| SPEI 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-evapotranspiration index (SPEI)
The standardized precipitation-evapotranspiration index (SPEI; Vicente-Serrano et al. 2010) was calculated for 30 main meteorological stations in Croatia at different time-scales: 1, 2, 3, 6, 9, 12, 18 and 24 months using monthly precipitation sums, mean maximum and minimum air temperature from the period 1981 - 2020.
First, the water balance was estimated for each month as the difference between precipitation (P) and potential evapotranspiration (PET), the letter was assessed using the method of Hargreaves - Samani (1985). The log-logistic probability density function was fitted to the P-PET series at different time scales and the distribution parameters were estimated by the L-moment method (Hosking and Wallis, 1997). The corresponding cumulative probability function was then transformed into the standard normal distribution with a zero mean and standard deviation equal to one. The obtained value represents the SPEI. The SPEI classification scale (Table) indicates near normal, moderately, severely and extremely wet/dry conditions. The bar colors indicate the corresponding SPEI category from Table
For the spatial interpolation of the SPEI, the inverse distance method was used. On each map, the corresponding histogram presents a percentage of Croatian territory affected by a particular category of SPEI (from the Table) for each time scale.
Hargreaves, G. H. & Z. A. Samani (1985): Reference crop evapotranspiration from temperature. Applied Engineering in Agriculture, 1, 96–99.
Hosking, J. R. M. & J. R. Wallis (1997): Regional Frequency Analysis. Cambridge University Press.
Vicente-Serrano, S. M., S. Beguería & J. I. López-Moreno (2010): A multiscalar drought index sensitive to global warming: the standardized precipitation evapotranspiration index. Journal of Climate, 23, 1696-1718.
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.
