A conceptual model of drought characterization across the climatic spectrum is formulated. The
model is particularly suited to subtropical and midlatitudinal regions. Drought duration, intensity, and recurrence
interval are expressed in terms of the ratio of mean annual precipitation to annual global terrestrial precipitation.
The model is useful as a framework for the systematic analysis of droughts and the assessment of changes in
drought characteristics due to climatic changes.
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1. INTRODUCTION
A drought at a given location or region is a period of time,
lasting weeks months, or years, during which the actual moisture
supply consistently falls short of the climatically expected
moisture supply. Droughts are better documented in semiarid
and subhumid regions, where humans tend to concentrate.
Drought data in extremely arid regions is scant, since very few
people are actually affected. Likewise, droughts in very humid
regions go largely unnoticed, since the supply of water usually
exceeds the actual demand. Here, the writers relate drought
characteristics to climatic parameters across the climatic spectrum.
The latter is defined in terms of mean annual precipitation
and cross-referenced to annual potential evapotranspiration.
Coping with droughts is possible through proper forecasting
and planning. To reduce the impact of drought, it is necessary
to develop the capability to forecast its characteristics, i.e., its
duration (How long will it last?), its intensity (How severe will it be?),
and its recurrence interval (How often will it recur?).
2. THE CLIMATIC SPECTRUM
Droughts are cyclical and regional in nature; their occurrence
is related to prevailing climatic parameters. A readily
available climatic parameter is mean annual precipitation,
which depends on: (1) latitude; (2) orographic factors; (3) mesoscale
ocean currents; (4) atmospheric wind circulation; (5)
proximity to oceans and large lakes; (6) atmospheric pressure;
(7) character of the Earth's surface, including color and texture;
and (8) presence of atmospheric particulates, both natural
and human-induced. Closely related to mean annual precipitation
is annual potential evapotranspiration, which is a function
of: (1) net solar radiation: (2) vapor-pressure deficit; (3)
surface roughness; and (4) leaf-area index.
For our purposes, the writers define the climatic spectrum
solely in terms of mean annual precipitation, an approach that
is particularly useful for subtropical and midlatitudinal regions.
The writers characterize the climatic spectrum in terms of the
ratio of mean annual precipitation Pma to annual global terrestrial
precipitation Pagt.
The amount of moisture stored in the atmosphere is a function
of latitude and climate, varying typically from 2-15 mm
in polar and arid regions to 45-50 mm in humid regions
(World 1978). A global terrestrial mean value of 25 mm is
assumed for the purpose of estimating annual global terrestrial
precipitation. The atmospheric moisture recycles every eleven
days on the average, for a total of 33 cycles per year,
which results in the annual global terrestrial precipitation
Pagt = 825 mm. Here, the writers assume a round number,
Pagt = 800 mm.
Globally, the middle of the climatic spectrum, i.e., the division
between semiarid and subhumid climates, corresponds
to Pma /Pagt = 1. Regions with Pma /Pagt < 1 have
less-than-average moisture; conversely, regions with Pma /Pagt > 1 have
greater-than-average moisture. Mean annual terrestrial precipitation
varies typically in the range of 100-6400 mm, with a
few isolated cases falling outside this range. This enables the
division of the climatic spectrum in subtropical and midlatitudinal regions into the following eight types:
Superarid, with Pma /Pagt < 0.125
Hyperarid, with 0.125 ≤ Pma /Pagt < 0.25
Arid, with 0.25 ≤ Pma /Pagt < 0.5
Semiarid, with 0.5 ≤ Pma /Pagt < 1
Subhumid, with 1 ≤ Pma /Pagt < 2
Humid, with 2 ≤ Pma /Pagt < 4
Hyperhumid, with 4 ≤ Pma /Pagt < 8
Superhumid, with Pma /Pagt ≥ 8
Table 1 shows the climate types with mean annual precipitation
Pma and corresponding Pma /Pagt ratios. To determine
suitable Eap /Pma ratios, the writers have approximately estimated
potential evapotranspiration across the climatic spectrum,
for subtropical and midlatitudinal regions. For instance,
the writers estimate Eap = 3,000 mm at the limit between superarid
and hyperarid regions. Corresponding estimates for
other regions led to the Eap /Pma ratios shown in Table 1.
TABLE 1. Conceptual model of drought characterization across the climatic spectrum.
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(1) |
(2) |
(3) |
(4) |
(5) |
(6) |
(7) |
(8) |
Climatic Spectrum |
Climatic type |
Super- arid ← |
Hyper- arid → |
Arid → |
Semi- arid → |
Sub- humid → |
Humid → |
Hyper- humid → |
Super- humid → |
Mean annual precipitation Pma (mm) |
|
100 |
200 |
400 |
800 |
1600 |
3200 |
6400 |
Pma /Pagt |
|
0.125 |
0.25 |
0.5 |
1 |
2 |
4 |
8 |
Annual potential evapotranspiration Eap (mm) |
|
3000 |
2400 |
2000 |
1600 |
1200 |
1200 |
1200 |
Eap /Pma |
|
30 |
12 |
5 |
2 |
0.75 |
0.375 |
0.1875 |
Length of rainy season L (mo) |
|
1 |
2 |
3 |
4 |
6 |
9 |
12 |
Drought Characteristics |
Duration (yr) |
|
1 |
2 |
4 |
6 |
4 |
2 |
1 |
Intensity (dimensionless) |
|
Moderate |
|
0.25 |
0.5 |
1.0 |
1.5 |
1.0 |
0.5 |
0.25 |
Severe |
|
0.5 |
1.0 |
2.0 |
3.0 |
2.0 |
1.0 |
0.5 |
Extreme |
|
0.75 |
1.5 |
3.0 |
4.5 |
3.0 |
1.5 |
0.75 |
Recurrence interval (yr) |
|
2 |
3 |
6 |
12 |
25 |
50 |
100 |
Note: Pagt = annual global terrestrial precipitation.
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3. CONCEPTUAL MODEL OF DROUGHT CHARACTERIZATION
Our conceptual model deals specifically with meteorological
droughts lasting at least one year, with an emphasis on subtropical
and midlatitudinal regions. Persistence is the property
of a drought event to last more than one year. For a given
drought event, intensity refers to the extent of the precipitation
deficit. To determine drought intensity, the moisture deficiency
is accumulated over the drought duration. Therefore, the
longer the duration, the greater the intensity. Since dry periods
are generally followed by corresponding wet periods, it follows
that the recurrence interval is always greater than the
duration. Thus, for meteorological droughts lasting at least one
year, the recurrence interval is at least two years.
A conceptual model works in the mean: i.e., it describes
general trends and not necessarily specific events. It is meant
to aggregate the deterministic and stochastic components of
the precipitation anomalies. Its value is that it provides a conceptual
framework for interpreting the regional variability of
drought phenomena.
Given a drought year with precipitation P where P < Pma,
the precipitation deficiency may be classified into three types:
(1) moderate, with P/Pma = 0.75;
(2) severe, with P/Pma = 0.5;
and
(3) extreme, with P/Pma = 0.25.
The writers define drought intensity as the ratio
of the deficit (Pma - P ) to the mean (Pma). For a drought
lasting more than one year, intensity is defined as the summation of the annual intensities:
Pma - P
I = ∑ __________
Pma
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in which I = drought-intensity index.
Therefore, average annual drought intensity is the total
drought intensity divided by the duration.
The writers base their conceptual model of drought characterization
on the following premises, amply supported by
observations:
Drought duration varies across the climatic spectrum,
reaching a maximum around the middle and decreasing
toward the extremes.
Since drought intensity is directly related to duration,
intensity also reaches a maximum around the middle of
the climatic spectrum and decreases toward the extremes.
The drought recurrence interval increases gradually from
the dry to the wet side of the climatic spectrum.
Table 1 summarizes the writers' conceptual model of
drought characterization. For drought duration, the expected
values vary between 1 and 6 years, with larger values toward
the middle of the climatic spectrum (6 years), decreasing toward
either extreme (1 year). The longer durations toward the
middle of the climatic spectrum are due to greater interannual
precipitation variability within the semiarid and subhumid
regions. Within these regions, drought duration is likely to be
the longest, approaching 4-6 years. The shorter durations toward
both extremes of the climatic spectrum are justified because
of smaller interannual precipitation variability. In superarid
regions, variability is reduced because the precipitation
amounts are small; in superhumid regions. variability is reduced
because of the length of the rainy season, which approaches 12 months.
The drought recurrence interval varies between 2 years on
the extreme dry side and about 100 years on the extreme wet
side, increasing in an approximate geometric progression.
Since recurrence interval decreases from wet to dry climates,
and since it must always exceed duration, it follows that duration
must decrease toward the dry side of the climatic spectrum (Table 1).
Thus, in hyperarid regions, droughts are short
and recur once every 2-3 years; in semiarid and subhumid
regions, droughts are long and recur once every 6-25 years;
in hyperhumid regions, droughts are short and recur once
every 50-100 years.
4. SUMMARY
Drought duration varies between 1 and 6 years across the
climatic spectrum, and reaches a maximum toward the middle.
Intensity varies directly with duration, and recurrence interval
increases approximately in a geometric progression, from 2
years on the extreme dry side to about 100 years on the
extreme wet side.
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