Journal Name 2016, x, x; doi:10.3390/ www.mdpi.com/journal/xxxx
Multi-Index Drought Assessment in Europe
Panagiotis D. Oikonomou 1,*, Christos A. Karavitis 2, and Elpida Kolokytha 3
Received: date; Accepted: date; Published: date
Academic Editor: name
1 Colorado Water Institute, Colorado State University, Campus Delivery 1033, Fort Collins, CO 80523-1033,
2 Water Resources Sector, Department of Natural Resources Development and Agricultural Engineering,
Agricultural University of Athens, 75 Iera Odos, 11855, Athens, Greece; email@example.com
3 Division of Hydraulics and Environmental Engineering, Department of Civil Engineering, Aristotle
University of Thessaloniki, University Campus, 54124 Thessaloniki, Greece; firstname.lastname@example.org
* Correspondence: email@example.com; Tel.: +1-970-491-6328
Abstract: Any attempt for the application of integrated drought management, requires identifying
and characterizing the event per se. The questions of scale, boundary, and of geographic areal
extend are of central concern for any efforts of drought assessment, impacts identification, and thus
of drought mitigation implementation mechanisms. The use of drought indices, such as
Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration
Index (SPEI), has often lead to pragmatic realization of drought duration, magnitude and spatial
extend. The current effort presents the implementation of SPI and SPEI on a Pan-European scale
and it is evaluated using existing precipitation and temperature data. The E-OBS gridded dataset
for precipitation, minimum temperature, and maximum temperature used covered the period 1969
– 2018. The two indices estimated for time steps of 6, and 12 months. The results for the application
period of recurrent droughts indicate the potential that both indices offer for an improvement on
drought critical areas identification, threshold definitions and comparability, towards contingency
planning leading to better mitigation efforts.
Keywords: Drought; precipitation; SPI; SPEI; Europe
Drought is a normal, periodic natural hazard, although often inaccurately pictured as an
unexpected and exceptional phenomenon. It strikes practically all the planet, but its characteristics
vary significantly from one region to another [1,2]. Drought is a temporary anomaly of the usual
climatic events and it is considered a creepy slow evolving natural hazard, quite different from
aridity, which is a long-term, permanent part of a climatic zone [3–9]. Droughts are generally caused
by a combination of natural events that many times are boosted by anthropogenic pressures. The
most common definition of drought is a rainfall deficiency, whose occurrence, distribution, and
magnitude affect the existing water supply, demand, and consumption. Such deficiency may lead to
in less than expected water quantities necessary for the natural and the societal systems.
Droughts can befall anywhere in both high and low rainfall areas, in any locale and in any
season. Drought impacts are exacerbated, when drought strikes a region with already limited water
resources, and/or misuse and mismanagement of water and with discrepancies between water
demand and water supply.
Since there is no single definition of drought, its beginning and ending points are d ifficult to be
accurately determined. Thus, it is difficult for decision makers and stakeholders to initiate measures
to confront drought timely and accurately. In this quest, a drought indicator may be proved a
valuable tool. Drought indicators are conveying objective information about a system’s status that
may aid decision makers to identify the onset, magnitude and duration of a drought. Nevertheless,
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the literature agrees that no single index alone can precisely describe the spatial extent, the duration
and the magnitude of the phenomenon. Given such characteristics, appropriate and effective drought
early-warning systems should be based on multiple indices and/or a synthesis of indicators to
sufficiently demarcate the drought events [5,6,8,10–16].
Currently very few indicators may appropriately illuminate all the drought dimensions at a
large scale. In addition, applying multiple and /or a combination of indicators provides crucial
information to monitor and categorize droughts. There exists a plethora of climatic, water supply and
demand indices to illustrate the drought dimensions and to portray them in a stochastic posture. Each
index has strengths and weaknesses, with none being superior to the other in its specific application.
In this regard, SPI and SPEI offer a very well tested and dependable combination of indicators, thus
they were chosen for application to describe Drought conditions in Europe during the latest decades.
Drought events have regularly occurred all over Europe and particularly in the last fifty years.
The spatial extent, the magnitude, the duration of such drought events, as well as the diversified
impacts inflicted on societies and the environment varied all over this period. Existing information
in the pertinent literature categorizes the most harsh events that distressed more than (30%) of the
EU territory as the ones in 1972-74, 1990-94, 2000, 2003, 2007 and 2011 with the most recent in 2018
Drought information in the literature exposed that there are two distinct geographical regions
in Europe reflecting mostly common meteorological, environmental and geomorphological
conditions: the southern Mediterranean corridor from the Atlantic Ocean to Asia Minor including
Portugal, Spain, southern France, Italy, Greece, and Cyprus); and the Northern one beyond the Alps
mountain chain having Belgium, UK, Finland, Germany, Hungary, Lithuania, Netherlands, Norway,
Slovakia [6,7,17–21]. It is within these two regions that drought dimensions namely spatial extent,
duration (temporal extent), and magnitude are markedly pronounced.
Drought spatial extent is closely associated to a country’s given geographical locale and total
area with the smaller countries to be usually devoured by the event per se (Cyprus, Greece, Italy,
Malta, Spain, Portugal, France, Ireland, Great Britain, Denmark, Latvia, Estonia, etc.). Drought
magnitude diversifies all over the continent with the most prominent the 1990-94, 2000 and 2007 ones
in Spain Italy, Greece, France and Hungary [4,17–19]. Drought duration is equally fluctuating from
country to country. In the Mediterranean area Cyprus, Greece, Italy, Malta, Southern France, Portugal
and Spain, are having an extended summer period annually with minimal rain. Thus, droughts may
only manifest themselves during the rainy winter months. In other words, a drought may have a six-
month duration which compounding to the arid summer period creates a full problematic year
[4,6,7,22,23]. In the northern countries, droughts occur primarily during the rainy summer season
having durations from one month (Germany, Hungary, and Lithuania) to two up to six months
(Northern France, Austria, Belgium). It is noted that Finland was distressed by a nine-month drought
from August 2002 to April 2003 [24,25]. The estimation of the foremost drought impacts usually
involves economic costs resulting from the various droughts. Such estimations depict the overall
economic impacts of droughts during the last fifty years to more than 100 billion € at EU level. They
also present that the annual average impacts doubled from the 1976-1990 period to the 1991-2006 one.
Overall, the impacts cost on the average 6.2 billion €/year up to 2003, with an escalation to 8.7 billion
€ during the 2003 drought . In the 2018 summer as shown in Figure 1, the majority of northern
Europe is under a drought spell, including Ireland, Great Britain, Netherlands, Belgium, Northern
France, Germany, Czech Republic, Denmark, Norway, Sweden, Estonia Latvia and Finland .
In this regard, drought impacts are already influencing the agricultural production in the region.
According to EC (2018) the decrease in crop yields will exceed 50% in the majority of these countries,
reaching up to 70% in Estonia. Hence, on August the 30th 2018, the European Commission offers
advanced payments to distressed farmers to receive up to 70% of their direct payment and 85% of
payments under rural development by mid-October 2018. It is pointed out that such compensations
refer to economic costs and do not incorporate social and environmental costs as relevant data are
not available. All in all, the improvement of the economic cost estimation has to comprise social and
environmental impact assessments in an EC level approach.
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Figure 1. Weather Situation in EU Europe during July and August 2018 .
2. Materials and Methods
To produce SPI and SPEI, the ensemble version of the E-OBS dataset , which covers the area
of 25N-71.5N x 25W-45E, in 0.25 degree regular latitude-longitude grid resolution, was used. The
period on record of the E-OBS dataset starts on January 1950 and extends until September 2018. The
information retrieved includes the following parameters: daily minimum temperature, daily
maximum temperature, and daily precipitation sum. The data files are in NetCDF-4 format and their
temporal resolution is daily following the regular calendar (including leap years). All data
manipulation was performed in R  utilizing ncdf4 , raster , plyr , abind , and SPEI
 R packages.
For the computation of the 6-month and 12-month SPI, daily precipitation for the study period
(Jan. 1969 - Sep. 2018) was converted to a monthly step. Missing value criteria for each one of the grid
cells' (93,264 in total) daily time series were set for quality control purposes. Such criteria are that the
missing daily values within a month should not exceed 35% or they should not exceed 30% if the
missing data are continuous. The minimum (maximum) daily temperature data were transformed to
monthly mean. Daily minimum (maximum) temperature also based on the aforementioned criteria.
Monthly evapotranspiration was computed for each grid cell based on the 1985 Hargreaves method
 in order to be used as input for the SPEI index calculation.
3. Results and Discussion
The resulting values were spatially visualized in a GIS environment. According to the
classification presented in Figure 2. The 1990, 1993, 2003, 2007, 2015 and 2018, droughts were
identified and spatially portrayed. From these droughts, the most intense drought periods were
chosen to be included in the current effort namely the 1990, 2007 and 2018 ones. These events are
presented in Figures 3, 4 and 5.
Figure 2. SPI Classification scale.
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Figure 3. SPI and SPEI for Europe on April and August 1990, a) 6-month step and b) 12-month step.
From Figure 3, it may be deduced that the drought was spread out all over Europe. The distinct
behaviour of southern Europe points out that drought is intensified at the end of the usually rainy
winter season. Such an event was recorded in the pertinent literature [4,5,7,19,22]. Particularly in
Greece precipitation was only 43% of the annual average , a fact also portrayed in Figure 3. On
north-western Europe drought reaches its peak at the end of the summer period, when the usual
rains are crucial also for agriculture. The pertinent literature reported that during 1989, the weather
all over Europe was unusually dry. This particular trend has continued in 1990, and drought alert
was issued in many European countries [5,35].
Figure 4. SPI and SPEI for Europe on April and August 2007, a) 6-month step and b) 12-month step.
From Figure 4 is more evident in North eastern Europe. Such an event was recorded by EEA 
and Spinoni et al. . Karavitis et al.  also report the manifestation of a rather minor drought in
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Figure 5. SPI and SPEI for Europe on April and August 2018, a) 6-month step and b) 12-month step.
The 2018 drought clearly manifests its spell on the Northern part of Europe as portrayed in
Figure 5. These facts are also shown in Figure 1, as well as in the pertinent literature [20,26,36]. By
comparing the various drought incidents as portrayed by SPI and SPEI, it may be derived that the
most intense drought was the greatest on record for the given time period.
Effective decision-making is paramount for improving the assessment and responses to drought.
In order that such Decision Making to take place the aid of indicators to pinpoint, the dimension of
drought phenomena is more than critical. The application of SPI and SPEI has led to clearly depict
drought events all over Europe with two distinct zones, the Mediterranean and the Northern one
beyond the Alps. It would seem that the 1990 drought was the greatest on record. Policy makers and
others must understand that drought is a normal climatic phenomenon, and its recurrence is
inevitable and the delineation of its dimension are fundamental for any drought contingency and
impact mitigation efforts.
Author Contributions: P.O. and C.K. conceived, designed and performed the experiments; P.O., C.K. and E.K.
analyzed the data; P.O., C.K. and E.K. wrote the paper.
Conflicts of Interest: “The authors declare no conflict of interest."
The following abbreviations are used in this manuscript:
SPI: Standardised Precipitation Index
SPEI: Standardised Precipitation Evapotranspiration Index
1. Van Lanen, H.A.J.; Wanders, N.; Tallaksen, L.M.; Loon, A.F.V. Hydrological drought across the world:
impact of climate and physical catchment structure. Hydrology and Earth System Sciences 2013, 17, 1715–
2. Grigg, N.S. The 2011–2012 drought in the United States: new lessons from a record event. International
Journal of Water Resources Development 2014, 30, 183–199, doi:10.1080/07900627.2013.847710.
3. Vlachos, E.C. Drought management interfaces. In; Annual American Society of Civil Engineers
Conference: Las Vegas, Nevada, USA, 1982; p. 15.
4. Karavitis, C.A. Drought and urban water supplies: the case of metropolitan Athens. Water Policy 1998, 1,
ECWS-3, 2018 6 of 7
5. Karavitis, C.A. Decision Support Systems for Drought Management Strategies in Metropolitan Athens.
Water International 1999, 24, 10–21, doi:10.1080/02508069908692129.
6. Karavitis, C.A.; Tsesmelis, D.E.; Skondras, N.A.; Stamatakos, D.; Alexandris, S.; Fassouli, V.; Vasilakou,
C.G.; Oikonomou, P.D.; Gregorič, G.; Grigg, N.S.; Vlachos, E.C. Linking drought characteristics to impacts
on a spatial and temporal scale. Water Policy 2014, 16, 1172–1197, doi:10.2166/wp.2014.205.
7. Karavitis, C.A.; Alexandris, S.; Tsesmelis, D.E.; Athanasopoulos, G. Application of the Standardized
Precipitation Index (SPI) in Greece. Water 2011, 3, 787–805, doi:10.3390/w3030787.
8. Loukas, A.; Vasiliades, L.; Tzabiras, J. Evaluation of climate change on drought impulses in Thessaly,
Greece. European Water 2007, 17/18, 17–28.
9. Vasiliades, L.; Loukas, A.; Patsonas, G. Evaluation of a statistical downscaling procedure for the estimation
of climate change impacts on droughts. Natural Hazards and Earth System Sciences 2009, 9, 879–894,
10. Grigg, N.S.; Vlachos, E.C. Drought and Water‐Supply Management: Roles and Responsibilities. Journal of
Water Resources Planning and Management 1993, 119, 531–541, doi:10.1061/(ASCE)0733-
11. Karavitis, C.A.; Oikonomou, P.D.; Waskom, R.M.; Tsesmelis, D.E.; Vasilakou, C.G.; Skondras, N.A.;
Stamatakos, D.; Alexandris, S.; Grigg, N.S. Application of the Standardized Drought Vulnerability Index
in the Lower South Platte Basin, Colorado. In 2015 AWRA Annual Water Resources Conference,16-19
November 2015, Denver, CO; 2015.
12. European Environment Agency Environmental indicators: Typology and overview; Technical report No 25;
13. European Environment Agency EEA core set of indicators: Guide.; EEA Technical report No 1/2005;
Publications Office of the European Union: Luxembourg, 2005; ISBN 978-92-9167-757-3.
14. European Environment Agency Digest of EEA indicators 2014; Technical report No 8/2014; Publications
Office of the European Union: Luxemburg, 2014;
15. European Environment Agency Trends and projections in Europe 2015: Tracking progress towards Europe’s
climate and energy targets; Technical report No 4/2015; Publications Office of the European Union:
16. Vasiliades, L.; Loukas, A. Hydrological response to meteorological drought using the Palmer drought
indices in Thessaly, Greece. Desalination 2009, 237, 3–21, doi:10.1016/j.desal.2007.12.019.
17. Environment in the European Union at the turn of the century; European Environment Agency, Ed.;
Environmental assessment report; Off. for Official Publ. of the Europ. Communities: Luxembourg, 1999;
18. European Environment Agency The European environment — state and outlook 2015: Assessment of global
megatrends; Publications Office of the European Union: Luxemburg, 2015;
19. Spinoni, J.; Naumann, G.; Vogt, J.V.; Barbosa, P. The biggest drought events in Europe from 1950 to 2012.
Journal of Hydrology: Regional Studies 2015, 3, 509–524, doi:10.1016/j.ejrh.2015.01.001.
20. Di Liberto, T. A hot, dry summer has led to drought in Europe in 2018 Available online:
(accessed on Oct 18, 2018).
21. Karavitis, C.A.; Skondras, N.A.; Tsesmelis, D.E.; Stamatakos, D.; Alexandris, S.G.; Fassouli, V.P. Drought
impacts archive and drought vulnerability index. In Drought Management Centre for South-East Europe –
DMCSEE. Summary of the result of the project, co-financed by the South east europe transnational Cooperation
ECWS-3, 2018 7 of 7
programme (contract no. See/a/091/2.2/X); Gregorič, G., Ed.; Slovenian Environmental Agency, 2012; pp. 33–
22. Karavitis, C.A.; Chortaria, C.; Alexandris, S.G.; Vasilakou, C.G.; Tsesmelis, D.E. Development of the
standardised precipitation index for Greece. Urban Water Journal 2012, 9, 401–417,
23. Karavitis, C.A.; Vasilakou, C.G.; Tsesmelis, D.E.; Oikonomou, P.D.; Skondras, N.A.; Stamatakos, D.;
Fassouli, V.; Alexandris, S. Short-term drought forecasting combining stochastic and geo-statistical
approaches. European Water 2015, 49, 43–63.
24. Ciais, P.; Reichstein, M.; Viovy, N.; Granier, A.; Ogée, J.; Allard, V.; Aubinet, M.; Buchmann, N.; Bernhofer,
C.; Carrara, A.; Chevallier, F.; De Noblet, N.; Friend, A.D.; Friedlingstein, P.; Grünwald, T.; Heinesch, B.;
Keronen, P.; Knohl, A.; Krinner, G.; Loustau, D.; Manca, G.; Matteucci, G.; Miglietta, F.; Ourcival, J.M.;
Papale, D.; Pilegaard, K.; Rambal, S.; Seufert, G.; Soussana, J.F.; Sanz, M.J.; Schulze, E.D.; Vesala, T.;
Valentini, R. Europe-wide reduction in primary productivity caused by the heat and drought in 2003.
Nature 2005, 437, 529–533, doi:10.1038/nature03972.
25. Schär, C.; Vidale, P.L.; Lüthi, D.; Frei, C.; Häberli, C.; Liniger, M.A.; Appenzeller, C. The role of increasing
temperature variability in European summer heatwaves. Nature 2004, 427, 332–336,
26. DG AGRI Exchange of views with the European Commission (DG AGRI) on the drought situation in the
27. Cornes, R.C.; van der Schrier, G.; van den Besselaar, E.J.M.; Jones, P.D. An Ensemble Version of the E-OBS
Temperature and Precipitation Data Sets. Journal of Geophysical Research: Atmospheres 2018,
28. R Core Team R: A Language and Environment for Statistical Computing; R Foundation for Statistical
Computing: Vienna, Austria, 2018;
29. Pierce, D. ncdf4: Interface to Unidata netCDF (Version 4 or Earlier) Format Data Files. R package version 1.16;
30. Hijmans, R.J. raster: Geographic Data Analysis and Modeling. R package version 2.7-15; 2018;
31. Wickham, H. The Split-Apply-Combine Strategy for Data Analysis. Journal of Statistical Software 2011, 40,
32. Plate, T.; Heiberger, R. abind: Combine Multidimensional Arrays. R package version 1.4-5; 2016;
33. Beguería, S.; Vicente-Serrano, S.M. SPEI: Calculation of the Standardised Precipitation-Evapotranspiration
Index. R package version 1.7; https://CRAN.R-project.org/package=SPEI, 2017;
34. Hargreaves, G.H.; Samani, Z.A. Reference Crop Evapotranspiration from Temperature. Applied
Engineering in Agriculture 1985, 1, 96–99, doi:10.13031/2013.26773.
35. Hamer, M. New Scientist. August 18, 1990, pp. 20–21.
36. European Commission Drought in Europe Available online:
https://ec.europa.eu/commission/news/drought-europe-2018-aug-30_en (accessed on Oct 15, 2018).
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