Thomas Eugene Pond. AN ANALYSIS OF DROUGHT COMPONENTS IN THE
EASTERN UNITED STATES. (Under the direction of Richard A.
Stephenson, Department of Geography and Planning, November,
1986).
The purpose of this research is to study drought components
in the climatic regions of the United States east of the
Mississippi River. It is hypothesized that drought associated
with temperature variation exhibits unique spatial patterns.
This investigation adds a different spatial dimension related to
drought phenomenon. The areal association of temperature as an
altering factor upon environmental moisture status is evaluated.
Research related to drought analysis has traditionally
focused upon negative departures from precipitation normals.
Only in recent years has temperature variation as an aspect of
drought and its intensity been considered. These studies
incorporated temperature-induced évapotranspiration demands as an
element significant to either intensifying or ameliorating an
area's moisture status, but failed to separate the moisture
supply and moisture demand parameters as component elements.
This study shows that there is an areal differentiation
regarding the impact of above normal temperatures on the
phenomenon of drought.
AN ANALYSIS OF DROUGHT COMPONENTS
IN THE EASTERN UNITED STATES
A Thesis Presented to
the Faculty of the Department of Geography and Planning
East Carolina University
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts in Geography
by
Thomas Eugene Pond
November 1986
DI \nl^ V
AN ANALYSIS
OF DROUGHT COMPONENTS
IN THE EASTERN UNITED STATES
by
Thomas Eugene Pond
CHAIRMAN OF THE DEPARTMENT OF GEOGRAPHY AND PLANNING
DEAN OF THE GRADUATE SCHOOL
Joseph G. ^Pfdyette
ACKNOWLEDGEMENTS
Grateful acknowledgement is extended to Dr. Richard A.
Stephenson for his assistance and encouragement in directing this
thesis. Special thanks also go to Dr. Donald Steila for his
guidance and support throughout this study.
Additional thanks go to Suzanne Reynolds for help in
preparing the final copy; and to my parents and employer for
their encouraging support.
TABLE OF CONTENTS
Page
L
CIIIIST OF Th.IaDIIp..DITtSeRHrCOInDDDDEThMEThUUASBGLSHEISTO: ANND FIGURES vCONCEPTS AND MEASUREMENT TECHNIQUES 1troduction 1rought Definitions 3rought Measurement: Past 5rought Measurement: Recent 8rought and the Concept of PotentialEvapotranspiration 9STUDY AREA AND METHODOLOGY 12e Study Area 12ethodology 12xplanation of the Sample Water Budget 13e Steila Drought Index Methodology ISOF SELECTED EXAMPLE 23
The Mountain Climatic Division of South Carolina . . . .23
The Central Climatic Division of Georgia 25
The Southern Coastal Plain Climatic
Division of North Carolina 25
The Western Climatic Division of Kentucky 28
The Northeastern States 30
The Southeastern Climatic Division of Vermont
and the Southern Climatic Division of
New Hampshire 30
The Northern Climatic Division of New Hampshire .... 33
The Saint Lawrence Valley Climatic
Division of New York 33
The Southwest Climatic Division of Illinois 36
IV. ANALYSIS OF DROUGHT COMPONENTS 39
Precipitation 39
Potential Evapotranspiration 41
Available Soil Moisture 42
V. POTENTIAL EVAPOTRANSPIRATION DROUGHT
, INTENSIFICATION: AN AREAL ANALYSIS 44
Winter (January-March) 45
Spring (April-June) 49
Summer (July-September) 51
Autumn (October-December) 51
General Patterns 54
VI. SUMMARY AND CONCLUSIONS 5 8
BIBLIOGRAPHY 61
LIST OF TABLES AND FIGURES
Table Page
1. Climatic Divisions by Numbers 13
2. Sample Thornthwaite Water Balance 16
3. Steila Drought Indices 19
4. Occurrence-percentages of Drought Intensification
Attributable to Above Normal PE by
Climatic Division 46
Figure
1. The Study Area 14
2. Relationship of PE Departure Upon EMS 22
3. The Mountain Climatic Division of
South Carolina, 1954 24
4. The Central Climatic Division of Georgia, 1954 .... 26
5. The Southern Coastal Plain Climatic Division
of North Carolina, 1954 27
6. The Western Climatic Division of Kentucky, 1954 ... 29
7. The Southeastern Climatic Division of
Vermont, 1964 31
8. The Southern Climatic Division of
New Hampshire, 1964 32
9. The Northern Climatic Division of
New Hampshire, 1968 34
10. The Saint Lawrence Valley Climatic Division of
New York, 1963 35
11. The Southwest Illinois Climatic Division, 1953 .... 37
12. The Southwest Illinois Climatic Division, 1954 .... 38
13. Monthly Drought Occurrence-Percentages
Due to > PE (Winter) 4 7
vi
14. Monthly Drought Occurrence-Percentages
Due to > PE (Spring) 50
15. Monthly Drought Occurrence-Percentages
Due to > PE (Summer) 52
16. Monthly Drought Occurrence-Percentages
Due to > PE (Autumn) 53
17. Monthly Drought Occurrence-Percentages
Due to > PE (Autumn/Winter) 55
18. Monthly Drought Occurrence-Percentages
Due to > PE (Spring/Summer) 56
CHAPTER I
DROUGHT: CONCEPTS AND MEASUREMENT TECHNIQUES
Introduction
Moisture has long been recognized as being extremely
important to man as a necessary resource. Because it is a
necessary resource, major departures in the amount of available
water from that normally expected have often proved disastrous to
man and his environment. The measurement and prediction of these
departures have been of keen interest to researchers for decades.
Much effort has gone into the examination of positive
moisture anomalies, yet there exists only meager research
regarding the negative aspect of drought. No doubt the positive
deviations are more dramatic in terms of swift response, for
example, the fluctuations of stream head. Yet, the deviation
caused by the lack of moisture, such as damaging and destroying
property, crop, and livestock should be considered of major
significance. Of all insured crop claims in the United States,
drought accounts for 39 percent of indemnities paid (Annual
Report, Federal Crop Insurance Corp., 1963).
Departures from the normal precipitation for any location or
area for any period of time are expected. Seldom do average
climatic data express persistent conditions over extensive time
periods. If a particular moisture departure does not
2
significantly deviate from the mean moisture conditions, the
physical environment may well accommodate that departure without
deleterious affects, but deviations that reach the stress limits
of the physical environment are of major consequence to man
(Wang, 1963).
While drought or a negative moisture departure has
widespread geographic ramifications, the subject, with few
exceptions, has not been investigated in geographic detail. The
topic has received some attention in terms of cultural perception
and agricultural impacts; but the actual identification of its
onset, termination, degree of intensity, and magnitude has been
almost solely limited to the hydrologists, agriculturists,
meteorologists, and foresters (Saarinen, 1966; Hewes, 1963; and
Visher, 1943). Rarely have the investigations been applicable or
utilized for broadscale areal characterization of drought
patterns. However Shear and Steila (1971), utilizing the
Thornthwaite Water-Budget technique, have combined certain
Thornthwaite indices and arrived at an index of drought. The
Shear-Steila method has been applied throughout the southeastern
United States as well as Arizona and has proven to function quite
well in the recognition of drought and its intensity.
Clearly the identification of drought is of significance,
but also of importance is the impact of the individual components
of drought upon moisture status departures and intensities. The
impact of a moisture deficiency was vividly demonstrated during
the dust-bowl era, and since then dry periods have been a threat
to the forests and food supplies. The Shear-Steila Index will be
used in this study in the recognition of drought's areal
patterns, intensity, and components.
Drought Definitions
Drought is a phenomenon of widespread significance. The use
of the term in both professional and non-professional literature
is extremely varied. The following comments and definitions
demonstrate a lack of agreement regarding terminology and an
absence of reference to above normal temperatures being a
contributor to drought.
(1) Drought (common usage) reflects the relative
disparity of mankind in the face of natural
phenomenon that he does not understand thoroughly
and for which, therefore, he has not devised
adequate protective measure (Thomas, 1963).
(2) ... a spell of dry weather (Tannehill, 19A7).
(3) A period of ten days with a total rainfall not
exceeding a fifth of an inch of rain
(Tannehill, 1947).
(4) A valley of rain deficiency in the broad sweep of
time and weather (Tannehill, 1947).
(5) ... 15 days with no rain (Coble, 1933).
(6) ... period of thirty days or more with deficient
rainfall and not in excess of a fourth of an inch
in any 24 hours (Tannehill, 1947).
(7) An "absolute" drought ... a period of 14 consecutive
days without a hundredth of an inch in any one day
(Tannehill, 1947).
(8) drought ... a meteorological phenomenon which
occurs during a period when precipitation is
significantly less than the long term average
and when this deficiency is great enough and
continues long enough to affect mankind
(Thomas, 1963).
(9) Drought ... a relatively temporary departure
of the climate from the normal or average toward
aridity (Palmer, 1957).
(10) When precipitation is insufficient to meet the
needs of established human activities, drought
conditions may be said to prevail (Hoyt, 1938).
(11) drought ... is considered to be a period when
pasture growth is so far below normal that it
affects adversely the animals grazing in
particular areas (Everist and Moule, 1952).
(12) Any period in which tree growth was reduced for
five or more years has been considered to be a
drought period (Weakly, no date).
(13) drought . . . refers to a specific period of
time during which the total amount of rainfall
recorded at a station is deficient to the extent
that, more often than not, the corn yield falls
below normal for the county in which the station
is located (Barger and Thom, 1949).
(14) ... a period of deficient rainfall that is
seriously injurious to vegetation (Tannehill,
1947).
(15) a period of 21 days or more with rainfall 30
percent or more below the normal (Tannehill,
1947).
(16) A droughty condition is created if, in the
economic development of a region, man creates
a demand for more water than is normall
available (Hoyt, 1938).
(17) Drought is a phenomenon taking many forms,
but its most specific and most essential
feature is a disparity between the plant's
requirements of moisture and the latter's supply
from the soil (Kulik, 1965).
5
Clearly, the preceding examples show the concept of drought
is variable. To foresters, drought is the susceptibility of
surface conditions to the threat of fire. The concern of the
agriculturist, centers about his crop's water-needs and soil
conditions. To the hydrologist, drought is evidenced by dropping
stream and groundwater levels. However, basic to each of the
previously mentioned physical definitions is the fact that
drought represents a negative departure from the expected
moisture availability of an area.
Drought Measurement; Past
The measurement of drought intensity and the determination
of the threshold value which initiates the onset of the "dry
spell" have been no less varied than the many definitions
previously presented.
Between 1900 and 1910 the U.S. Weather Bureau tested for
drought utilizing rainfall records. "Any period of twenty-one
days or more with rainfall 30 percent or more below normal
(Tannehill, 19A7)," was considered a drought. In an application
of this measure to the District of Columbia, it was found that 62
cases of drought had occurred in 33 years. Further examination
of rainfall data indicated that in most cases cited, "there had
been ample or heavy rainfall preceding the dry period, and there
was enough soil moisture to support vegetation" (Tannehill,
19A7).
Though the status of available moisture for vegetation is
not the sole factor to consider with respect to dry periods, the
foregoing findings do reveal the inadequacy of several drought
indices. There is the failure to take into consideration the
water reservoir within the soil. As Jen-Yu Wang states, "during
drought (moisture deficient) soil moisture acts as a limiting
factor" (1963). Whether a drought is preceded by a wet or dry
period can add to the intensity of a given drought or aid in
relieving it. The level of available soil moisture at any time
provides an indicator to the moisture-status of the preceding
period.
Kulik (1965) described moisture deficiency with the use of
two terms: 1) a drought period occurs when the available
moisture reserves in the first twenty centimeters of soil are
less than 20 millimeters for a period of 10 days; and 2) a dry
period occurs when the available moisture reserves in the first
twenty centimeters of soil are less than 10 millimeters for a
period of 10 days.
A major drawback to Kulik's work is the soil moisture
requirement of 20 millimeters or more in the upper 20 centimeters
for drought not to exist. Under this assumption, the more porous
soils would always experience drought as would soils in arid
climatic zones. Though the more porous soils may be droughty in
nature, the climatic environment need not be experiencing dry
conditions. Soils nearby may be well above field capacity due to
recent precipitation. His methodology does not identify drought
but, in particular cases, identifies droughtiness of soils.
Kulik also classified soils with normal moisture retention
of less than 20 millimeters as continually experiencing drought,
thus, including the soils of arid climatic regimes. Similarly,
Thornthwaite (1963) has coined the term perennial drought for
those areas where potential évapotranspiration needs are not
sufficiently met by precipitation. However, can an arid climatic
regime actually experience drought? The answer should be
apparent. If Phoenix, Arizona expects to receive an average of
eight inches of precipitation a year and only receives three or
four inches in a given year, it is experiencing drought.
Utilizing Thornthwaite's terminology, however, Phoenix would be
experiencing perennial drought plus additional drought of 4 to 5
inches.
Referring to the list of drought definitions mentioned
previously, the one point of agreement is that drought represents
a period of time wherein moisture status is below that normally
expected. Phoenix does not experience perennial drought, but
does undergo period of drought.
Barger and Thom (1949) attempted a drought prediction study
in Iowa by correlating rainfall data with corn yields. Even
though this study produced positive results for Iowa, the study
8
was extremely limited in areal application. The great weakness
of this endeavor was that it measured drought solely in terms of
one crop--corn, and was useful only during the growing season.
Drought Measurement ; Recent
Keetch and Byrum (1968) developed an index for use by fire
control managers in estimating degree of drought. Their index
was based on soil moisture status and its relationship to the
flammability of organic fuels. This index portrayed, as
intended, a measure of flammability of fuels, but it is not
useful for portraying total environmental stress.
Carreker and van Bavel (1957), although not really developing a
drought index in the purist sense, have devised a methodology for
calculating drought probability. They are primarily concerned
with "agricultural drought" which they define as "a condition in
which sufficient soil moisture is not available in the root zone
for plant growth and development." They use four major governing
factors within their system: rainfall, évapotranspiration,
available soil moisture, and the moisture requirements of the
plant.
The most widely applied drought measure was devised by
Palmer (1965), who characterized drought solely in terms of
meteorological phenomena. His definition of drought is:
An interval of time, generally on the order of months
or years in duration, during which the actual moisture
supply at a given place rather consistently falls short
9
of the climatically expected or climatically appropriate
moisture supply. Further, the severity of drought may
be considered as being a function of both the duration
and the magnitude of the moisture deficiency.
Evapotranspiration is a variable in the Palmer Index, which
classifies a station into degrees of wetness (surplus moisture)
as well as into degrees of dryness (drought). The Palmer Index
breaks down moisture availability into eleven different
categories, perhaps unnecessarily detailed.
Palmer's methodology has gained a high degree of popularity
among U.S. Weather Service personnel involved with climatological
work and, though tedious and involving numerous variables has
been applied to several states. To the author's knowledge, the
following states have had the index applied: Alabama,
California, Georgia, Idaho, Kansas, Nevada, North Carolina, South
Carolina, Tennessee, Washington, and West Virginia.
The accomplishment of Palmer was without question a
monumental task which has stimulated further research into the
drought phenomenon. He expanded the scope of the drought concept
and provided a standardized definition that is applicable to
several aspects of the physical environment.
Drought and the Concept of Potential Evapotranspiration
In order to recognize drought as occurring at a given time
and place, there must first exist a water-need; since if no
water-need existed, negative departures would be of questionable
significance.
Solar heating induces a loss of moisture from the surface by
means of evaporation and transpiration. Transpiration maintains
temperature balance on leaf surfaces and is vital to plant
respiration. It prevents the surface of the leaf from over-
heating and represents a transfer of water to the atmosphere
through living plant cells. Evaporation is that amount of water
returned to the atmosphere from open areas and represents a
conversion in the state of water from its liquid to gaseous form.
In understanding the water status of an area, knowledge of
the balance between incoming moisture and that lost through
evaporation and transpiration must be considered. Lowery and
Johnson 91942) recognized the above fact and coined the term
"consumptive use." They defined "consumptive use" as being "the
quantity of water, in acre-feet per cropped acre per year,
absorbed by the crop and transpired or used directly in the
building of plant tissue, together with that evaporated from the
crop producing land."
Based on the foregoing and similar studies, attempts to
determine "consumptive use" followed. Thornthwaite (1948)
redefined the term, labelled it "potential évapotranspiration",
and stated:
Potential évapotranspiration is an index of thermal
efficiency. It possesses the virture of being an
expression of day length as well as of temperature.
It is not merely a growth index but expresses growth
in terms of the water that is needed for growth. Given
the same units as precipitation it relates thermal
efficiency to preciptation effectiveness.
11
Temperature is the principal factor in determining the
amount of "consiomptive use" and has been used in various formulas
to obtain potential évapotranspiration or water-need values for a
particular site. Holdridge (1964) notes that when temperature is
used alone as an indicator of potential évapotranspiration, it is
difficult to account for differences produced by local soils,
vegetation, and atmospheric conditions. However, if the
"specifications of a zonal soil and a zonal climate are included
in the definition of potential évapotranspiration, the
utilization of a nomogram or formula based on temperature alone
for obtaining a precise value becomes possible."
In sximmarizing the views on evaporation and transpiration
measurement, potential évapotranspiration may be defined as the
total amount of water which could be evaporated and/or transpired
under optimum conditions of soil moisture and vegetation on a
zonal soil and in a zonal climate.
Once the potential évapotranspiration requirements of an
area are established and subsequently balanced with incoming
precipitation over a long time period, it is possible to
determine the normal water status of the location. Negative
departures from the normal, or expected, water status provide a
measure of drought.
CHAPTER II
The Study Area
The area for this research includes the selected states
located east of the Mississippi River (Figure 1). To cover the
area adequately without using data from all climatic division
(155), approximately twenty-five percent of the divisions were
chosen for analysis through use of a random numbers table. All
states are represented except Delaware, Michigan, Massachusetts,
Ohio, Maine, and New Jersey. The climatic divisions chosen are
listed by an identification number in Table 1 with their
approximate locations are illustrated in Figure 1. Climatic
divisions are multicounty areas having basically the same
temperature and precipitation characteristics as defined by the
National Climatic Data Center.
Methodology
To determine the impact of moisture demand and availability
of moisture in this drought study, the Shear-Steila Drought Index
is used in conjunction with the Thornthwaite Water Budget Model.
Forty-seven years of monthly climatological data (1931-1977) were
analyzed via the Thornthwaite Model to establish moisture demand
and supply. These parameters developed within the Thornthwaite
Water Budget were then utilized to calculate the Shear-Steila
Index to determine the intensity of actual moisture-status
departure from normal.
13
TABLE 1
Climatic Divisions by Numbers
Alabama New Hampshire
1. Upper Plains 19. Northern
2. Piedmont Plateau 20. Southern
3. Coastal Plain
New York
Connecticut 21. Hudson Valley
4. Coastal 22. St. Lawrence Valley
Florida North Carolina
5. Northwest 23. Central Piedmont
6. Lower East Coast 24. Southern Coastal PI.
25. Northern Coastal PI.
Georgia
7. West Central Pennsylvania
8. Central 26. Central Mountains
Illinois South Carolina
9. Northeast 27. Mountain
10. Central 28. Southern
11. Southwest
Tennessee
Indiana 29. Middle
12. West Central
13. Southwest Vermont
30. Southeastern
Kentucky
14. Western Virginia
31. Tidewater
Maryland 32. Southwestern Mountain
15. Northeastern Shore
16. Allegheny Plateau West Virginia
33. Central
Mississippi
17. Upper Delta Wisconsin
18. Southwest 34. Northwest
35. Central
36. Southwest
Source: U.S. Department of Commerce, Weather
Climatological Data
14
THE STUDY AREA
(Numbers correspond to Climatic Divisions listed in Table 1.)
Figure 1
15
The data base was obtained from the Environmental Sciences
Service Administration and NOAA. The Thornthwaite Water Budgets
and Shear-Steila Indices were generated with a computer program
developed by Dr. Donald Steila ( 1971). An additional computer
program was developed, to determine the percent of departure from
normal moisture status attributable to each of the primary
components of drought: precipitation and évapotranspiration via
temperature.
A sample water budget of the South Carolina Mountain
Division-1965 (Table 2) illustrates how the Thornthwaite and
Steila methodologies interact. The first thirteen rows of the
Water Budget represent the Thornthwaite Model while the last row
depicts Steila Drought indices.
Explanation of the Sample Water Budget
The first row (T) contains a record of the average monthly
temperature. These averages were applied to the Thornthwaite
tables (Thornthwaite 1957) to obtain heat indices (l) represented
in row two.
Summing the monthly heat indices yields the annual heat
index. Using this total, unadjusted potential évapotranspiration
ratings (UNAD PE) were determined with Thornthwaite Tables and
recorded in row three.
Row four contains the duration of sunlight or latitudinal
correction factors related to variations in length of daylight
THORNTHWAITE WATER BALANCE*
SOUTH CAROLINA: MOUNTAIN DIVISION (1965)
IAN FEB MAR APR MAY lUN JUL AUG SEP OCT NOV DEC
Temperature (T) 38.30 38,.10 43,.60 57,.60 65.90 67.60 74,,40 71,.70 67 .60 54,.20 47,,70 40,.00
Heat Index (I) 0.58 0..55 1,.47 4,.87 7.45 8.02 9,.71 9,.46 8 .02 3,.92 2,.32 0,.84
Duration Sunlight (DUR SUN) 0.87 0,.85 1,.03 1,.10 1.21 1.22 1,.24 1,.16 1 .03 0,.97 0,.86 0,.84
Unadjusted PE (UNAD PE) 0.32 0,.30 0,.74 2,.24 3.31 3.54 4,.22 4,.12 3 .54 1..83 1,.13 0..44
Potential Evapotr2inspiration (PE) 0.28 0,.26 0,.77 2,.46 4.00 4.32 5,,23 4,.78 3 .65 1..78 0,.97 0,.37
Precipitation (P) 3.01 4,.84 7,.16 4,.15 4.19 4.02 4,.66 4,.33 4..01 3.,96 2,,14 0..36 Table
Precipitation-PE (P-PE) 22.73 4,.58 6..39 1,.69 0.19 -0.30 -0,.57 -0,.45 0,.36 2,,18 1,.17 -0.,01
Accumulated Water Loss (AWL) 0.00 0..00 0,.00 0,.00 0.00 -0.30 -0,.87 -1,.32 -0,.88 0,.00 0,,00 -0.,01
Soil Storage (ST) 6.00 6,.00 6,.00 6..00 6.00 5.71 5,.18 4,.80 5,.16 6.,00 6,.00 5..99
Surplus (SUR) 2.73 4,.58 6..39 1..69 0.19 0.00 0,,00 0.,00 0,.00 1.,34 1.,17 0,,00
Storage Change (ST CHG) 0.00 0,,00 0.,00 0,.00 0.00 -0.29 -0,,53 -0,.38 0,.36 0.,84 0.,00 -0,,01
Actual EvapotrMspiration (AE) 0.28 0.,26 0.,77 2..46 4.00 4.31 5,,19 4..71 3,.65 1..78 0,,97 0.,37
Deficiency (DEF) 0.00 0..00 0.,00 0,.00 0.00 -0.01 -0,.04 -0.,07 0,.00 0.,00 0.,00 0.,00
Drought Index (I) -1.75 0.,27 1..88 -0,.64 -0.26 0.03 -1,.16 -1.,87 -1,.15 -0.,18 -1..55 -4..39
^Temperatures are in degrees Farenheit, all others are in inches.
cr\
17
(DUR SUN). Unadjusted potential évapotranspiration values were
multiplied by correction factors to calculate potential
évapotranspiration (PE).
Row six is a record of average monthly precipitation (P) as
intercepted by standard rain gauges. Available moisture (P-PE)
is represented in the following row, a result of subtracting
potential évapotranspiration figures from precipitation.
Negative values in this row indicate that precipitation was
insufficient to meet the évapotranspiration demands for those
months.
Accumulated water loss (AWL) is the amount of unfulfilled
water demand. It is calculated by summing the increase of
potential évapotranspiration demands in excess of precipitation.
Stored soil moisture (ST) has the ability to offset some
moisture deficiency from inadequate precipitation and must be
considered as part of the available moisture.
In months where there is more moisture than demanded by
potential évapotranspiration, and the soil moisture is at
capacity, there is a surplus (SUR).
Changes in the soil storage (ST CHG) occur when soil
moisture is depleted by évapotranspiration or when there is a
soil moisture recharge taking place from excess precipitation.
Actual évapotranspiration (AE) is the amount of moisture
that is actually evaporated and transpired at a location. Actual
évapotranspiration equals potential évapotranspiration when
precipitation and/or soil moisture can meet potential
18
évapotranspiration needs. However, if precipitation and/or soil
moisture are insufficient to meet potential évapotranspiration
requirements, then actual évapotranspiration equals precipitation
received plus released soil moisture.
Differences between actual and potential évapotranspiration
are deficiencies (DEE). A negative value indicates that
precipitation and released soil moisture were unable to meet
potential évapotranspiration needs.
The Steila Drought Index Methodology
The last row on the sample water budget indicates the Steila
Drought Indices (DI). These values indicate departure from the
mean moisture status, either positive (excess moisture) or
negative (moisture shortage). The mean moisture status is the
average monthly moisture status for each climatic division for
every month from 1931 through 1977.
The Steila Drought Index ranks the derived values of
environmental moisture status into six categories (Table 3).
These categories demonstrate the severity of a drought condition
based on the amount of negative departure from mean moisture
status.
Positive measurements of the Index occur when actual
moisture status exceeds mean moisture status. During these times
moisture status equals soil storage and existing surpluses.
19
Table 3
Stella Drought Indices
Above Normal Moisture > -1.00
Near Normal Moisture 0.99 to -0.99
Mild Drought -1.00 to -1.99
Moderate Drought -2.00 to -2.99
Severe Drought -3.00 to -3.99
Extreme Drought < -4.00
20
Negative indices computed in the sample identified in Table
2 resulted from moisture deficiencies. For the dry months,
actual moisture status equaled the deficiencies subtracted from
soil storage moisture. Drought intensity is determined in terms
of the magnitude of the negative departure from mean moisture
status (zero). A drought index value of -4.86 is more intense
than a reading of -0.97.
Temperatures which deviate from those normally expected play
an important role in the intensity of drought. A situation can
exist in which normal amounts of precipitation are insufficient
to keep the moisture status of an area at expected levels due to
an accompanying situation of higher than normal potential
évapotranspiration. Another situation can exist in which below
normal precipitation may not constitute a negative departure in
moisture status because of below normal potential
évapotranspiration demands. Furthermore, if both precipitation
and potential évapotranspiration are below normal, the intensity
of negative moisture departure may be reduced significantly by
the below normal potential évapotranspiration factor. Finally,
it is also possible to have both above normal precipitation and
potential évapotranspiration, resulting in a condition of below
normal moisture status.
Figure 2 facilitates an understanding of these relationships
between precipitation and potential évapotranspiration deviations
from normal Environmental Moisture Status (EMS). In example "A",
above normal moisture status is associated with above normal
21
precipitation and below normal potential évapotranspiration.
Example "B" identifies an above normal moisture status existing
due to above normal precipitation; yet, environmental wetness is
reduced by the above normal potential évapotranspiration.
Examples "C", "D" and "E" illustrate the impact of potential
évapotranspiration during periods of below normal moisture
status. "C" depicts the ameliorating effect of below normal
potential évapotranspiration on drought intensity. In "D" the
effect of above normal potential évapotranspiration demonstrates
the increased intensity of drought. Example "E" portrays below
normal moisture status existing solely due to deficient
precipitation, with potential évapotranspiration having
absolutely no effect on either ameliorating or intensifying the
situation; i.e. below normal precipitation and normal potential
évapotranspiration.
RELATIONSHIP OF P.E. UPON EMS
4.0 —
MOISTURE
STATUS
DEPARTURE
-5,0—
Figure 2
ru
ro
CHAPTER III
DISCUSSION OF SELECTED EXAMPLES
After reviewing the data generated for the study area,
several drought periods have been selected for discussion.
Criteria used for selection included a drought period of at least
two months duration and a Steila Drought Index intensity level of
-3.00 or greater.
The Mountain Climatic Division of South Carolina
In the s\immer and autumn of 1954 many areas of the
southeastern United States experienced severe to extreme drought
for several months. One area severely affected was the Mountain
Climatic Division of South Carolina. From July through November
the Drought Index exceeded -3.00 and for September as -6.00.
Figure 3 depicts the actual moisture departure effect on the
intensities. From July through October higher than normal
temperatures increased the negative moisture departure. Below
normal temperatures for November reduced the intensity of
negative departure slightly. In September above normal potential
évapotranspiration accounted for 6.7 percent of drought
intensity. During July above normal potential évapotranspiration
was significant, in as much as, the high temperature effect
caused a change in the Drought Index category for the month.
Without the high temperature effect, the month would have been
4.0 —
3.0 —
2.0 —
DROUGHT 1.0 —
0 —
INDEX
-1.0—
(INCHES) -2.0 —
-3.0 —
-4.0 —
-5.0 —
-6.0 —
ro
25
classified as having Moderate Drought; but the above normal
potential évapotranspiration caused the intensity of drought to
be significantly higher, classifying the month in the Severe
Drought category.
The Central Climatic Division of Georgia
Another area suffering drought in 1954 was the Central
Georgia Climatic Division (Figure 4). Only August and September
exceeded -3.00 on the Drought Index but September was as severe
as in the South Carolina Mountain Division, recording a -6.12.
In both months higher than normal temperatures increased the
intensity of the moisture deficiency considerably. The higher
than normal potential évapotranspiration was sufficiently
significant to cause August to be classified as having Severe
Drought instead of Moderate Drought, and September's deficiency
to be intensified by 13.2 percent or 0.81 inches. The above
normal temperatures accounted for 11 percent of the moisture
deficiency for the month of August.
The Southern Coastal Plain Climatic Division of North Carolina
The Southern Coastal Plain Climatic Division of North
Carolina experienced drought conditions from July through
September in 1954 (Figure 5). The departure from Mean Moisture
Status ranged from -4.18 in July to -6.07 inches in September.
Although above normal temperatures contributed to the increased
GEORGIA CENTRAL
4.0 CLIMATIC DIVISION. 1954
Actual
3.0 ~ Moisture Status
Below Normal
2.0 — Temperature EFFECT
DROUGHT 1.0 —
0 --
INDEX
-1.0 —
(INCHES) -2.0 —
-3.0 —
-4.0 —
-5.0 —
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
Figure 4 ro
cr\
N.C. SOUTHERN COASTAL PLAIN
CLIMATIC DIVISION. 1954
Actual
Moisture Status
Below Normal
Temperature EFFECT
DROUGHT
INDEX
(INCHES)
ro
28
moisture deficiencies for August and September (7.4 percent for
August and 5.4 percent for September), it was not as significant
a factor as in the previous two examples. Note that temperatures
for July were virtually normal and all moisture deficiency is
attributable to below normal precipitation.
The Western Climatic Division of Kentucky
For the last seven months of 1954 the Western Climatic
Division of Kentucky suffered moderate to extreme drought (Figure
6). In each of these months drought intensity was increased by
above normal potential évapotranspiration, significantly so for
June. Of the
-3.62 moisture deficiency, -1.24 inches (34%) was attributable to
the above normal potential évapotranspiration component. With
the exception of April and May, temperatures were above normal
for the entire year. These above normal temperatures accounted
for increased drought intensities ranging from 8 percent to 34
percent for the months of June through September. The drought
intensity in this climatic division attained its most extreme
condition in November and December, but above normal temperatures
were not significant as 98 percent of the negative moisture
status was a result of below normal precipitation.
KENTUCKY WESTERN
4.0 CLIMATIC DIVISION, 1954
Actual
Moisture Status
Below Normal
2.0 — Temperature EFFECT
DROUGHT Above Normal1.0 — Temperature EFFECT
0 —
INDEX
-1.0 —
(INCHES) -2.0 —
-3.0 —
-4.0 —
-5.0
-6.0
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
ro
Figure 6 'O
30
The Northeastern States
During 196A and 1968 the New England States experienced some
drought conditions, but not to the extent of the Southern States
in 1954. The dry conditions seemed to be more spotty in this
area while in the Southern States it was widespread. As an
example, the Southern Climatic Division of New Hampshire
experienced moderate drought in 1964, but the Northern Climatic
Division did not have a drought.
The Southeastern Climatic Division of Vermont and
The Southern Climatic Division of New Hampshire
In 1964 the Southeastern Climatic Division of Vermont
(Figure 7) and the Southern Climatic Division of New Hampshire
(Figure 8) had similar patterns of drought. The most significant
months were the autumn months of September, October, and November
as opposed to the summer and autvimn months in the Southeastern
States. Another major difference between the two regions was the
effect of potential évapotranspiration on the drought intensity.
While above normal temperatures were significant in increasing
drought intensities in the South; below normal temperatures
played the role of reducing drought intensities in the Northeast.
Drought intensity was ameliorated on an average of about 7
percent for these climatic division in Vermont and New Hampshire.
VERMONT SOUTHEASTERN
4.0 CLIMATIC DIVISION. 1964
Actual
Moisture Status
2.0 —
DROUGHT 1.0 —
0 —
INDEX
-1.0—
(INCHES) -2.0 —
-3.0 —
-4.0 —
-5.0 —
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
Figure 7
NEW HAMPSHIRE SOUTHERN
4.0 CLIMATIC DIVISION, 1964—
Actual
3.0 — Moisture Status
Below Normal
2.0 —
Temperature EFFECT
DROUGHT 1.0 —
0 —
INDEX
-1.0 —
(INCHES) -2.0 —
-3.0 —
-4.0 —
-5.0 —
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
UJ
Figure 8 ro
33
The Northern Climatic Division of New Hampshire
In 1968 the Northern Climatic Division of New Hampshire
experienced dry conditions for September and October (Figure 9).
Although the drought intensity was not as great as in southern
Vermont and New Hampshire in 196A, there is an important
difference in the two drought periods. The 1968 northern New
Hampshire dry period was increased in intensity as a result of
above normal potential évapotranspiration, indicating that higher
than normal temperatures can contribute to drought intensity in
the more northern latitudes. Although the increase in the
intensity was only four to five percent, it is worth noting.
More often than not, however, the temperature effect in this
area was one of reducing drought intensities rather than
increasing them.
The Saint Lawrence Valley Climatic Division of New York
The Saint Lawrence Valley Climatic Division of New York
experienced its driest month for the 47 years of data in October
1963 (Figure 10). The Drought Index for that month was -4.02, an
extreme drought. However, since only September and November
experienced mild drought, the severity of drought was limited to
a short time. During September the negative moisture status was
significantly ameliorated (0.95 inches, -41.3 percent) by
temperatures well below the normal expected. Had it not been for
NEW HAMPSHIRE NORTHERN
4.0 — CLIMATIC DIVISION. 1965
3.0 —
2.0 —
DROUGHT 1.0 —
0 —
INDEX
-1.0 —
(INCHES) -2.0 —
-3.0 —
-4.0 —
Above Normal
-5.0 Temperature EFFECT—
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
Figure 9
NEW YORK ST. LAWRENCE VALLEY
4.0 CLIMATIC DIVISION, 1963
Actual
Moisture Status
Below Normal
Temperature EFFECT
DROUGHT Above NormalTemperature EFFECT
INDEX
(INCHES)
-3.0 —
-4.0 —
-5.0 —
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
Figure 10 OJui
the below normal potential évapotranspiration, September would
have been classified as having moderate drought.
The Southwest Climatic Division of Illinois
A much different situation can be noted for the Southwest
Climatic Division of Illinois for 1953 and 1954 (Figures 11 and
12). A severe to extreme drought condition endured for ten
consecutive months, from June 1953 through March 1954. In every
one of these months above normal potential évapotranspiration
contributed significantly to the drought intensity. The degree
of impact on the drought intensity ranged from a low contribution
of approximately 6 percent in December 1953 and January 1954 to a
maximum effect of 30 percent in June 1953. This particular
drought would have been severe to extreme with normal
temperatures, but the higher than normal temperatures made the
drought more intense. Over the twenty-four month period of 1953
and 1954 this climatic division experienced below normal
potential évapotranspiration for only four months; a significant
factor with regard to moisture demand. Since both precipitation
and potential evaporation was above normal for only four months
during this period, the area was not able to recharge the
accumulated moisture losses. This was a contributing factor for
drought continuing into the second year.
ILLINOIS SOUTHWEST
4.0 — CLIMATIC DIVISION, 1953
Actual
3.0 — Moisture Status
Below Normal
Temperature EFFECT
DROUGHT
INDEX
(INCHES)
-6.0—
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
LO
Figure 11
ILLINOIS SOUTHWEST
4.0 CLIMATIC— DIVISION, 1954
3.0 —
2.0 —
DROUGHT 1.0 —
0 —
INDEX
-1.0 —
(INCHES) -2.0 —
-3.0 —
Actual
Moisture Status
-4.0 —
Below Normal
Temperature EFFECT
-5.0 —
Above Normal
Temperature EFFECT
-6.0 —
JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC
UJ
Figure 12 oo
CHAPTER IV
ANALYSIS OF DROUGHT COMPONENTS
The three basic components of drought are: (1)
precipitation, (2) potential évapotranspiration, and (3)
available soil moisture; with the latter being dependant to a
significant degree on the first two.
PRECIPITATION
As noted earlier, many drought definitions are based solely
on deficient precipitation. Precipitation is the single most
important determining component of environmental moisture status.
Whenever precipitation amounts fall below the normal expected,
environmental moisture status usually diminishes.
If there is little or no rainfall during the summer months,
the environment becomes noticeably "dry", however, during the
cooler winter months the "dryness" may not be noticed as much if
precipitation is below normal. For most humid areas the cooler
winter months are times for soil moisture recharging, and if
precipitation is below normal at this time, the long range effect
can be devastating. If there as been sufficient precipitation to
recharge the soil moisture to capacity during the cooler winter
months, then shortages of rainfall during the summer months will
not be as detrimental to environmental moisture status unless the
dry period is for an extended period of time.
40
Can drought occur during periods of surplus environmental
moisture status? Many qualitative definitions of drought refer
to the phenomenon as a "state of dryness" (McBryde, 1982).
Quantitative research clearly identifies periods wherein an
environment has surplus moisture availability; yet, at the same
time, total water supplies are appreciably below those normally
associated with the same region. Under such conditions the area
is not "dry", but it is below its expected norm with respect to
water supply (Palmer, 1965; Steila, 1971, 1974; Sanders, 1972).
In short there is drought with regard to expected environmental
moisture status.
Petterson (1980) substantiated the consequences of these
statements in her analysis of the 1977 drought in the
Southeastern United States. In several parts of the study area,
water budget analysis and hydrological data indicated surplus
water to be available during the winter of 1977. Precipitation
amounts, however, were well below the normal expected. This is
also the case for the 1954 drought of the same area. Although
these environments were not "dry", they received too little
precipitation to adequately recharge groundwater and soil
moisture reserves to normal levels.
In all cases evaluated in this study where drought was
deemed to be extreme or severe, the chief component was below
normal precipitation. As the drought periods intensified and
became more severe, the percentage of departure from normal
moisture status attributable to deficient precipitation
41
increased. This substantiates that below normal precipitation is
the primary drought component.
POTENTIAL EVAPOTEANSPIRATION (TEMPERATURE)
The monthly contributions of above normal potential
évapotranspiration have been evaluated as a percentage of the
total drought status. Findings for the 195A drought in the
southeastern United States indicate that the above normal
potential évapotranspiration demand accounted for as little as 2
percent to as much as 35 percent of total drought status during
given months. The 35 percent figure is quite high considering
that the only months closely examined had a Drought Index value
of -3.00 or greater.
Steila (1971, 1983) found that above normal potential
évapotranspiration accounted for as much as 75 percent of drought
status for a given month during the 1936 drought in the
southeastern United States. However, the largest percentages in
his study were for months with minor negative environmental
moisture status departures.
As the examples discussed have shown, above normal potential
évapotranspiration can occur for any month and will have a
negative effect on environmental moisture status for that month.
During the normally cool winter months above normal potential
évapotranspiration can negate surplus precipitation and therefore
42
impede the recharging processes for groundwater and soil moisture
storage.
Potential évapotranspiration demands are not always above
normal and in many instances may be below normal. When
temperatures are significantly below the normal expected a
noticeable reduction in drought intensities occurs. This
ameliorating effect is documented in the examples of the more
northern latitudes. This was particularly significant in the
Saint Lawrence Valley examples where negative moisture status was
ameliorated by 41.3 percent.
Even in the southern latitudes below normal potential
évapotranspiration can have a significant ameliorating effect.
However, below normal potential évapotranspiration tends to have
a greater impact on environmental moisture status when the
negative departures are less than -3.00 inches on the Drought
Index.
AVAILABLE SOIL MOISTURE
When a drought occurs the most noticeable effect is that
which is observed on vegetation. When available moisture in the
rhyzosphere has been used by the plants and rainfall is absent,
the area's vegetation suffers and may die. Some agricultural
crops may survive but produce significantly lower yields or none
at all.
A significant factor is the ability of the soil to store
moisture effectively. The characteristics of the soil determine
its moisture holding capabilities. For example, soil texture is
extremely important regarding soil moisture retention. Sandy
porous soils will lose their moisture rather quickly while those
soils with a higher concentration of clays will retain moisture
over a longer period of time. The fine texture of the clay soils
enable them to release their stored moisture to vegetation over a
longer period of time than the sandy soils. However, these clay
soils are also more difficult to recharge with moisture since
moisture absorption is also at a slower rate.
During the cooler winter months when soil moisture is
recharged, adequate precipitation must fall to bring soil
moisture to capacity. If this fails to occur and precipitation
continues to be below normal throughout the spring and sximmer
months, then the onset of drought will be earlier and more
severe. Should temperatures be above normal at this time, the
evaporation demand from the soil will be higher, depriving the
vegetation of some of the available soil moisture.
In short, the characteristics of the soil, particularly
texture, have an effect on how significant deviations from normal
precipitation and potential évapotranspiration will be for a
particular area for a given length of time.
CHAPTER V
POTENTIAL EVAPOTRANSPIRATION DROUGHT INTENSIFICATION:
AN AREAL ANALYSIS
The concept of temperature variation as a significant
parameter of drought and its intensity is relatively new. In
particular, studies conducted by Palmer (1958), and Steila (1971)
incorporated temperature-induced évapotranspiration demands as an
element significant to either intensifying or ameliorating an
area's environmental moisture status.
This study shows that above normal potential
évapotranspiration is not only a significant component of drought
and its intensification; but that there is an areal
differentiation of the potential évapotranspiration factor
through time, in as much as, the areal patterns of significance
shift through the seasons of the year.
A 20 year period of monthly data were randomly selected for
each climatic division within the study area to evaluate
potential évapotranspiration as a factor altering moisture status
a twenty year period of monthly data were randomly selected for
each climatic division within the study area. Sabin and Shulman
(1985) concluded that ten years of data were probably as suitable
as thirty-five or forty years of data for evaluating
climatological phenomena. Data from all months for the twenty-
year period from all the climatic divisions randomly selected for
this study were examined, and only those months with a Drought
45
Index value of -1.00 or greater were selected for further
evaluation. This data was then grouped by seasons for analysis.
The months which exhibited an increased drought intensity
attributable to above normal potential évapotranspiration were
tabulated and an occurrence-percentage calculated against all
months for the period. The calculated drought-intensification,
monthly occurrence-percentages for each season are listed in
Table 4 by climatic division number.
These percentages were then used to construct a series of
seasonal isoline maps to illustrate the areal patterns associated
with above normal temperatures and their potential
évapotranspiration equivalents (PE). It is important to note
that these maps offer predictive indices of drought recurrence in
association with potential évapotranspiration deviation from
normal. For example areas showing an occurrence-percentage of 18
to 20 percentage means that above normal potential
évapotranspiration contributed to drought intensity 18 to 20
percent of the time.
WINTER (January-March)
The most significant influence of potential
évapotranspiration during winter is along the south Atlantic
coastal region and at the confluence of the Ohio and Mississippi
Rivers (Figure 13). There is no effect across the northern tier
of states and only slight influence along the western slopes of
46
TABLE 4
OCCURRENCE-PERCENTAGES OF DROUGHT INTENSIEICATIQN
ATTRIBUTABLE TO ABOVE NORMAL PE
BY CLIMATIC DIVISIONS
AUTUMN SPRING
DIV# WINTER SPRING SUMMER AUTUMN WINTER SUMMER
1. 13.3 23.3 23.3 15.0 1A.2 23.3
2. 11.7 21.7 25.0 16.7 1A.2 23.3
3. 16.7 20.0 26.7 15.0 15.8 23.3
A. 3.3 18.3 23.3 20.0 11.7 20.8
5. 16.7 23.3 26.7 16.7 16.7 21.7
6. 16.7 16.7 21.7 20.0 18.3 19.2
7. 13.3 21.7 23.3 16.7 15.0 22.5
8. 11.7 21.7 23.3 16.7 1A.2 22.5
9. 10.0 18.3 25.0 15.0 15.0 20.8
10. 15.0 20.0 25.0 18.3 20.0 22.5
11. 21.7 28.3 25.0 23.3 20.8 26.5
12. 18.3 18.3 21.7 16.7 17.5 20.0
13. 18.3 21.7 23.3 20.0 19.2 22.5
lA. 20.0 26.7 23.3 28.3 2A.2 25.0
15. 11.7 20.0 21.7 18.3 15.0 20.8
16. 6.7 20.0 21.7 15.0 11.7 20.8
17. 13.3 25.0 25.0 20.0 16.7 25.7
18. 15.0 18.3 25.0 13.3 1A.2 21.7
19. 0.0 16.7 23.3 10.0 5.0 20.0
20. 1.7 16.7 23.3 15.0 8.3 1A.2
21. 0.0 18.3 23.3 15.0 7.5 20.8
22. 0.0 11.7 25.0 10.0 5.0 18.3
23. 11.7 18.3 21.7 13.3 9.2 20.0
2A. 15.0 16.7 23.3 18.3 16.7 22.5
25. 15.0 21.7 23.3 18.3 16.7 22.5
26. 5.0 20.0 18.3 13.3 9.2 19.2
27. 15.0 18.3 23.3 13.3 1A.2 22.5
28. 18.3 23.3 23.3 18.3 18.3 23.3
29. 10.0 18.3 25.0 11.7 10.8 21.6
30. 1.7 18.3 25.0 15.0 7.5 20.8
31. 13.3 20.0 21.7 16.7 15.0 20.8
32. 6.7 20.0 20.0 13.3 10.0 20.0
33. 5.0 20.0 18.3 11.7 8.3 19.2
3A. 5.0 15.0 20.0 13.3 9.2 17.5
35. 6.7 15.0 20.0 15.0 10.8 17.5
36. 10.0 18.3 23.3 15.0 12.5 20.8
47
MONTHLY DROUGHT
OCCURRENCE—PERCENTAGES DUE TO > PE
WINTER (JAN/MAR)
Figure 1 3
48
the Appalachian Mountains. Note the general pattern of a
Northeast to Southwest trend in intensification, and a trough of
lower values paralleling the Appalachian Plateau.
The lack of potential evapotranspiration-departure influence
along the northern tier of states is attributable to low mean
monthly temperatures. Variations in temperature from means of
20° to 30° Farenheit have essentially zero significance in
raising the potential évapotranspiration impact. Indeed, the
Thornthwaite model--the basis for this analysis, does not account
for évapotranspiration occurring at temperatures below freezing.
The trough of low occurrence along the Appalachians is likewise
related to high-altitude/low-temperature complexes.
As noted above, high frequency nodes occur in two locations.
It is hypothesized that the Mississippi-Ohio River "high
frequency" node is associated with a "backing effect" on low
level high pressure systems that stagnate westward of the
Appalachian Mountains. While intensifying to develop sufficient
depth to cross the mountain chain, clear skies with surface
warming conditions are experienced. The latter can provide for
above normally expected temperatures and associated above normal
potential évapotranspiration. Likewise, the South-Atlantic Coast
"high frequency" node can be linked to oscillating strengthening-
weakening patterns of the North Atlantic Subtropic-High Pressure
Cell.
SPRING (April-Jime)
The map for Spring (Figure 14) demonstrates complex changes
taking place relative to potential évapotranspiration influences.
The impact at the Ohio and Mississippi Rivers is significantly
strengthened and the south Atlantic influence split and shifted
northward and toward the southwest, dividing the region into
three core-areas of high intensity with intervening depressions
of lower intensity. Whereas the northern states had 0 percent
effect during Winter, the occurrence-percentages of potential
évapotranspiration contribution now totals as much as 14 percent.
Clearly, the surface warming and rising air temperatures are
responsible for the increased occurrence of above-normal
temperature-induced drought throughout the northern tier of
states. The southwest-northeast trend of lowest occurrence
appears well situated with respect to the dominant spring track
of cyclonic storms. Greater cloudiness and precipitation suggest
less temperature variability than to the west or east. The Gulf
Coast "high frequency" node also reflects the northward storm-
track shift and a period of decreased cloudiness/precipitation
and consequently more frequent high-intensity solar-heating
episodes; whereas the Mid-Atlantic States "high frequency" node
is associated with the northward shift of the North Atlantic
Subtropic High and the stabilizing effect of the Newfoundland
MONTHLY DROUGHT
OCCURRENCE—PERCENTAGES DUE TO > PE
SPRING (APR—JUN)
Figure 1 4
51
Current. The Mississippi-Ohio River node bears the continuing
pattern of winter, but as will be shown, weakens rapidly with the
onset of summer.
SUMMER (July-September)
During the summer months the cells of potential
évapotranspiration influence are less well-defined than during
the other seasons (Figure 15). The patterns have largely merged
and there is as much differentiation from East to West as there
is from North to South. As is the case with Winter and Spring,
the impact of potential évapotranspiration during Summer is not
as high along the western slopes of the Appalachian Mountains,
possibly as a result of a higher percentage of cloudy days
associated with orographic precipitation.
The patterns for Sximmer parallel the circulation pattern
that has weakened due to high-latitude solar heating. Although
the percentages are relatively high, they basically follow a
South to North diminishing trend. The single exception is the
increased values of the New England States, reflective of high
pressure system recurrence associated with the Arctic and lack of
the Great Lakes moderating influence.
AUTUMN (October-December)
With the onset of cooler temperatures and fewer hours of
sunlight the patterns of potential évapotranspiration influence
begin to shift southward during Autumn (Figure 16). Significant
52
MONTHLY DROUGHT
OCCURRENCE—PERCENTAGES DUE TO > PE
SUMMER (JULY—SEP)
Figure 1 5
53
MONTHLY DROUGHT
OCCURRENCE-PERCENTAGES DUE TO > PE
AUTUMN (OCT—DEC)
Figure 1 6
54
coastal influence is prevalent along the Atlantic seaboard
associated with periods of minimal hurricane activity; and the
strong, well-defined cell of influence at the Ohio and
Mississippi Rivers confluence reappears. Again the lowest
occurrence-percentages are across the northern states southward
through the upper Ohio Valley and along the western Appalachian
Plateau.
Autumn is almost a mirror image of Spring with minor
exception. Occurrence of Gulf and Atlantic Coast hurricane
activity can diminish coastal potential évapotranspiration
effects; the lack thereof, results in the intensification shown,
and the re-establishment of the cyclonic storm track provides a
southwest-northeast corridor of lower potential
évapotranspiration induced drought occurrence. Relatively, the
low altitude high pressure cells once again are being blocked
west of the Appalachians and the emerging winter "high frequency"
node becomes entrenched at the Mississippi-Ohio River confluence.
GENERAL PATTERNS
Two summary maps have been generated grouping Autumn and
Winter (Figure 17) and Spring and Summer (Figure 18). These
illustrate patterns similar to the seasonal maps, depicting the
strong influence from the West as an area capable of generating
high occurrence-percentages of temperature-induced drought
intensification. Also the seasonal, shifting-influence of the
55
MONTHLY DROUGHT
OCCURRENCE—PERCENTAGES DUE TO > PE
AUTUMN/WINTER (OCT—MARCH)
Figure 1 7
56
MONTHLY DROUGHT
OCCURRENCE—PERCENTAGES DUE TO > PE
SPRING/SUMMER (APR—SEP)
Figure 1 8
57
Bermuda High is well illustrated, as well as the tempering impact
of the Appalachian Moiintains.
It is apparent that potential évapotranspiration induced
drought intensification changes with season, and exhibit
distinctly different areal patterns of the drought phenomenon.
Also noteworthy is the predictive value of these maps. The
monthly drought recurrence associated with greater than normal
potential évapotranspiration indices enable projection of
drought-incidence associated with above normal potential
évapotranspiration.
CHAPTER VI
SUMMARY AND CONCLUSIONS
Identification and evaluation procedures of drought have
evolved slowly from very simplistic approaches that considered
the phenomenon to be a rainfall deficiency, to problem-specific
models of limited applicability, and finally, to sophisticated
techniques that quantitatively appraise total environmental
moisture status and its departure from normality. The latter
have been available for research use for less than twenty years,
but have already afforded valuable insights into the intensity,
duration, and areal distribution of drought events. They have
provided a means which enables a researcher to assess the
relative severity of a given drought occurrence.
Of the three major components contributing to drought, below
normal precipitation is certainly the chief factor. During
periods of extreme and severe drought it accounts for 75 to 98
percent of the total negative moisture status. It is not
unreasonable to understand why the early definitions of drought
were based only on the parameter of precipitation.
Overlooked in those early definitions of drought was the
impact of temperature variation from the normal expected as a
component of drought. No doubt comments were made to the effect
that a particular time seemed to be "hot and dry"; but beyond
that, temperature was not considered to be an important factor.
59
This study has demonstrated that potential
évapotranspiration is an important component in assessing a
drought and its intensity. It has been proven quantitatively
that departures from normal potential évapotranspiration is a
significant contributing component to drought intensity. When an
extreme or severe drought month has as much as one-third of its
total negative departure from normal moisture status attributable
to above normal temperatures, it is a significant factor.
Equally significant is the ameliorating effect on drought
intensities of below normal potential évapotranspiration. With
ameliorating percentages ranging from five to forty-one percent,
it is clearly shown that temperatures less than normal have a
significant impact on drought intensity.
In most cases the impact of potential évapotranspiration
departure from normal was larger when total negative moisture
status was less than -3.00 inches on the Drought Index. The
temperature effect was more apparent during months of Mild or
Moderate Drought than during months of Extreme or Severe Drought.
The role of available soil moisture as a component of
drought is discussed briefly. The availability of soil moisture
is directly related to precipitation and potential
évapotranspiration along with the characteristics of the soil
itself. In order for the soil to provide maximum available
moisture there must be a complete recharging of soil moisture
during the cooler winter months. Inadequate precipitation and/or
above normal potential évapotranspiration demand can prevent
60
recharging of the soil to its moisture holding capacity. How
much available moisture the soil is able to provide will have an
impact on the threshold of drought as well as the intensification
of drought.
Throughout the study, the average annual contribution of
above normal potential évapotranspiration to drought amounted to
no more than fourteen to fifteen percent of total drought status.
These findings suggest that negative precipitation amounts and
soil-moisture-storage departures from normal may dominate the
drought status of hiimid climates. These results, however, may
not apply to other climatic regimes. Detailed research regarding
the contribution of soil-moisture-storage has not yet been
conducted and/or published, and this remains an area of much
needed attention.
Regional causes for the development of the drought
phenomenon are well known. Unknown, however, are the global
dynamics that generate regional circulation patterns which can
initiate drought conditions. This, therefore, places great
limitations on when and where a drought is likely to occur.
Additional research regarding both components of drought and
specific causes of drought will aid in the future development of
ways to cope with the phenomenon and to better utilize our
environmental resources.
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