3. CHARACTERISTICS OF DROUGHT
The characteristics of droughts are expressed in terms of: (1) drought index,
3.1 Drought Index
A drought index assimilates thousands of data on rainfall, snowpack,
streamflow and other water-supply indicators into a comprehensible picture.
A drought index is typically a single number,
far more useful than raw data for decision making.
There are several indices that measure how much precipitation for a given
period of time has deviated
from historically established norms.
Some indices are better suited than others for certain uses. For example,
the Palmer Drought Severity Index (PDSI) has been widely used by the U.S. Department
of Agriculture to determine
when to grant emergency drought assistance. The PDSI is better suited for
large areas with uniform topography.
Western states, with mountainous terrain and the resulting complex regional
microclimates, find it useful to supplement the PDSI with other indices
such as the Surface Water Supply Index (SWSI),
which takes snowpack and other unique conditions into account.
The National Drought Mitigation Center is using a new index, the Standardized
Precipitation Index (SPI),
to monitor moisture-supply conditions. The distinguishing features of this index are
that it identifies emerging droughts months sooner than the Palmer Index, and that it is computed on
various time scales.
Most water-supply planners find it useful to evaluate one or more indices before making a decision.
Important drought indices used in the United States and elsewhere (Australia) are detailed below.
- Percent of normal precipitation (PNP)
The percent of normal precipitation is the ratio of actual to normal precipitation for a given location
and a given period, expressed as a percentage.
Analyses using the percent of normal are effective when used for a single
region and a single season.
In other applications, the index can vary depending on the choice of period,
including monthly, seasonal, or annual.
- Standardized Precipitation Index (SPI)
The Standardized Precipitation Index (SPI) was developed in the understanding
that a certain deficit of precipitation has different impacts on the soil moisture, ground water,
reservoir storage, snowpack, and streamflow.
The SPI was designed to quantify the precipitation deficit for multiple time scales.
These time scales reflect the
impact of drought on the availability of the various types of water resources.
Soil moisture conditions respond to
precipitation anomalies on a relatively short scale, while ground water, streamflow,
and reservoir storage
reflect long-term precipitation anomalies. For these reasons, the SPI was originally calculated
for 3-, 6-,12-, 24-, and 48-month time periods.
The SPI is an index based on the precipitation record for a location and chosen period (months or years).
The record is fitted to a probability distribution which is then transformed into
a normal distribution so that the mean SPI for the location and period is zero.
The index is negative for drought and positive for non-drought conditions.
Table 1 shows a classification system linking SPI's with drought intensities.
A drought event occurs any time the SPI is continuously
negative and reaches an intensity less than or equal to -1.0.
The event ends when the SPI becomes positive.
Each drought event has a duration defined by its beginning and end,
and an intensity for each month that the event lasts.
The sum of the SPI's for all the months within a drought event is the drought magnitude.
|Table 1. SPI classification |
|2.0 or more||Extremely wet|
|1.5 to 1.99||Very wet|
|1.0 to 1.49||Moderately wet|
|-0.99 to 0.99||Near normal|
|-1.0 to -1.49||Moderately dry|
|-1.5 to -1.99||Severely dry|
|-2.0 and less||Extremely dry|
- Palmer Drought Severity Index (PDSI)
In 1965, Palmer developed an index to measure the departure of the moisture supply.
Palmer based his index on the supply-and-demand
concept of the water-balance equation, taking into account the
precipitation deficit. The objective of the
Palmer Drought Severity Index (PDSI) is to provide standardized
measurements of moisture conditions, so
that comparisons can be made between locations and between durations.
The PDSI is a meteorological drought index that is responsive to abnormal weather conditions,
either on the dry or wet side. For example, when conditions
change from dry to normal or wet, the drought measured by the PDSI ends
without taking into account streamflow, lake and reservoir
levels, and other long-term hydrological aspects.
The PDSI is calculated based on
precipitation and temperature data, as well as the local
Available Water Content (AWC) of the soil. From the inputs, all the basic terms of the
water balance equation can be determined, including
evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer.
Human impacts on the water balance, such as irrigation,
are not considered.
Palmer developed the PDSI to include the duration of a drought or a wet spell.
wet month in the middle of a long-term drought should not have a major impact on the
index, or a series of months with near-normal precipitation
following a serious drought does not mean that the drought is over. Therefore, Palmer
developed criteria for determining when a drought or a wet
spell begins and ends, and to adjust the PDSI accordingly. In near-real time, Palmer's
index is no longer a meteorological index but becomes a
hydrological index, properly referred to as the Palmer Hydrological Drought Index (PHDI). This index
is based on moisture inflow (precipitation), outflow,
and storage, and does not take into account the long-term trend.
|Table 2. PDSI classification|
|4 or more||Extremely wet|
|3.0 to 3.99||Very wet|
|2.0 to 2.99||Moderately wet|
|1.0 to 1.99||Slightly wet|
|0.5 to 0.99||Incipient wet spell|
|0.49 to -0.49||Near normal|
|-0.5 to -0.99||Incipient dry spell|
|-1.0 to -1.99||Mild drought|
|-2.0 to -2.99||Moderate drought|
|-3.0 to -3.99||Severe drought|
|-4.0 or less||Extreme drought|
- Surface Water Supply Index (SWSI)
The Surface Water Supply Index (SWSI) complements the Palmer Index for moisture condition.
The Palmer Index is basically a
soil-moisture algorithm calibrated for relatively homogeneous regions. It is not
designed for large topographic variations across a region and it
does not account for snow accumulation and subsequent runoff.
The SWSI was designed to be an indicator of surface water conditions, including
The objective of the SWSI is to incorporate both hydrological and climatological
features into a single index resembling the Palmer Index, and applicable to
major river basins. The SWSI values are standardized to allow comparisons between basins.
Four inputs are required: snowpack, streamflow, precipitation, and
reservoir storage. The SWSI is dependent on the season; therefore, it
is computed with only the snowpack, precipitation, and reservoir storage in the winter.
During the summer months, streamflow replaces snowpack as a
component of the SWSI.
The procedure to determine the SWSI for a particular basin is as follows.
Monthly data are collected and summed up for all precipitation stations,
reservoirs, snowpack and streamflow measuring stations across the basin. Each summed
component is normalized using a frequency analysis gathered
from a long-term data set. The probability of nonexceedence
is determined for each component based on the frequency analysis.
This allows comparisons of the probabilities between the components.
Each component has a weight assigned to it depending on its typical
contribution to the surface water within that basin. These
weighted components are summed to determine a SWSI value representing the entire basin.
Like the Palmer Index, the SWSI is centered on zero and has a
range between -4.2 and +4.2.
- Reclamation Drought Index
The Reclamation Drought Index (RDI) was developed as a tool for defining
drought severity and duration, and for predicting the beginning
and end of a drought period.
As with the SWSI, the RDI is calculated at a river basin level.
It incorporates the supply components of precipitation, snowpack, streamflow,
and reservoir levels. The RDI differs from the SWSI in that it builds a temperature-based
demand component and a duration into the index. The RDI is
adaptable to each particular region and its main strength is its ability to account for
both climate and water supply factors. As shown in Table 3, the RDI values and severity designations
are similar to the SPI, PDSI, and SWSI.
|Table 3. RDI classification
|4 or more||Extremely wet|
|1.5 to 4||Moderately wet|
|0 to 1.5||Normal to mild wetness|
|0 to -1.5||Normal to mild drought|
|-1.5 to -4||Moderate drought|
|-4 or less||Extreme drought|
- Deciles (monthly drought)
Another drought-monitoring technique consists of arranging
the monthly precipitation data into deciles.
This technique was developed to avoid
some of the weaknesses of the "percent-of-normal" approach.
The technique divides the distribution of occurrences over a
long-term precipitation record into tenths of the distribution.
Each of these categories is a "decile."
The first decile is the rainfall amount not exceeded by the lowest 10%
of the precipitation occurrences. The second decile
is the precipitation amount not exceeded by the lowest 20% of occurrences.
These deciles continue until the rainfall amount identified by
the tenth decile is the largest precipitation amount
within the long-term record.
By definition, the fifth decile is the median, and it is the
precipitation amount not exceeded by 50% of the occurrences over the period of record.
The deciles are grouped into five classifications.
|Table 4. Decile Classification
|Much below normal|
Next lowest 20%
Next highest 20%
|Much above normal|
3.2 Intensity-duration-frequency |
The relations between drought intensity, duration and frequency can be
studied with conceptual models, which deal with meteorological droughts lasting at least one year,
with specific applicability to subtropical and midlatitudinal regions.
The climate types are defined across the climatic spectrum
in terms of the ratio
of mean annual precipitation to annual global terrestrial precipitation
Pma / Pagt , and additionally,
on the ratio of annual potential evapotranspiration to mean annual precipitation
Eap / Pma . To complete the description, the length
of rainy season Lrs across the climatic spectrum is also indicated.
For any year with precipitation P,
drought intensity is defined as the ratio of the deficit (Pma - P)
to the mean (Pma) .
For drought events longer than one year, intensity is the summation of the annual intensities.
The conceptual model of drought intensity-duration-frequency is
shown in Table 5.