OCR for page 44
OCR for page 45
-45-
These and other problems raise such questions as who
is using these forecasts, what are their actions as a
result of the forecasts, and do the long-term benefits
exceed the potential short-term costs? That is, can
many years of benefits be countered by a single lawsuit
from a single faulty forecast? Or should these
forecasts be made by the private, rather than the
public, sector, which would assume liability via a
corporate structure?
-
CONCLUDING REMARKS
This chapter, which focuses on the causes and
occurrence of droughts, deals with the technical aspects
of this subject. However, in considering this topic and
the broader issue of drought management and its impact
on public water systems not only the engineering and
technological aspects of the problem should be analyzed,
but also the areas of economics, finances, legalities,
politics, society, and the environment. For example,
suppose an elaborate international network for drought
prediction were developed, which included ground
sensors, report collection stations, telemetering
satellites, simulation models, and information
dissemination centers. Such a system would be required
to answer such questions as do the benefits exceed the
costs, who will finance such an enterprise, is such a
system completely legal under present laws, is it
supported politically, will society accept the end
results, and is it environmentally feasible? Only if
such a drought prediction system can successfully pass
each of these feasibility tests will it actually be
established.
REFERENCES
Beran, M. A., and J. A. Rodier. 1985. Hydrologic Aspects
of Drought, Studies and Reports in Hydrology Series,
UNESCO/WMO Panel, Rep. 39.
Buchanan, T. J., and B. K. Gilbert. 1977. The drought:
A pervasive problem. Water Spectrum 9~3~6-12.
California Department of Water Resources. 1985. Water
Conditions in California. Bull. 120-85. Sacramento
Calif.
OCR for page 46
—46—
California Department of Water Resources. 1985. State
Water Project Water Delivery Rule Curve and Criteria
for 1985. Sacramento, Calif.
California Department of Water Resources. 1985.
California Water Supply Outlook. Sacramento, Calif.
Central Intelligence Agency. 1974. A Study of
Climatological Research as it Pertains to Intelligence
Prob lems. Office of Research and Development, CIA,
Washington, D.C.
Charney, J. G. 1975. Droughts in the Sahara. Science
187:435-436.
Dracup, J. A., K. S. Lee, and E. G. Paulson, Jr. 1980a.
On the definition of droughts. Water Resour. Res.
16~2~:297-302.
Dracup, J. A., K. S. Lee, and E. G. Paulson, Jr. 1980b.
On the statistical characteristics of drought events.
Water Resour. Res. 16~23:289-296.
Dracup, J. A., D. L. Haynes, and S. D. Abrams on. 1985.
Accuracy of Hydrologic Forecasts, Proceedings of the
53rd Annual Meeting, Western Snow Conference, Boulder,
Colo.
Findlater, J. 1977. A Numerical Index to Monitor the
Afro-Asian Monsoon During the Northern Summer.
Meteorol. Mag. 105:134-143.
Fritz, H. C. 1977. Cited in Drought--Is Stable Climate
at an End?, Los Angeles Times. Pp. 1, 20, 21.
Glantz, M. H. 1982. Consequences and responsibilities in
drought forecasting: the case of Yakima, 1977. Water
Resour. Res., 17~1~:3-13.
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Representative terms from entire chapter:
drought definition
