Droughts are complex natural hazards that, to a varying degree, affect some part of Europe almost every year. The temporary shortage of water poses a great threat to nature, the quality of life and the economy. Any future increases in the demand for water will be most critical in periods of severe low flow and extensive droughts. As drought is a slowly developing phenomenon, only indirectly affecting human life, its impacts are often underestimated in relatively rich regions such as Europe. However, recent years have shown how vulnerable countries can be to drought, as they often cover larger regions and extend for longer periods than floods. Unlike aridity, drought is a naturally occurring phenomenon and can be character-ised as a deviation from normal conditions of such variables as precipitation, evaporation, soil moisture, groundwater or streamflow. The last two quantities, used to define a hydrological drought, represent the available water resources at a site or in a region, and their low flow and drought characteristics provide threshold values for different water based activities.
It is difficult, because of its complexity, to define precisely the onset, duration and severity of a drought. As a consequence there are numerous definitions available and a lack of consistent analysis tools. This has hampered the progress of research and limited operational applications, which favour standardised measures. Traditional low flow measures, defined as one characteristic of the low flow domain (e.g. the mean annual minimum flow), do not suffer the same constraints. Low flow processes and different low flow measures have been discussed thoroughly in earlier FRIEND reports. The focus in this phase of FRIEND has been on hydrological drought: its definition, extreme characteristics, variability in time and space and synoptic behaviour across Europe. Common to many of the studies is the use of the threshold level method to define drought events from a time series of observations. A sequence of drought events is obtained when the variable is below a threshold level, with each event being characterised in terms of its duration and deficit volume. The European Water Archive has offered a unique opportunity to study the regional behaviour of streamflow droughts.
Time series of precipitation, and groundwater recharge and heads have also been analysed.
The work in the Low flow group has ranged from statistical multivariate analysis at the regional and European scale to physically-based and conceptual modelling studies at the catchment scale. A common objective has been to characterise the spatial and temporal variability of low flow and hydrological droughts and to identify the main factors governing low flow and droughts at different scales. As the Low flow group has continued to grow, cooperation has evolved around smaller groups, such as the European Union supported project ARIDE and the Eastern European group. This has proved an efficient way to organise the work, with common meetings or workshops every 12-18 months. More recently the Textbook group was established. Based on the vast experience of the members of the Low flow group, it was decided to produce a synthesis of this knowledge and publish a textbook on low flow and hydrological drought. The book is aimed at both professionals and students and will be published by Elsevier Science Publications in 2003. In addition to subgroup and common meetings, exchange of students and joint excursions have also proved to be beneficial.
The key objectives of the ARIDE project (1998-2000) were to improve the understanding of processes that control European droughts, our ability to predict their duration, magnitude and extent for a given frequency and the sensitivity of droughts to environmental changes (Demuth
& Stahl, 2001). Consistent drought definitions were derived and applied to quantify temporal and spatial variations in meteorological droughts in terms of lack of precipitation, as well as hydrological droughts in terms of streamflow and groundwater deficits (Hisdal & Tallaksen, 2000; van Lanen & Peters, 2000; Tate & Gustard, 2000). The main objectives of the Eastern European group (established in 1999) have been to exchange data, harmonise methods and software tools for drought analysis, and jointly synthesise the results. The focus has been on regional statistical methods, identification of important catchment and climate characteristics influencing severe droughts and long-term fluctuations in time series of drought. A regionalisation program for low flow and drought analysis has been developed and applied to a joint east-European dataset.
The research results of the low flow group have been published in international journals and conference proceedings and cover a wide range of low flow topics, e.g. the link between streamflow drought and atmospheric circulation patterns (Stahl & Demuth, 1999), impact assessment of drought mitigation measures (Querner & van Lanen, 2001), flow variables for ecological studies (Clausen & Biggs, 2000), assessment of small-scale hydropower potential (Rees, Croker, Prudhomme & Gustard, 2000), hydrological drought and climate variability (Kašpárek & Novický, 2002), frequency analysis of droughts (Tallaksen, 2000), seasonality aspects of low flows (Laaha, 2002), changes in water balance and hydrological drought in Poland (Kasprzyk & Pokojski, 1998), and changes in groundwater runoff in Slovakia (Fendekova, 1999). Two regional drought studies are presented in more detail below.
Have droughts in Europe become more severe or frequent?
As a part of the ARIDE project, a study was carried out to investigate whether droughts in Europe have become more severe or more frequent (Hisdal et al., 2001). The topic is important because many experts cite the occurrence of recent droughts as evidence of climate change.
More than 600 daily records from the EWA were analysed to detect spatial and temporal changes in drought patterns. Figure 2.4 shows the results obtained by applying the non-parametric Mann-Kendall trend test on annual maximum series of drought deficit volumes (AMV) for the period 1962-90. For most stations, there are no significant changes. However, distinct regional differences can be seen. Examples of increasing drought deficit volumes are found in Spain, the western part of Eastern Europe and in large parts of the UK, whereas, decreasing drought deficit volumes occur in large parts of Central Europe and in the eastern part of Eastern Europe.
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Despite anecdotal evidence of increased drought frequency in Europe, the results showed that it is not possible to conclude that drought conditions, in general, have become more severe. It is important to realise that what is seen as an increased trend in droughts, based on a ‘short’ period of observations, might turn out to be a part of a long-term fluctuation and cannot be seen as evidence of human-induced climate change nor be used as a basis for drought prediction in the future. The period analysed and also the selection of stations strongly influenced the regional pattern. 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 controlled by the combined effect of climate and catchment characteristics such as storage capacity.
Monitoring droughts
Characterising the dynamic development of historic drought events could help to predict how drought conditions might spread during future events. This would be important for early warning systems, mitigation and improved drought management. A methodology for deriving Figure 2.4 Spatial distribution of the Mann-Kendall test statistic for the period 1962-90
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Mann-Kendall test statistic for AMV
time series of daily flow exceedance from records held on the EWA has been developed within the ARIDE project, including facilities to retrieve time-series of daily mean flow and transformation of daily flow time series to daily exceedance series for selected drought years.
A prototype of the electronic drought atlas ElectrA has been developed (Zaidman et al., 2000), based on a regularly spaced grid of 0.25° × 0.25° resolution (see Figure 2.5). Different mapping procedures, based on catchment area, have also been investigated. A pilot software application to evaluate the possibilities of a near-real-time drought monitoring and forecasting system for Europe was also tested, based on near-real-time data from over 50 gauging stations in Europe.
In future this system could result in a geographical map of Europe displaying daily streamflow conditions as exceedance values, but there are still some political and technical issues to be resolved before such a system can be realised. The work conducted within the ARIDE project represents an important step towards delivering a tool for a European Drought Watch System for the water industry, but a considerable amount of further research and development is still needed to establish an operational system.
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Figure 2.5 Electronic drought atlas