Have streamflow droughts in Europe become more severe or frequent?

Abstract

Changes in the magnitude and frequency of droughts will have extensive impacts on water management, agriculture and aquatic ecosystems. With the projected global temperature increase, scientists generally agree that the global hydrological cycle will intensify and suggest that extremes will become or have already become more common. In this study, a pan-European dataset of more than 600 daily streamflow records from the European Water Archive (EWA) was analysed to detect spatial and temporal changes in streamflow droughts. Four different time periods were analysed: 1962-1990, 1962-1995, 1930-1995 and 1911-1995. The focus was on hydrological droughts derived by applying the threshold level approach, which defines droughts as periods during which the streamflow is below a certain threshold. The Annual Maximum Series (AMS) of drought severity and the frequency of droughts in Partial Duration Series (PDS) were studied. Despite several reports on recent droughts in Europe, the non-parametric Mann-Kendall test and a resampling test for trend detection showed that it is not possible to conclude that drought conditions in general have become more severe or frequent. The period analysed and the selection of stations strongly influenced the regional pattern. For most stations, no significant changes were detected. However, distinct regional differences were found. Within the period 1962-1990 examples of increasing drought deficit volumes were found in Spain, the eastern part of Eastern Europe and in large parts of the UK, whereas decreasing drought deficit volumes 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 influences in the catchment. Changes in the number of drought events per year were determined by the combined effect of climate and catchment characteristics such as storage capacity. The importance of the time period chosen for trend analysis is illustrated using two very long time series.

Determination of the Jenkinson and Collison’s weather types for the western Mediterranean basin over the 1948-2009 period. Temporal analysis

“…Trigo and DaCamara (2000) showed a decrease in the frequency of cyclonic weather types in Portugal, and Piervitali et al (1997) and Brunetti et al (2000) did the same for Italy. Corte-Real et al (1998) indicated that the general decrease in precipitation identified in the Iberian Peninsula from 1960 to 1995 (Hisdal et al, 2001) coincides with the decrease in the frequency of weather types shown to generate precipitation (cyclonic, westerly and south-westerly) and also with an increase in anti-cyclonic weather types. Vicente-Serrano and López-Moreno (2006) found trends of types A and C opposite to the ones found herein, but for a grid centered over the Iberian Peninsula and for the 1952-1999 period, and a previous reclassification of type U days.…”

Observed drought and wetness trends in Europe: an update

“…In the international literature there are several papers addressing the trend detection in drought episodes both at regional and large-scale level in Europe, using different data sources and methodologies (see for example Hisdal et al, 2001;Lloyd-Hughes and Saunders, 2002;Moreira et al, 2006;Briffa et al, 2009;Trnka et al, 2009), but no comprehensive study has been carried out with updated data. In agreement with our findings, all these efforts demonstrate the spatial variability of the detected trends and the crucial influence of the time records selected for the analysis.…”

Drought Regionalization Based on Low Flow Trends in Selected River Basins in Slovakia

“…Hydrological drought analyses in terms of stream flow deficits are said to be studies over a season or longer time periods and in a regional context. A stream flow drought event definition quantitatively defines whether the flow can be regarded as being in a drought situation or not and gives the duration of a drought, whereas low flow indices characterise specific features of the low flow range [9].…”

“…During the past decades, many parametric and nonparametric techniques for the detection of long-term trends in time series were developed and applied [10]. A discussion on whether to choose a parametric or non-parametric test can be found in [11].…”

“…Despite several reports on recent droughts in Europe, the non-parametric Mann-Kendall test and a resampling test for trend detection in the paper [10] was showed that it is not possible to conclude that drought conditions in general have become more severe or frequent. The period analysed and the selection of stations strongly influenced the regional pattern.…”