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.
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:
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:
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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