The "Standardized Precipitation Index" (SPI) is used to describe extremely dry or wet climate situations.
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 reference period used for calculation of all distributions is 1901-2020.
The SPIs (1, 3, 6, 9, 12, 24, 48) were calculated from the Climate Research Unit (CRU) precipitation data set, Version: CRU TS 4.05 for the period 1901 - 2020 for Europe and USA. It is an update and replaces the SPI from CRU by Frank Sienz. As various changes were made to the scripts, comparisons with examples of the results were made to ensure the quality of the data. The date specified in the files always indicates the end of the period under consideration.
