The Ojos Negros Research Group


The characteristics of droughts are expressed in terms of: (1) drought index, (2) intensity-duration-frequency.

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 moreExtremely wet
1.5 to 1.99Very wet
1.0 to 1.49Moderately wet
-0.99 to 0.99Near normal
-1.0 to -1.49Moderately dry
-1.5 to -1.99Severely dry
-2.0 and lessExtremely 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. An abnormally 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 moreExtremely wet
3.0 to 3.99Very wet
2.0 to 2.99Moderately wet
1.0 to 1.99Slightly wet
0.5 to 0.99Incipient wet spell
0.49 to -0.49Near normal
-0.5 to -0.99Incipient dry spell
-1.0 to -1.99Mild drought
-2.0 to -2.99Moderate drought
-3.0 to -3.99Severe drought
-4.0 or lessExtreme 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 mountain snowpack.

    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 moreExtremely wet
1.5 to 4Moderately wet
0 to 1.5Normal to mild wetness
0 to -1.5Normal to mild drought
-1.5 to -4Moderate drought
-4 or lessExtreme 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
Deciles 1-2
Lowest 20%
Much below normal
Deciles 3-4
Next lowest 20%
Below normal
Deciles 5-6
Middle 20%
Near normal
Deciles 7-8
Next highest 20%
Above normal
Deciles 1-10
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.


Definition of Drought Impacts of Drought Drought Mitigation

Strategies for the Ojos Negros Valley Summary Drought Facts

http://ponce.sdsu.edu/three_issues_droughtfacts03.html 021104