Have streamflow droughts in Europe become more severe or frequent?

Changes in the magnitude and frequency of droughts will have extensive impa cts on water management, agriculture and aquatic ecosystems. With the proje cted global temperature increase, scientists generally agree that the globa l hydrological cycle will intensify and suggest that extremes will become o r have already become more common. In this study, a pan-European dataset of more than 600 daily streamflow records from the European Water Archive (EW A) was analysed to detect spatial and temporal changes in streamflow drough ts. Four different time periods were analysed: 1962-1990, 1962-1995, 1930-1 995 and 1911-1995. The focus was on hydrological droughts derived by applyi ng the threshold level approach, which defines droughts as periods during w hich the streamflow is below a certain threshold. The Annual Maximum Series (AMS) of drought severity and the frequency of droughts in Partial Duratio n Series (PDS) were studied. Despite several reports on recent droughts in Europe, the non-parametric Mann-Kendall test and a resampling test for tren d detection showed that it is not possible to conclude that drought conditi ons in general have become more severe or frequent. The period analysed and the selection of stations strongly influenced the regional pattern. For mo st stations, no significant changes were detected. However, distinct region al differences were found. Within the period 1962-1990 examples of increasi ng drought deficit volumes were found in Spain, the eastern part of Eastern Europe and in large parts of the UK, whereas decreasing drought deficit vo lumes occurred in large parts of Central Europe and in the western part of Eastern Europe. Trends in drought deficit volumes or durations could, to a large extent, be explained through changes in precipitation or artificial i nfluences in the catchment. Changes in the number of drought events per yea r were determined by the combined effect of climate and catchment character istics such as storage capacity. The importance of the time period chosen f or trend analysis is illustrated using two very long time series. Copyright (C) 2001 Royal Meteorological Society.