Drought monitoring in Croatia using the Standardized Precipitation-Evapotranspiration Index

Several droughts during the last decade seriously affected large parts of Europe, including Croatia, causing significant economic losses, particularly in the agriculture and energy sectors. Though rainfall is the main driver of drought, high temperatures during summer months may intensify its development with devastating consequences. Such was the case in 2022 when long dry spells were accompanied by several heat waves. In this study, climatological drought monitoring with the Standardized Precipitation-Evapotranspiration index (SPEI) was analysed with the primary aim of finding the best theoretical distribution for fitting the water balance in Croatia before implementing the index in the official operational procedure for drought monitoring. Monthly precipitation amounts and air temperature values were employed at 31 main meteorological stations for the period 1961-2022 and the water balance (defined as a difference between precipitation and potential evapotranspiration) was calculated for different time scales (1 - 24 months). Among five selected distributions, a three-parameter generalized logistic (GLO) distribution was found as the most appropriate one. There is a general agreement between SPEI and SPI time series, both in sign and intensity of droughts. However, a tendency toward higher drought intensity prevailed with the SPEI, particularly in periods with a light to moderate lack of precipitation and high air temperature. Comparison in precipitation and water balance in the two reference climate periods (1961-1990 and 1991-2020) revealed: a consistent trend toward drying conditions from March to August; an increase in the frequency of dry spells; and a tendency toward longer dry spells (for time scales up to 9 months). A comparative analysis of the 2022 drought in Croatia confirmed the ability of SPEI to detect drought earlier than the SPI, also suggesting that a larger area of the country was affected by drought due to high-temperature anomalies.