The Spatial Coherence of European Droughts – Final Report

Drought can cause serious problems across much of Europe. Many droughts are localised and short, but others are widespread and cause environmental and social effects that cross international boundaries. Some of the most important UK droughts were also significant droughts across much of Europe. Intuitively, it would seem that there may be considerable potential for developing improved drought monitoring and forecasting tools by examining the spatial coherence of droughts on a continental scale. This project has considered the potential for developing new approaches to forecasting drought by asking the following research questions: • Is there any systematic time lag between the onset and development of droughts in different parts of Europe? • Can the onset and development of droughts in some parts of Europe provide an early warning for the development of droughts in other parts of Europe, and in particular, in the UK? • Can these relationships be used to build reliable and robust operational tools for UK drought forecasting? The method, which draws on a unique archive of flow and rainfall data from across much of Europe, involved the following steps. 1. Calculate a normalised deficiency index for each site – a measure of drought that allows comparison between locations with different climatological and hydrological regimes, and between different seasons 2. Group catchments with similar drought characteristics into regions 3. Develop standardised flow and rainfall deficiency indices for these regions 4. Analyse relationships between regions and develop statistical models to predict drought. Twenty-four homogenous regions were identified across Europe; catchments within these groups frequently experience simultaneous streamflow deficiencies. Four distinct geographical regions emerged in the UK. A further group, comprising very slow-responding catchments (Base Flow Index > 0.8), was identified in southeast England. For each of these regions, time series of regional streamflow and rainfall deficits were defined and a catalogue of regional drought severity developed, spanning 1901 – 2005 for meteorological droughts, and 1961 – 2005 for hydrological droughts. This enabled a characterisation of major drought periods, in terms of duration, seasonality and spatial coherence in the various regions. This drought catalogue is a major deliverable of this project, and will be of considerable practical utility for drought management and future research in the UK and in Europe. For major post-1961 streamflow droughts, a comprehensive description of the extent and spatio-temporal development of the drought was provided. A standalone publication has been produced, which illustrates the evolution of streamflow and rainfall anomalies, along with climatic drivers and large-scale atmospheric circulation anomalies for major droughts (e.g. 1975 – 76; 1988 – 1992). From an appraisal of these events, it is clear that most droughts appear to have different characteristics, in terms of their duration, spatial coherence and seasonality. For example, a contrast was found between the 1976 drought, which was spatially consistent across much of Europe and was combined with a rainfall deficiency the preceding winter and a heat wave in the summer, and the 1995-1997 drought, which was interspersed by wet episodes and had little long-lasting spatial coherence over Europe. In most historical events, the UK experienced drought simultaneously with other European regions, or earlier; there was little evidence of any systematic lag time which could be readily exploited in the development of early warning systems for the UK based on conditions in other parts of Europe. An exploratory data analysis was then carried out, to determine whether there are relationships in the drought indicators which could be exploited to develop forecasting tools. Correlation analysis, multidimensional scaling and statistical modelling were applied to find relationships, which were generally fairly weak. Low correlations exist between regional drought deficiency time series of different regions, and the correlation patterns for hydrological and meteorological droughts are similar, albeit slightly higher for the latter. Correlations with the rest of Europe are stronger in winter than in summer for northern and western Britain, but are of similar magnitude all year round for southeast England. Although a relationship was identified between the length of a UK drought and the number of regions contemporaneously experiencing drought elsewhere in Europe, it was found that this relationship was not statistically significant. Following these exploratory analyses, statistical models were built for each UK region, which predict the number of drought months that may occur in the next 6 months. Predictions are based on streamflow deficiencies in other European regions, so the models essentially predict ‘drought from drought’ – i.e. they use the spatial coherence of anomalies to derive forecasts for the UK based on deficiencies on the continent. The models forecast droughts in groundwater-dominated catchments in southeast England reasonably well. In northwest Britain, however, the predictive capability is poor. Importantly, the models have some significant benefits when compared to previous seasonal forecasting studies – in particular, the approach is based on large regions, rather than being ‘tuned’ to particular catchments, and they enable forecasting of winter anomalies rather than just summer flows. Furthermore, the models perform reasonably well at forecasting the cessation of drought conditions. These attributes mean that the models could potentially be of high utility during long, multi-season drought events, to determine whether a drought is likely to intensify or to diminish. Whilst the predictive capacity is modest in some regions, the models clearly have potential for application in UK drought management, although there are also important practical considerations – in particular, the need for timely data supply from across Europe – which would need to be examined in further research before they could evolve into an operational tool. Further analysis concentrated on attempting to explain observed patterns of spatial coherence, by linking drought indicators to large scale modes of atmospheric variability (e.g. the North Atlantic Oscillation and the East Atlantic-West Russia pattern). In some regions and some seasons, these predictors clearly play an important role in determining the spatial coherence of droughts. Whilst their predictive capability is relatively weak at present, there is undoubtedly scope for refining these relationships into tools for monitoring and providing indicative forecasts. An advantage of this approach is that some climatological indicators are routinely forecast (although the modest skill levels are a further obstacle to application at present). The regional drought indicators are shown to be powerful tools for illustrating the dynamics of rainfall and streamflow deficiencies. They could therefore find application in UK and European drought monitoring systems. Again, there would be important practical limitations to consider, and further research would be needed to optimise the indicators for use in monitoring. However, they could potentially fill an important gap; existing monitoring European drought monitoring systems lack a streamflow component, whilst UK approaches (e.g. CEH’s monthly Hydrological Summaries) consider runoff deficiencies but do not use any metrics tailored specifically to drought.