The EDII contains report entries since 1900 and Fig. 1 shows the annual time series of the content of the database (as of March 2015). Impact reports prior to 1970 correspond to well-known European drought years, such as the early 1920s, the second half of the 1940s, and 1959 (Fig. 1a). However, this earlier time period prior to 1970 is only sparsely covered by report entries in contrast with more recent times (Fig. 1b). Consequently, we focused our closer inspection of report sources and impact distributions on the period from 1970 to 2013, and specifically on years with at least 20 drought impact reports (Fig. 1b–e).

Within this 43-year study period, the largest numbers of reports refer to impacts reported in the year 2003, followed by 1975–76, 2011–12, and 2005–06 (Fig. 1b). Besides those events, the number of annual reports generally appears to have increased since the 1990s. The bulk of drought impact reports from the last 15 years (2000–2015) are reports published by governmental authorities or other institutions, whereas a considerable fraction of the earlier drought impact reports are based also on academic work (Fig. 1c). The fraction of media and internet sources becomes larger in more recent years, with the most current entries almost entirely based on these sources.

The collected impact reports cover many European countries and regions (Fig. 1d). Impacts in specific geographical regions, which we grouped based on simplified agro-climatic zones proposed by Bouma (2005), reflect the occurrence of regional drought events: for instance, reports in 2001 are mostly from southeastern European countries, in 2005 mostly from Spain and Portugal, and in 1976 mostly from maritime/western European countries (Fig. 1d). By contrast, the period from 1989 to 1995 is characterized by impact reports across several regions in Europe with reports in maritime/western Europe dominating the early years of this period and a high number of drought impact reports from Portugal and Spain as well as from southeastern European countries in the early 1990s (Fig. 1d). The fraction of impact reports from Northeastern European countries is relatively small.

The yearly distributions of reported impact categories (Fig. 1e) illustrate the characteristic diversity of drought impacts. Throughout, a substantial fraction of the reported drought impacts were assigned to the category “agriculture and livestock farming”, “public water supply”, and other impact categories directly linked to water resources. Impacts on “energy and industry” appear prominently during the early 2000s and reports on “freshwater ecosystems” comprise a large fraction since 2003. Reports in the category “terrestrial ecosystems” or those addressing social consequences, i.e., the categories “conflicts” and “human health”, were only found for few years and are limited in number. Overall, recent years show a slightly more diverse and balanced distribution of the reported categories.

The annual distribution of impact categories and regions (Fig. 1d and e) reveals some agreement between a large proportion of reports from southeastern Europe, i.e., drought centered on this region, and increases in reports on “agriculture and livestock farming” (i.e., 1983, 1994, 2000–01). By contrast, large proportions of reports from western European countries generally coincide with a lower fraction of agricultural impacts (i.e., 1975, 1984, 1995, 2010) and more varied impact categories. In years with few impact reports, particular sources, regions, and impact categories often dominated the reported impacts in contrast to the wide range of reported impacts in other years.

To date, drought impact reports in the EDII cover 33 countries, but there is a strong imbalance in the distribution of entered reports per country (Fig. 2). This geographical imbalance is explained by a number of restrictions and biases regarding the availability of information to populate this database and Sect. 5 provides a more detailed discussion of the resulting limitations. The map in Fig. 2 illustrates some important differences regarding the composition of information sources within the current EDII. The UK, Germany, France, Portugal, the Netherlands, Spain, Bulgaria, Switzerland, Italy, and Norway are the 10 countries with the largest numbers of impact report entries (in order of decreasing numbers). Most of these countries are located in western Europe. Bulgaria is the only country located in eastern Europe. For Bulgaria, a particular academic information source (Knight et al., 2004), which had previously compiled Bulgarian drought impact information over a long historical period was available. In countries with many reported drought impacts, often governmental reports comprise a high fraction of the EDII entries. Such reports are mostly systematic reviews of a past drought situation and thus provide all necessary information for the EDII (time, location, etc.). In countries with generally fewer impact reports, reports by non-governmental sources as well as internet and (international) media sources comprise a higher fraction of the information sources of drought impact reports (Fig. 2).

Figure 3 shows the relative ranking of the three most frequently reported impact categories for each country across Europe. The relative frequency within each country is independent of the absolute number of reports and hence less affected by quantitative report bias. The category “agriculture and livestock farming” ranks highest most often and ranks among the top three most frequently reported impact categories for all countries except Finland (Fig. 3). Only eight out of the 15 available EDII impact categories are represented in these top three impact categories. Besides agriculture, these are most often the impact categories “public water supply”, “energy and industry”, and “wildfires”, followed by the categories “water quality”, “freshwater ecosystems”, and “human health and public safety”. Categories that are nowhere represented among the three most frequent categories are “freshwater aquaculture and fisheries”, “waterborne transportation”, “tourism and recreation”, “terrestrial ecosystems”, “soil systems”, “air quality”, and “conflicts”.

The high fraction of impacts on “public water supply” in southern (Mediterranean) countries is in agreement with the common perception of susceptibility to quantitative water supply problems due to water scarcity in the region. The fraction of reports in the “forestry” category is greater in forest-rich countries in northern and eastern Europe. Impacts on “wildfires” also rank highly in these countries, although in absolute numbers, available EDII data are scarce. Impacts on energy and industry rank highly in Norway, Belgium, and several countries in southeastern Europe. Impacts on “water quality” rank highly, in terms of relative frequency, for the UK, the Netherlands, and Germany. “human health and public safety” impacts were identified in five countries. Nevertheless, the latter category includes a wide range of impact type subcategories. An analysis of the impacts' text descriptions in the EDII revealed that, in the case of France, the high fraction of reported impacts results mainly from a multitude of reports of increased human mortality rates during the associated heat wave in 2003. However, for Moldova, the “human health and public safety” category ranks second with 2 out of a total number of only 14 drought impact reports. Here, the reports deal with food security issues in response to the droughts in 1946 and 2007. These examples demonstrate the diverse nature of reported impacts within one broad category.

The EDII's two-level categorization scheme of assigning main impact categories and more detailed impact types within each category (see Table A2) facilitates a further differentiation of the differences of reported impacts across Europe. The category “agriculture and livestock farming” is separated into nine impact types. Their relative fractions were assessed for 21 countries with high data availability (Fig. 4, upper panels). The proportion of impact types were similarly assessed for the two impact categories “public water supply” and “water quality” across a selection of seven countries with high data availability in the EDII (Fig. 4, lower panels).

For “agriculture and livestock farming”, reported impacts that relate to a reduced agricultural production and to crop yield losses comprise the largest fraction of the overall agricultural reported impacts. A considerable fraction of reported impacts also relates to livestock farming, particularly in France, Hungary, and Moldova. The reported impact types also demonstrate the importance of irrigation in some regions through a high fraction of impact types related to irrigation (e.g., “Reduced availability of irrigation water”) in the Mediterranean countries (Spain, Greece, Italy, and Portugal). Compared to other impact categories, the impact type describing economic losses was more often reported in the “agriculture and livestock farming” category, even though losses were rarely quantified.

For the category “public water supply”, the distribution of impact types shows some differences among countries. Within this category, the different impact types categorize increasing impact severity ranging from a mere reporting of shortages to imposed restrictions (e.g., for outdoor water use such as gardening or pools) to actual limitations in the public water supply (e.g., water supply could no longer be provided). In general, the majority of reports describe local or regional water shortages and thus primarily aim to raise awareness. France and the UK have a high total number of reported water supply impacts, with a relatively high fraction of use bans aiming to reduce demand and conserve water. Bulgaria, Spain and Italy have the largest fractions of reports on actual supply limitations during droughts. Reports on increased costs for the public water supply sector were only available for Portugal, Spain, and the UK.

Within the category “water quality”, impact types are classified by their relevance to particular water quality aspects such as temperature, the impaired water service (ecological status, drinking water, irrigation water, etc.), and general consequences. The type “water quality deterioration in surface waters” typically constitutes a substantial fraction of impact types in all countries, whereas other impact types appear to be more specific to certain countries. In Germany and France, a significant fraction of the reports relates to increased surface water temperature, as the majority of reported water quality impacts refer to the severe summer droughts of 1975–76 and 2003 that were associated with very hot weather conditions. High surface-water temperatures during these droughts heavily impacted the energy sector due to an impaired water use for cooling of thermal power plants (fossil and nuclear). In Germany, bathing water quality impairments affected recreational activities during the summer holiday time, but drinking water was rarely impaired. In the Netherlands, a comparatively large fraction of reported impacts in the “water quality” category deal with increased salinity of surface waters as a result of direct saltwater intrusion in the river mouths and a higher proportion of brackish groundwater infiltration into the rivers. Increased salinity (of surface water or groundwater) also appears to be a threat for Portugal, Spain, and the UK. Water quality problems with drinking water comprise considerable fractions of reports in Portugal, Spain, and particularly in Bulgaria.

The EDII database was developed and populated as part of a 3-year research project (Stahl et al., 2012; Van Lanen et al., 2015). The EDII's classification scheme of impact categories and subordinate impact types (Table A2) was modeled after the US Drought Impact Reporter (DIR), but was adapted to the project's needs and thus has a more detailed classification scheme. The process of describing, assigning an impact to a category and associated subtype, and entering time and space information may present some challenges if it is to be part of a real-time monitoring tool such as the DIR, but for a research database, it has the advantage of allowing an unprecedented detailed stratification and assessment of the gathered data. So far, the scheme has proven to be suitable for capturing impact details and all impacts found in the collected reports could be categorized. It should be kept in mind that the EDII is limited to only considering and categorizing negative impacts of drought. Drought can also have positive impacts. For instance, in the agricultural sector, there may be winners and losers during the same drought (e.g., Ding et al., 2011, Musolino et al., 2015). Furthermore, text descriptions in the EDII were not standardized; the only requirement is that the text descriptions are in English and reflect the text of the original information source as closely as possible. Although the text descriptions often contain common terminology, differences in translation or synonyms may have introduced uncertainty. We identified the correct categorization of reported impacts as the critical step to enable an objective comparative assessment and concluded that a moderator who performs a screening and quality assessment of the data is crucial before impact reports are accepted into the database.

Governmental reports and documents make up a high fraction of referenced sources in the EDII (Fig. 2). Governmental reports by national or regional authorities involved in the monitoring and management of droughts were found to provide rather detailed information on the occurrence of past drought impacts, and thus proved to be particularly useful for the historical reconstruction of drought impact distributions. It was crucial that a clear association between drought impact and event was given in the referenced source. Lackstrom et al. (2013) named this clear association as one of the major difficulties in drought impact reporting owing to the complexity of drought impacts, especially the complexity of multiple stressors and temporal indefinability of drought. Governmental reports are usually edited in the respective national language. In addition, governmental reporting culture varies across Europe and hence the availability of such reports also differs. By contrast, academic output and reports by multinational organizations are available in English, but are often less detailed on the nature of the impacts. These academic articles and reports represent the major information sources of reported drought impacts in the EDII for countries in eastern Europe to date (Fig. 2).

During the limited time available within the 3-year project, the search for drought impact reports revealed a wealth of information on a variety of drought impacts across most of Europe, even though it was carried out by a group of people limited to searching in only a few European languages, particularly lacking Slavic languages. Generally, the requirement for and availability of data has increased with the emerging prevalence of digital documents and online dissemination techniques during the 20th century. This is reflected in the contents of the database (Fig. 1). There are more reports and categorized impacts for more recent droughts. In our assessment, this limits a quantitative comparison of the larger samples of impacts for the more recent drought events, e.g., 2003 and 2011–12 to those of earlier events (Fig. 5). That the number of drought impacts may be more strongly related to the availability of reports than to the severity of drought impacts confirms the relative approaches taken in this study.

Similarly, the identification of the time and location of reported drought impacts challenges the resolution and accuracy of the data set. The specific month in which an impact started and ended was often not detailed in the original information source and as a consequence, many database entries only state a season or even only the year of occurrence. For instance, for the selected drought events assessed in Sect. 4, between 15 and 41 % of the reported impacts did not specify the month or season of impact start (Fig. 5, top graphs). However, during a severe drought, similar impacts tend to be reported by many sources, and we assume that seasonal information still provides a valuable estimate of the onset of major impacts. The same issue applies to the end of an impact. Similar to difficulties to define the termination of drought as a natural phenomenon (Parry et al., 2015), the termination or recovery of drought impacts remains largely unknown, as recovery is rarely reported, which is an issue not yet widely addressed (Lackstrom et al., 2013). Introducing links between entries would allow the identification and tracking of multiple reports of the same impact – this would enable an accurate timeline of impact onset and end to be created. Currently, this information must be distilled manually from the database.

The spatial reference (and so the reported NUTS levels) also shows high variability in the level of information details provided. Often, the original source provides only vague location information; then, a larger spatial unit within which the reported impact occurred had to be selected for the categorization. Consequently, for some regions and events, especially those with sparser data coverage, reported impacts rarely referred to subnational scales (e.g., drought events in 1990 and 2007 in Fig. 5), whereas for other regions, more detailed spatial information was available (e.g., drought events 2004–08 in the Iberian Peninsula and generally in Germany). Occasionally, however, reports were very specific. For instance, in some cases energy production impacts were provided for specific hydropower sites or fish kills for particular sections of rivers. The flexibility to aggregate from lower (local) to higher (regional/national) levels is desirable but known to be challenging (Lackstrom et al., 2013).

The information gathered provides a number of opportunities for further assessment, provided the limitation and biases inherited in the collection of impact reports are kept in mind. One opportunity not yet explored, due to the low number of entries, is an investigation of associated secondary impacts and response measures that relate directly to reported impacts. A retrospective analysis for the few entries that are available, however, could be a starting point to trace the success or failure of these measures by cross-referencing with other documentation and information sources. Such an analysis has potential to help improve drought management. Another option is to investigate the link between monitored drought indices and the occurrence of drought impacts as demonstrated in several recent studies (e.g., Bachmair et al., 2015; Stagge et al., 2015). Such analyses may guide the choice of impact related indicators for risk assessment and drought monitoring and early warning. Given the current status of the EDII, the most reliable report samples for pan-European assessments are aggregated information on an annual scale and at the country scale or larger European regions. This is the resolution used by Blauhut et al. (2015a) to model annual likelihood of impact occurrence by the standardized precipitation and evapotranspiration index; and by Van Loon et al. (2015) to study the impacts of temperature-related drought types. However, the more detailed the spatial and temporal reference, the more details can be gained in the identification of similarities and differences across Europe. Bachmair et al. (2015) compared the statistical link between drought impacts and a number of different hydrometeorological drought indicators across Germany based on a higher spatial resolution (NUTS-1 regions); Stagge et al. (2015) used the available monthly impact occurrence for selected countries to identify the best hydrometeorological drought predictors of the likelihood of impact occurrence and addressed the database's biases by building trends into the model. All studies conclude that in order to derive a useful sector-specific operational drought index, future efforts to compile drought impact data should aim at using available higher resolution in time and space that is closer to the resolution of hydrometeorological drought indices.

Impact reports show a wide range of impacts locally and across Europe. Additionally, impact categories and types appear to have increased in diversity over time (e.g., Fig. 1). Whereas such patterns may be related to the overall increase of information availability over time, it may also reflect an increased public awareness (e.g., Wilhite, 2007; Dessai and Sims, 2010), or specific changes in the importance of the impact-related economic sector, both in governance or policies (De Stefano et al., 2012; Tánago et al., 2015; Blauhut et al., 2015b). The spatial distribution of the sheer number of reported impacts in the EDII differs to those recorded in other databases. For example the Emergency Events Database (EM-DAT, http://www.emdat.be) from the Université Catholique de Louvain in Brussels in Belgium generally returns few entries for Europe compared to other continents, but these are mainly from Mediterranean and (south)eastern European countries. However, with a focus on disasters, EM-DAT and other databases use other definitions for drought impacts.

Availability and accessibility of impact information from reports appears to vary across Europe and thus introduces bias in the volume of information on impacts. In some countries, regular reports detailing the status of a water-dependent sector or special reports detailing a particular drought event were rich sources that provided many database entries. Examples of the former type of regular reports that were used include the drought monitoring briefs for England and Wales produced by the Environment Agency for the 2004–6 and 2010–12 droughts in the UK. Such reports also highlight a sectoral relevance that differs regionally. Comprehensive reports or books on specific drought events for example covered the drought of 2003 in Germany and Switzerland, the event of 1992–94 in Bulgaria, and the event of 1975–76 in the UK. In other regions, such previously assembled information sources were not available and assembling impact information was more tedious and resulted in fewer database entries. In addition, the EDII still lacks targeted searches for information in several countries and languages. For example, data entries for many eastern European countries rely mostly on international report sources and media coverage available online. As a result of these regional differences, the volume of EDII contents is biased, and therefore our analyses focused instead on the relative fractions and composition of impact reports within each region.

Agricultural drought impacts represent the largest fraction of impacts in most regions across Europe (Fig. 3) and for most large-scale or regional drought events (Fig. 5). This explains why research on drought impacts across Europe has often used crop failures, losses or yields and other types of drought impacts have been considered less often. Consistent with the conceptual view of drought propagation from its origin as a precipitation deficit to a deficit in soil moisture and finally to hydrological deficits (in groundwater and streamflow, and related water resources), the dominance of agricultural reports may also reflect the typical association the public has with drought. In addition, where agriculture is irrigated, agriculture is often the major water user. This reliance on water for agriculture may explain the high visibility and awareness (Ding et al., 2011) and thus the increased report availability of agricultural impacts. The impact category “agriculture and livestock farming” also records the most entries for cost and losses, suggesting a high economic visibility in Europe (Fig. 4). The relevance is also reflected in the efforts of drought monitoring systems targeting agricultural drought, e.g., the European Drought Observatory (Sepulcre et al., 2012) or the Drought Managment Centre for Southeastern Europe (e.g., Ceglar et al., 2012).

Public water supply impacts have also been reported frequently. If considered together with impacts on water quality and freshwater ecosystems, they comprise an even larger fraction of impacts that relate to direct impacts on water resources than impacts on agriculture. In the Mediterranean countries, the fraction of reported impacts on public water supplies is particularly large. These numbers confirm general public perception of drought importance on public water supply impacts in the south. This was illustrated, for example, by stakeholders mapping drought impacts during the 2nd DROUGHT-R&SPI pan-European Dialogue Forum, where water supply was consistently labeled as a concern by Mediterranean stakeholders (Stahl et al., 2014, 2015). In addition, the details of reported impact types in this category revealed that water supply impacts in southern regions were more severe. They have been affected by actual limitations to households and drinking water supply, whereas other regions in Europe only reported preventive water use bans to decrease demand (Fig. 4). Future work could investigate the feasibility of assigning a relative severity to impact details to improve the ranking and analysis of such differences.

Our analyses show that that physical drought characteristics may be linked to the impact characteristics (Fig. 5). Summer droughts combined with heat waves, for example, have invoked unusual impacts on public health and on energy and industry by restricting river water use for cooling. Multi-year events in southern Europe (e.g., Estrela et al., 2000) and in the UK (e.g., Folland et al., 2015) have specifically reduced storage levels (surface reservoirs and groundwater), provoking conflicts among users. The factors controlling meteorological, hydrological, and soil moisture drought in Europe have been previously analyzed (Hannaford et al., 2011; Lloyd-Hughes, 2012; Van Loon and Van Lanen, 2012; Tallaksen and Stahl, 2014). The EDII provides a potential data source to investigate the importance of drought severity, surface characteristics, and water management in producing drought impacts. This data set, or similar products, may provide the necessary link to evaluate whether drought metrics can predict impacts, a research goal consistently stated (e.g., Kallis, 2008).