The European 2015 drought from a hydrological perspective
Abstract. In 2015 large parts of Europe were affected by a drought. In two companion papers we summarize a collaborative initiative of members of UNESCO’s EURO FRIEND-Water program to perform a timely pan-European assessment of the event. In this second paper, we analyse the event of 2015 relative to the event of 2003 based on streamflow observations. Analyses are based on range of low flow and hydrological drought indices for about 800 records across Europe that were collected in a community effort based on a common protocol. We compare the hydrological footprints of both events with the meteorological footprints presented by Ionita et al. (2016), in order to learn from similarities and differences of both perspectives and to draw conclusions for drought management. Overall, the hydrological drought of 2015 is characterised by a different spatial extent than the drought of 2003. In terms of low flow magnitude, a region around the Czech Republic was most affected with annual low flows that exhibited return intervals of 100 years and more. In terms of deficit volumes, the geographical centre of the event was in the area of Southern Germany where the drought lasted particularly long. A detailed assessment at various spatial and temporal scales showed that the different behaviour in these regions was also a result of diverging wetness preconditions in the catchments. Extreme droughts emerged where antecedent conditions were particularly dry. In regions with wet preconditions, low flow events developed later, and were mostly less severe. The space-time patterns of monthly low flow characteristics show that meteorological and hydrological events spread differently across Europe, and they evolved differently in regard to extent and severity. The results underline that drought is a hazard that leaves different footprints on the various components of the water cycle, on different spatial and temporal scales. The different dynamic development of major hydrometeorological characteristics, temperature and precipitation anomalies versus the streamflow magnitude, duration and deficit volume also determine differences in the impacts of hydrological drought on various water use sectors and on river ecology. For an assessment of drought impacts on water resources, therefore, hydrological data is required in addition to the hydro-meteorological drought indices. Additional efforts with a pan-European dimension need to be undertaken to make timely hydrological assessments more operational in the future.
