| SPI Values | Category |
|---|
| ≥ 2 | Extremely wet |
| 1.5 to 1.99 | Very wet |
| 1.0 to 1.49 | Moderately Wet |
| -0.99 to 0.99 | Near normal |
| -1.0 to -1.49 | Moderately dry |
| -1.5 to -1.99 | Severely dry |
| ≤ -2 | Extremely dry |
Standardized Precipitation Index (SPI)
The standardized precipitation index (SPI) was calculated for 31 main meteorological stations in Croatia at different time-scales: 1, 2, 3, 6, 9, 12, 18 and 24 months.
The gamma probability distribution was fitted to the precipitation sums at different scales and the parameters were estimated by the maximum likelihood method (Wilks, 2006). The corresponding cumulative probability function was then transformed into the standard normal distribution with a zero mean and standard deviation equal to one providing SPI values. The SPI classification scale (Table) indicates near normal, moderately, severely and extremely wet/dry conditions (WMO, 2012). The bar colors indicate the corresponding SPI category from Table.
For the spatial interpolation of the SPI, the inverse distance method was used. On each map, the corresponding histogram presents a percentage of Croatian territory affected by a particular category of SPI (from the Table) for each time scale.
WMO (2012): Standardized Precipitation Index User Guide, (WMO-No. 1090), World Meteorological Organization, Geneva, Switzerland.
Wilks, D.S. (2006) Statistical Methods in the Atmospheric Sciences. 2nd Edition, Academic Press, London.
