Explaining reported drought impacts in the European Alpine region with selected drought indices

Even across Europe’s generally water-rich Alpine region the number of reports on negative drought impacts increased recently. The Alpine Drought Impact report Inventory EDII_ALPS archives information of more than 3,200 specifically reported impacts with a majority in the last decade underlining the need for region-specific drought monitoring and adaptation strategies. The relation between drought conditions and drought impact occurrence has not been analyzed systematically in this heterogeneous mountain terrain. This study aims to improve such systematic understanding through the analysis of selected drought characteristics and reported impacts. Therefore, we assigned EDII_ALPS’ reported impacts as soil-moisture drought impacts (SMD) and hydrological drought impacts (HD) and explored statistically the relation of these two impact groups to the following drought indices: Soil Moisture Anomalies, Standardized Precipitation Index, Standardized Precipitation Evapotranspiration Index, Vegetation Condition Index and Vegetation Health Index. The density of the reported SMD impacts and HD impacts increased clearly, the stronger the index’ value indicates drought conditions - apart from the vegetation indices. However, the correlation tests between reported impacts and indices did not identify explicit linear relations. To capture non-linear effects and differences between reported SMD impacts and HD impacts we applied decision trees using recursive partitioning. This way, we identified the Standardized Precipitation and Evapotranspiration Index to be most important for reported HD impacts and the Soil Moisture Anomalies to be most important for reported SMD impacts. To predict impact occurrence we recommend to model and evaluate a combination of drought indices allowing non-linearities in order to improve drought impact monitoring and early warning.