Drought: A Paleo Perspective

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Standard Precipitation Index (SPI)


SPI Map Example The The Standard Precipitation Index (SPI) was designed to enhance the detection of onset and monitoring of drought (McKee et al, 1993). The SPI is a simpler measure of drought than the Palmer Drought Severity Index (PDSI) and is based solely on the probability of precipitation for a given time period. A key feature of the SPI is the flexibility to measure drought at different time scales. Because droughts vary greatly in duration, it is important to detect and monitor them at a variety of time scales. Short-term droughts are measured by meteorological instruments and are defined according to specific regional climatology. Agriculturally important droughts result in deficits in soil moisture and three- to six-month droughts can have a great impact. Longer-term droughts (months to years) can have important impacts on surface and ground water supplies.

Values of SPI are derived by comparing the total cumulative precipitation for a particular station or region over a specific time interval (for example: the last month, the last 3 months, the last 6 months) with the average cumulative precipitation for that same time interval over the entire length of the record. For example, total precipitation in May of any given year for the northwestern Kansas climate region would be compared to average total precipitation for that region for all Mays in the record, 1895-1998. The severity of a drought can be compared to the average condition for a particular station or region. Values range from 2.00 and above (extremely wet) to -2.00 and less (extremely dry) with near normal conditions ranging from 0.99 to -0.99.

The classification values for SPI values are:

SPI Value:

Drought Category:

2.00 and above

Extremely wet

1.50 to 1.99

Very wet

1.00 to 1.49

Moderately wet

-0.99 to 0.99

Near normal

-1.00 to -1.49

Moderately dry

-1.50 to -1.99

Severely dry

-2.00 and less

Extremely dry

A drought event is defined when the SPI is continuously negative and reaches a value of -1.0 or less, and continues until the SPI becomes positive. Drought duration is defined by the interval between the beginning and end of that period. The magnitude of the drought event is measured by the sum of the SPI values for the months of the drought.


For more information on the Standard Precipitation Index, click here

To view current SPI maps of the U.S., click here.

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