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Standard
Precipitation Index (SPI) 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.
To view current SPI maps of the U.S., click here. Back to... What is Drought?. |
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