1. A Drought Catalogue for Europe: A tool for examining the spatial coherence of drought Jamie Hannaford, Simon Parry, Ben Lloyd-Hughes, Christel Prudhomme, Caroline Keef Centre for Ecology and Hydrology, Wallingford Walker Institute, University of Reading JBA Consulting, Skipton European Geosciences Union - General Assembly Session HS5.15: Large Scale Hydrology 24 April 2009

2. Background Major historical droughts have been documented previously
but not systematically (unlike floods)
Droughts vary in their spatial extent, duration, intensity, seasonality
Impacts can vary significantly from region to region…
Therefore an objective classification of droughts is preferred.
Why do we need a catalogue?
enable consistent comparison of drought characteristics between regions
examine temporal and spatial evolution of major European droughts
identify patterns of spatial coherence common to major droughts. Can they be explained?
facilitate the development of improved forecasting models
National/ regional drought assessments
Dartmouth Flood Observatory

3. Hydrological and Meteorological Data
Hydrological Data
Minimal artificial influence (generally small catchments)
Data
579 catchments with daily river flow
Most data from the FRIEND European Water Archive
France: Banque Hydro
UK: National River Flow Archive
Meteorological Data
From British Atmospheric Data Centre , CRU TS 3 monthly precipitation time series,
0.5 degree grid cells (~ 55km)
Data
A very important part of the work of quality check and updating has been done actually within the FRIEND framework by Dr. Stalh, for a separate project funded by UNESCO on trends in low flows, and we are extremely indebted to her to have shared with us the up-dated series as this was a very time consuming task. Also, Dr. Benjamin Renard in Cemagref in France has done a very thorough work of quality check of French catchments during his PhD , and we used the list of catchments he recommended as good quality in our study. Without the FRIEND network and these external collaborations, it would have been very difficult to successfully collate so many data.
Such project would not be possible without any data available accessible to us, free of charge. The UNESCO-FRIEND European Water Archive has been the major data provider for this project.
Here is a map of gauging stations where some daily river flow is stored in the EWA data base. The coverage is exceptional, in particular in western Europe, but also in eastern Europe. However, a some of the records held have not been updated in the database for a few years. This is a real problem for research, when the most recent events are not captured fully in the database.
The EWA data catalogue was updated the French time series as the Banque Hydro in France now provides free access to its river flow data for research, and from the NRFA in the UK.
We are extremely grateful to these organisation that share data.
However, there are still some places where we could not get information, such as for example Italy, and we could only insure records up to It means that we could not investigate the drought of fully due to lack of data coverage.

4. Hydrological Drought: RDI
For daily time series at each station, compare the measured river flow with a daily varying 90% percentile (Q90)
Flow time series transformed into binary series, or Deficit Index:
1 for deficit, or flow under threshold: yellow periods
0 for non deficit, or flow over threshold: white periods
For a region, the Regional Deficiency Index (RDI) is the proportion of stations which are below the threshold on any one day
Developed during ARIDE project (Stahl & Demuth, 1999)

5. River Pang, Berks Downs. Aug 2005
Homogeneous regions
The regions were derived using the Deficiency Index series of all gauging stations, and regrouping the stations with time series were low flows (1 in the DI) occur at the same time, while non low flow periods (0) occurred at the same time.
Even if no information about the location of the gauges was input in the algorithm used for the groups, the groups were naturally formed in distinct geographic areas. This reflect the very strong spatial characteristics of drought development, that needs some large-scale, long-lasting climatic conditions to develop into a drought. For an easier interpretation, and in order to derive the rainfall drought index RSPI, we also smoothed some of the regions boundaries, especially for areas where data from a dense river flow network was available. On purpose, we followed the administrative boundaries for certain regions where we did not have information on the country, as for example can be seen for the southern Alps and the border with Italy. This is because we did not have any data to corroborate any regional extent of our clusters.
Independently to the regions, we also looked at the hydrological regimes of the regions, to gain a better understanding of how drought might develop.
Regions based on previous work:
ARIDE project (Stahl & Demuth, 1999) and Prudhomme & Sauquet (2006)
River Pang, Berks Downs. Aug 2005

6. Meteorological Drought: RSPI
Standardised Precipitation Index (SPI) time series
Specifically designed to enable comparison between regions with different rainfall regimes
For each region, quantify proportion of grid cells under drought: Regional Standardised Precipitation Index (RSPI)

7. Example of Drought Catalogue Page (S. Germany)
RDI = River flow
RSPI = Rainfall
SPI – Rainfall time series
Flow Regime Plots
Seasonality Plots

8. Comparing Drought Characteristics of two UK regions
2003
1984
Catalogue can be used to compare the spatial extent and duration of droughts

9. European Catalogues Spain, 2005 Norway, 2002
More work needed to corroborate
regional droughts
Spain, 2005

10. Meteorological Drought – a longer view
Southern France
Pont du Gard in Aug 2003
(photo: H. Van Lanen)
1941 – 1948 drought (Annales de Géographie)

11. Coherence across Europe?

12. Application 1: examine evolution of major droughts

13. Application 1: examine evolution of major droughts

14. Application 1: examine evolution of major droughts

15. Application 1: examine evolution of major droughts

16. Application 1: examine evolution of major droughts

17. Application 1: examine evolution of major droughts

18. Application 1: examine evolution of major droughts

19. Application 1: examine evolution of major droughts

20. Application 1: examine evolution of major droughts

21. Application 1: examine evolution of major droughts

22. Application 1: examine evolution of major droughts

23. Application 1: examine evolution of major droughts

24. Application 1: comparing evolution of major droughts
1975/1976: multi-season drought. Coherent drought in NE Europe
2003: summer drought. UK generally not coherent with Europe

25. Application 2: examining spatial coherence
Correlation in RSPI between two British regions & other European regions
NW Great Britain
SE Great Britain

26. Application 2: explaining spatial coherence?
North Atlantic Oscillation
East Atlantic/West Russia Circulation
Can we provide a physical interpretation for spatial coherence?

27. Application 3: forecasting models
Question: can we predict length of drought given:
average RDI in other ‘predictor’ regions
RSPI in target region
Red lines: predicted
Black lines: observed
NW Britain
SE Britain
1976
2003
A work in progress – but a potential new method of forecasting

28. Concluding remarks
Drought catalogue provides a means of comparing drought characteristics across Europe using a consistent dataset and methodology
Considers both hydrological (1961 – 2005) and meteorological (1901 – 2005) drought
For 24 regions, characterises: occurrence, duration, spatial coherence, seasonality
Evidence suggests there is some synchronicity in major European drought episodes
Method provides a framework for visualising the development of major droughts
Research Applications
Explore regional coherence in drought indicators
Examine relationships between regional indicators and atmospheric circulation
Develop forecasting tools based on regional coherence
Major issue:
Lack of coverage in some areas of Europe