Abstract. How drought is characterised depends on the purpose and region of the study and the available data. In case of regional applications or global comparison a standardisation of the methodology to characterise drought is preferable. In this study the threshold level method in combination with three common pooling procedures is applied to daily streamflow series from a wide range of hydrological regimes. Drought deficit characteristics, such as drought duration and deficit volume, are derived, and the methods are evaluated for their applicability for regional studies. Three different pooling procedures are evaluated: the moving-average procedure (MA-procedure), the inter-event time method (IT-method), and the sequent peak algorithm (SPA). The MA-procedure proved to be a flexible approach for the different series, and its parameter, the averaging interval, can easily be optimised for each stream. However, it modifies the discharge series and might introduce dependency between drought events. For the IT-method it is more difficult to find an optimal value for its parameter, the length of the excess period, in particular for flashy streams. The SPA can only be recommended as pooling procedure for the selection of annual maximum series of deficit characteristics and for very low threshold levels to ensure that events occurring shortly after major events are recognized. Furthermore, a frequency analysis of deficit volume and duration is conducted based on partial duration series of drought events. According to extreme value theory, excesses over a certain limit are Generalized Pareto (GP) distributed. It was found that this model indeed performed better than or equally to other distribution models. In general, the GP-model could be used for streams of all regime types. However, for intermittent streams, zero-flow periods should be treated as censored data. For catchments with frost during the winter season, summer and winter droughts have to be analysed separately.

Concern over the potential impact of anthropogenic climate change on flooding has led to a proliferation of studies examining past flood trends. Many studies have analysed annual-maximum flow trends but few have quantified changes in major (25-100 year return period) floods, i.e. those that have the greatest societal impacts. Existing major-flood studies used a limited number of very large catchments affected to varying degrees by alterations such as reservoirs and urbanisation. In the current study, trends in majorflood occurrence from 1961 to 2010 and from 1931 to 2010 were assessed using a very large dataset (>1200 gauges) of diverse catchments from North America and Europe; only minimally altered catchments were used, to focus on climate-driven changes rather than changes due to catchment alterations. Trend testing of major floods was based on counting the number of exceedances of a given flood threshold within a group of gauges. Evidence for significant trends varied between groups of gauges that were defined by catchment size, location, climate, flood threshold and period of record, indicating that generalizations about flood trends across large domains or a diversity of catchment types are ungrounded. Overall, the number of significant trends in major-flood occurrence across North America and Europe was approximately the number expected due to chance alone. Changes over time in the occurrence of major floods were dominated by multidecadal variability rather than by long-term trends. There were more than three times as many significant relationships between major-flood occurrence and the Atlantic Multidecadal Oscillation than significant long-term trends.

Abstract:Links between severe hydrological droughts and weather types (WTs) were explored to improve the understanding of hydroclimatological processes involved in the development of regional hydrological drought in north-western Europe. A new Regional Drought Area Index (RDAI) was developed, using daily streamflow, to represent the drought-affected area. Daily RDAI series were created for two regions with homogeneous drought behaviour in Denmark and four regions in Great Britain. An objective version of the Hess-Brezowsky Grosswetterlagen yielding 29 WTs was used. Regional drought characteristics, including duration and frequency, were found to vary considerably between regions. However, in 1976 and 1996, all regions experienced severe events, and these years were found to be the most severe drought years across the study region as a whole. The hydrological response time (i.e. the time over which WTs influence drought development) was found to vary markedly (45-210 days) between regions according to basin storage properties. WT-frequency anomalies (FAs) before and during the onset of the five most severe droughts were identified for each region. The dominant drought-yielding WTs changed between regions and between events within each region. High-pressure systems centred over the respective region were most frequently associated with droughts as well as WTs with a northern (N, NE or NW) or a southern (S, SE or SW) airflow over the Danish and British regions. Five of the six WTs associated with drought for all regions represented a northern high-pressure system (i.e. over Great Britain, Fennoscandia or the Norwegian Sea). This article demonstrates (1) hydrological response time to be fundamental in moderating drought response to mesoscale climatic drivers and (2) severe hydrological droughts may be caused by a complex set of hydroclimatological processes that vary between regions and events.

This review outlines the use of documentary evidence of historical flood events in contemporary flood frequency estimation in European countries.The study shows that despite widespread consensus in the scientific literature on the utility of documentary evidence, the actual migration from academic to practical application has been limited. A detailed review of flood frequency estimation guidelines from different countries showed that the value of historical data is generally recognised, but practical methods for systematic and routine inclusion of this type of data into risk analysis are in most cases not available. Studies of historical events were identified in most countries, and good examples of national databases attempting to collate the available information were identified. The conclusion is that there is considerable potential for improving the reliability of the current flood risk assessments by harvesting the valuable information on past extreme events contained in the historical data sets.