In Europe, it is estimated that around 65 % of
drinking water is extracted from groundwater. Worryingly, groundwater
drought events (defined as below normal groundwater levels) pose a threat to
water security. Groundwater droughts are caused by seasonal to
multi-seasonal or even multi-annual episodes of meteorological drought
during which the drought propagates through the river catchment into the
groundwater system by mechanisms of pooling, lagging, and lengthening of the
drought signals. Recent European drought events in 2010–2012, 2015 and
2017–2018 exhibited spatial coherence across large areas, thus
demonstrating the need for transboundary monitoring and analysis of
groundwater level fluctuations. However, such monitoring and analysis of
groundwater drought at a pan-European scale is currently lacking, and so
represents a gap in drought research as well as in water management
capability. To address this gap, the European Groundwater Drought Initiative
(GDI), a pan-European collaboration, is undertaking a large-scale data
synthesis of European groundwater level data. This is being facilitated by
the establishment of a new network to co-ordinate groundwater drought
research across Europe. This research will deliver the first assessment of
spatio-temporal changes in groundwater drought status from
Groundwater is the primary source for public water supply, agricultural irrigation and industry in many countries. It also supports flow and water levels in rivers, lakes, and wetlands, maintaining healthy ecosystems, and providing amenity value and supporting livelihoods associated with these areas. In the European Union (EU), about three quarters of public water supplies come from groundwater, but some areas, such as Denmark, are entirely dependent on groundwater as drinking water source (EEA, 1999). When managed appropriately, groundwater generally is a reliable resource that can sustain water demand throughout the year, and that is relatively independent from short-term climatology. However, recharge of this valuable resource depends on natural year-on-year variations in rainfall (particularly meteorological conditions during the wet season), and drought signals from major meteorological droughts may propagate into the subsurface, causing groundwater heads to fall below normal (Van Lanen and Peters, 2000). These periods of below normal groundwater levels, termed groundwater droughts, have a number of unwelcome effects, such as reduced production of groundwater from boreholes, and the drying up of groundwater-dependent ecosystems with resulting implications for wildlife and livelihoods.
Major episodes of drought often affect large areas across national
boundaries. For example, the long groundwater drought in the UK from
2010–2012 showed the same drought effects over large parts of continental
Europe, particularly France. Increasing awareness of transboundary drought
events in Europe has resulted in a series of international research
initiatives. For example, Phase 8 of the UNESCO IHP Programme has a focus on
water security in a changing environment. In Europe, projects such as the
ongoing EURO FRIEND-Water Low Flow and Drought
(
In this context, a new pan-European project, the Groundwater Drought Initiative (GDI) has been established to bring together data holders of existing groundwater level data and researchers with an interest in groundwater drought across Europe. The main objectives of the GDI are to (i) assess groundwater drought status across Europe from 1960 to the present, (ii) assess and analyse the impacts of major groundwater droughts at the European scale, and to (iii) establish enduring international collaborations and partnerships to enable the development of impactful research into groundwater drought. This paper describes the background to the GDI and introduces the methodological approach that is being developed to assess groundwater status at the European scale (i.e. the first objective of the GDI), using selected groundwater level data from across Europe.
Hydrological droughts propagate from a meteorological signal (precipitation deficit), through soils (soil moisture deficit), and, via reduced recharge, cause lowered groundwater levels. The spatio-temporal characteristics of the drought signal change as it passes through the terrestrial water cycle (Changnon, 1987; Peters et al., 2003; Tallaksen and Van Lanen, 2004; Van Loon, 2015). Due to the potentially large storage changes in aquifers, groundwater systems act to attenuate and add lags to meteorological drought signals, thus mitigating the effects of relatively short monthly to sub-seasonal droughts (Tallaksen and Van Lanen, 2004). However, groundwater resources are highly susceptible to prolonged (multi-seasonal to multi-annual) episodes of drought, where pooling, lagging and lengthening of multiple consecutive sub-seasonal or sub-annual drought periods can lead to extended drought events in groundwater systems (Van Loon and Van Lanen, 2012; Van Loon, 2015). The efficacy of these pooling, lagging and lengthening effects depends on catchment and aquifer characteristics (Van Lanen et al., 2013) and may be impacted by anthropogenic influences such as groundwater abstraction (Bloomfield and Marchant, 2013; Bloomfield et al., 2015; Tallaksen et al., 1997; Tallaksen and Van Lanen, 2004; Van Loon and Laaha, 2015; Van Loon et al., 2016).
The impacts of groundwater drought are wide-ranging and have important consequences for society and ecosystems. For example, public water supply, energy production and industry, agriculture and livestock farming, freshwater and terrestrial ecosystems, tourism and recreation may all be adversely impacted by major droughts (Lange et al., 2017; Stahl et al., 2016). Specifically, a reduction in deployable output from boreholes due to lower than normal groundwater levels during major droughts may also lead to costly restrictions to public supply, or to reduced abstractions for agriculture and industry, thus causing economic strain to society (Van Loon, 2015). Good examples of this are the last major groundwater drought in the UK in 2010–2012 when seven water companies in south and east England had to impose temporary use bans (colloquially known as “hosepipe bans”) on about 20 million people, including the farming sector in spring of 2012, and the 2015 drought in Europe with dried up wells and boreholes leading to issues providing drinking water for cattle (e.g. in Germany; Van Lanen et al., 2016).
Improved understanding of drought propagation in similar aquifers (such as the Chalk of the Anglo-Paris Basin) and in a wider range of hydrogeological settings across Europe would provide insights into generic groundwater drought processes, opening the possibility for a systematic analysis of drought propagation from meteorological, soil moisture and surface water to groundwater droughts at the continental scale (Van Lanen et al., 2016), and thus contribute to improved preparedness and more informed decisions associated with the use of drought triggers.
During the 2015 event, the EURO FRIEND-Water LFD-network collated and
analysed data to characterise meteorological and streamflow drought
conditions, and impacts across Europe (Ionita et al., 2017; Laaha et al.,
2017; Van Lanen et al., 2016). However, an equivalent data synthesis and
analysis of the 2015 groundwater drought situation was not possible due to
the lack of co-ordination of groundwater level data and research across
Europe (Laaha et al., 2017; Van Lanen et al., 2016; Van Loon et al., 2017).
This highlights a major gap in European drought research capability. In
addition, there have recently been initiatives to better inform policy
makers, water managers, and the general public of drought status, for
example in Europe through on-line monitoring services such as the European
Drought Observatory (EDO,
Site map of groundwater gauges categorized by
In summary, despite some early attempts to highlight the importance of research into groundwater droughts across Europe (Hisdal et al., 2004; Peters et al., 2001; Van Lanen and Peters, 2000) recent projects and initiatives mentioned above lack a systematic approach of clearly integrating groundwater droughts into their drought analysis. With few exceptions (Van Loon et al., 2013), they focus instead on droughts in the driving meteorology and consequences of droughts only in the soil and surface water compartments of the terrestrial water cycle. Therefore, the following research gaps can been identified: (i) although the skills are present across Europe, there is a major gap in European drought research capability related to groundwater droughts, primarily due to a lack of co-ordination between hydrogeologists and hydrologists with appropriate expertise (Staudinger et al., 2019); (ii) there is a gap in research capability to provide a timely, integrated and consistent overview of groundwater level data at the continental scale under conditions of drought, and hence to assess the status of groundwater drought in a manner comparable with other hydrometeorological drought signals; and, (iii) there has to date been no systematic attempt to undertake an assessment of the impacts of groundwater droughts at the continental scale.
The GDI aims to deliver the first pan-European assessment of groundwater
drought status (
Table showing aquifer types per site, average water level in metres
below ground level (m bgl; average depth was calculated based on the
interpolated groundwater levels 1955–2018), maximum cross-correlation (CC)
between SPI and SGI, autocorrelation range (
Sixteen sites from four countries (Austria, France, UK, and Sweden) have
been chosen for analysis and to illustrate the GDI workflow. The main
selection criteria for these sites were the availability of relatively long,
mostly continuous series of groundwater level data from unconfined aquifers
and the representation of different hydrogeological settings. For a brief
comparison between aquifer settings, we additionally chose two sites from
confined aquifers (SW3 and SW4). The spatial distribution of the chosen
sites, Fig. 1, covers different parts of France and Austria (though with one
pair of neighbouring boreholes respectively, namely FR1 and FR2 in France,
and AU3 and AU4 in Austria), and the southern parts of the UK and Sweden.
The aquifer types differ across the sites (Table 1), and include fissured
igneous and metamorphic rocks, fractured limestone aquifers such as the
Chalk of the Anglo-Paris basin, consolidated sandstones and superficial
glacio-fluvial and alluvium deposits. All groundwater level data were
obtained from publicly available databases of groundwater levels for each
country respectively (Austria:
Figure 2 shows the overall workflow of the GDI. It is based on the analysis method developed by Marchant and Bloomfield (2018) that enables irregularly observed groundwater level data from a range of locations and variety of periods of observations to first be regularised into a common analysis frame, then standardised, and finally then to be used for characterisation of spatio-temporal variation in extracted episodes of groundwater drought. The raw time series of groundwater level data for each site is first pre-processed to provide a regularised monthly groundwater level time series using newly developed R-scripts run on a SHINY-interface. The regularisation step entails harmonisation of the observed groundwater levels to regular monthly time steps and stochastic modelling of groundwater levels (Marchant et al., 2016) to infill data gaps using the temporal correlation amongst the data and a modelled relationship with monthly precipitation data, obtained from the E-OBS gridded dataset (Haylock et al., 2008). This is achieved by estimating and predicting from a linear mixed model of the groundwater levels where the fixed effects are an impulse response function of rainfall plus a seasonal term. Random effects are temporally correlated according to a Matérn function – see Marchant and Bloomfield (2018) for a fuller description.
Methodology of data analysis.
A variant of the Standardised Precipitation Index (SPI), the Standardised
Groundwater level Index (SGI) approach (Bloomfield and Marchant, 2013; McKee
et al., 1993; Svoboda and Fuchs, 2016) is then used to standardise the
groundwater level data. The SGI is a non-parametric approach, which assigns
a standardised value to the reconstructed groundwater levels for each month,
thereby resulting in normalised monthly indices that allow for comparison
across the sites (see Bloomfield and Marchant, 2013). Additionally, cluster
analysis using the
Data availability for the selected sites varied and ranged between near-complete time series from 1955–2018 (UK1) to datasets with observations starting only in the 1990s (AU3, AU4, UK3). An example of the driving precipitation data, the observed groundwater levels, and the regularised groundwater level time series for three of the sites is shown in Fig. 3, demonstrating different degrees of flashiness (response of the aquifer to precipitation events) across the sites.
Table 1 shows cross-correlation (CC) between the estimated SGI for each of
the sixteen sites and the corresponding SPI for the period 1955 to 2018.
There was a reasonably strong correlation (
Combined graphic showing
The SGI time series for the sixteen sites (Fig. 4a) demonstrate the
different response patterns to driving meteorology. Some of the sites are
characterised by fewer, but longer droughts (e.g. SW1, SW2, UK2) and others
exhibit more frequent, but shorter drought periods (e.g. AU1, UK3). This is
also reflected in differences in
Geographical distribution of determined clusters. Map image is the intellectual property of Esri and is used herein under license. Copyright © 2019 Esri and its licensors. All rights reserved.
The
Summary table of number of droughts and drought duration (in months) for each site for the period 1955–2018.
Drought statistics related to drought frequency, duration and intensity can be extracted from the prepared SGI series. For illustration, Table 2 summarises drought frequency and duration for each site. It shows a total number of droughts per site between as few as four 4 (SW1) to as many as 40 (UK3). It is noteworthy that the median drought duration differs substantially between aquifer types, ranging for example from only 6–7 months at limestone (FR3 and UK3) and gravel/sand sites (AU1 and AU2), to 11–22 at Chalk sites (FR1, FR2, UK1, and UK4), and 24–64 months in glaciofluvial sand aquifers (SW1, SW2); thereby confirming the importance of aquifer type in drought propagation as previously observed in other studies (Bloomfield and Marchant, 2013; Bloomfield et al., 2015; Tallaksen et al., 1997; Tallaksen and Van Lanen, 2004; Van Lanen et al. 2013; Van Loon and Laaha, 2015; Van Loon et al., 2016).
In this paper, the methodological approach for a pan-European groundwater drought assessment was presented and applied to selected sites across four countries and different hydrogeological settings across Europe. The analysis shows that groundwater level time series can be successfully regularised using a stochastic model, and that the standardisation procedure using the Standardised Groundwater level Index (SGI) allows for comparison between sites. Varying strength of observed relationships between drought characteristics of the sites were found in terms of geographical proximity and between common types of aquifers; thereby supporting the theory that hydrogeological settings play an important role in groundwater drought propagation.
Further improvement to the analysis procedure will be undertaken in the future workflow of the GDI, such as using more advanced clustering techniques and advances in the methodology for groundwater drought event extraction from the dataset. This can be done for example by using sequent peak algorithms to collate separate drought events during which the aquifer has not fully recovered yet from the first drought episode. An improved analysis of drought events, for example by using mid-points of droughts and calculating the percentage of sites in drought at a given time, will allow for a better description of their spatio-temporal distribution and evolution.
Although the focus of the analysis is on sites relatively unaffected by abstraction or irrigation returns, identification of sites with poor correlation between precipitation and groundwater will present an opportunity for research into anthropogenic effects on groundwater droughts to allow distinction between climate and human affected droughts. In addition, it is hoped that comparative studies of the impacts of groundwater drought between semi-arid regions, e.g. southern Europe, and more temperate north-western Europe could provide additional, complementary insights into to the impacts of extremely low groundwater levels during droughts, thus informing future drought management planning across Europe.
The methodology used in the GDI is generic and could also be employed beyond Europe to other geographic regions. The approach is equally applicable to high groundwater level stands. Consequently, generic insights from the GDI will be of interest to the international groundwater research community, particularly to those interested in better understanding of extreme events in groundwater.
The data presented here are from an ongoing and yet incomplete project. The datasets generated during the current study are available on request from the corresponding authors.
JPB, DMH and AFVL conceptualised overarching research aims and secured funding, BPM, JPB and DC developed methods and models, DC performed numeric modelling and statistical analysis, HB, BT, and GH provided data and expertise, BB collated and visualised data and wrote the original draft, all co-authors reviewed and provided feedback.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Hydrological processes and water security in a changing world”. It is a result of the 8th Global FRIEND-Water Conference: Hydrological Processes and Water Security in a Changing World, Beijing, China, 6–9 November 2018.
Bentje Brauns, John P. Bloomfield, Christopher Jackson, Ben P. Marchant, and Daniela Cuba publish this
paper with the permission of the Executive Director of the British
Geological Survey (UK Research and Innovation, UKRI). We acknowledge the
E-OBS dataset from the EU-FP6 project ENSEMBLES
(
This research has been supported by the Natural Environment Research Council (grant no. NE/R004994/1).