SPI - Standardized Precipitation Index from CRU / ECAD for EU and USA

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.

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:

1. European Climate and Data Assessment (ECA&D), E-OBS gridded dataset Version 4.0 (1951 - 2010) for Europe
2. Climate Research Unit (CRU), Version: CRU TS 2.1 (1901 - 2002) for Europe and USA