SPI - Standardized Precipitation Index

CliSAP Product (P2)

: SPI3 November 2000

: SPI3 August 2003

: SPI1 August 2002

: SPI9 July 2005

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data access via file system:

/data/icdc/climate_indices/standard_precipitation_index_SPI

Description

The "Standardized Precipitation Index" (SPI) is used to describe extremely dry or wet climate situations. The World Meteorological Organization (WMO) recommends, that all national meteorological and hydrological services should use the SPI for monitoring of dry spells (Press report December 2009, WMO No. 872).

The advantages of SPI usage are:

Only precipitation data are needed for the calculation of the index.
The index is a standardized measure for precipitation in different climatic regions and for seasonal differences.
Calculated for different time scales: meteorological, agricultural-economic and hydrological.

Definition of SPI classes

SPI ≤ -2	Extremely dry
-2 < SPI ≤ -1.5	Severely dry
-1.5 < SPI ≤ -1	Moderately dry
-1 < SPI ≤ 1	Near normal
1 < SPI ≤ 1.5	Moderately wet
1.5 < SPI ≤ 2	Severely wet
SPI ≥ 2	Extremely wet

Calculation

The SPI, presented here, is different from the original SPI definition of McKee et al. 1993. An enhanced SPI is used, that significantly reduces errors resulting from the determination of the precipitation's distribution (Sienz et al. 2011). MC Kee et al. 1993 shifted the time series of the SPI one time step into the future, but this is not done for the calculation of the SPI presented here.

The SPI was calculated from two precipitation data sets:

European Climate and Data Assessment (ECA&D), E-OBS gridded dataset Version 4.0
Climate Research Unit (CRU), Version: CRU TS 2.1

Parameters

Name	Unit
Standardized Precipitation Index	dimension less
(Time scales: 1, 3, 6, 9, 12, 24 and 48 months)

Spatial and temporal resolution

SPI (ECA&D Precipitation), Europe
Time period:

January 1951 to December 2010, monthly

Geographically:

Longitude 11W to 60E (Resolution: 0.5 degree)
Latitude 35N to 72N (Resolution: 0.5 degree)

SPI (CRU Precipitation), Europe and USA
Time period:

January 1951 to December 2010, monthly

Geographically (Europe):

Longitude 11W to 60E (Resolution: 0.5 degree)
Latitude 35N to 72N (Resolution: 0.5 degree)

Geographically (USA):

Longitude 126W to 60W (Resolution: 0.5 degree)
Latitude 24N to 50N (Resolution: 0.5 degree)

Data quality

The quality of the SPI can not be better than the quality of the precipitation data used for its calculation. Inhomogeneities in the observed time series, the interpolation to the grid, as well as the irregular distribution of stations in space and time also affect the data presented here. On the quality of ECA & D data, see Haylock et al. (2008) and van den Besselaar et al. (2011) and for CRU data see Mitchell and Jones (2005).