2. Supporting Information

2.1 Indicator definition

The Groundwater Level Trend (GLT) indicator measures the average change of the annual groundwater level over the monitoring period. The result is expressed in metres per year (m/y). Positive values (higher than zero) indicate an increase of the groundwater level, whereas negative values (lower than zero) indicate a decrease of the groundwater level. GLT is based on the methods of multi-year groundwater level trend analysis (e.g. Nguyen et al., 2021; Le Duy et al., 2021) and the Groundwater Table Declining Trend indicator (Hofste et al., 2019; Galvis Rodríguez et al., 2017).

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2.2 Unit of measure

metre per year [m/y]

range of values used for the classification:

Increasing groundwater level trend: between +0.01 m/y and more than +2 m/y

Decreasing groundwater level trend: between - 0.01 m/y and more than - 2 m/year

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2.3 Policy/environmental relevance

The Water Framework Directive emphasises the importance of sustainable water use based on long-term protection of water resources and the need for sufficient supply of good quality surface water and groundwater (Directive 2000/60/EC; Article 1). The EU Biodiversity Strategy 2030 aims at preserving and protecting the natural capital in the EU, including groundwater associated surface waters and dependent terrestrial ecosystems (EC, 2019). The EU groundwater policy requires that groundwater bodies achieve and maintain good chemical and quantitative status (Directive 2006/118/EC). The reporting from the second River Basin Management Plans showed that 27% of the groundwater bodies in EU27 failed to achieve either good quantitative or chemical status, with 4% failing to achieve both good quantitative and chemical status (Psomas et al, 2021).

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2.3 EEA topics

  1. Nature - European freshwater
  2. Air and climate - Impacts, vulnerability and risks / Policies and actions
  3. Economic sectors - Greening Europe’s agriculture

 DPSIR - Driving force – Pressure – State – Impact – Response + possible second choice


Typology - Descriptive indicators (type A), Performance indicators (type B), Efficiency indicators (type C), Policy effectiveness indicators (type D), Total welfare indicators (type E) [1]

  1. Descriptive indicators (type A)

[1] EEA indicators — European Environment Agency (europa.eu)

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2.4 Methodology

The analysis of the groundwater level trends is described as a supporting element for the water balance test, conducted as part of the process for determining the groundwater quantitative status (CIS, 2009; EPA, 2010).

Since WISE SoE - Water Quantity (WISE-3) provides the possibility to report in more than one temporal resolutions (daily, monthly, seasonal and annual), an aggregation approach has been followed to optimise the use of reported groundwater level data. The aggregation started with daily data and aggregated all the available daily groundwater level values to monthly, seasonal and finally annual values. A minimum set of criteria were met in all intermediate steps, in order to ensure adequate representativity of all seasons on the annual aggregate value.


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The interquartile range (IQR) method was applied to detect erratic outliers. Values were only removed in case of an apparent error: e.g. the difference in groundwater level between two consecutive annual values is more than an order of magnitude and the change in groundwater level involves a singular erratic outlier. The figure 3 below provides an example of an outlier that was removed.

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Figure 3: Groundwater level measurement values from monitoring station DK201-3795-2, including an erratic error (within red circle) that was removed

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The minimum length of measurement series used for the calculation of the GLT is 5 years within a 10-year time window, based on the recommendation of the WFD CIS document of quantitative status assessment (CIS, 2009). Years are not necessarily consecutive. No gap filling was applied.

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After the temporal aggregation and quality checks, the dataset consisted of 3907 monitoring stations covering 19 countries (Austria, Belgium, Bulgaria, Cyprus, Denmark, Estonia, France, Ireland, Italy, Latvia, Lithuania, Netherlands, North Macedonia, Poland, Slovakia, Slovenia, Sweden, Switzerland).

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The Groundwater Level Trend (GLT) is computed by fitting averaged annual groundwater level records with linear regression. The slope of this regression line (in metres per year; m/y) indicates the magnitude and direction (downward or upward) of the GLT. The significance of the trend is estimated with the Pearson’s p-value. If p > 0.05, then the calculated GLT is not significant. GLT is based on the methods of multi-year groundwater level trend analysis (e.g. Nguyen et al., 2021; Le Duy et al., 2021) and the Groundwater Table Declining Trend indicator (Hofste et al., 2019; Galvis Rodríguez et al., 2017).

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The GLT is presented in three classes that indicate the statistical significance (p-value < 0.05) and direction of the groundwater level trends.

GLT index classes

GLT ranges (m/y); p-value

Color scheme

Increasing groundwater level trend

GLT > +0.01 m/y and p-value < 0.05


No significant trend observed (NT)

p-value > 0.05


Decreasing groundwater level trend

GLT < -0.01 m/y and p-value < 0.05


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Bar-charts are used to visualize the distribution of the groundwater level trend at the EU level and regional level. In this bar-chart, the percentages of monitoring stations that fall within the three GLT classes are shown (orange-grey-blue colour scheme), as well as the number of monitoring locations for Europe and for each region.

At country level, the groundwater level trends are visualized at 3 aggregation levels:
• Level 1: map of Europe with for each country a pie-chart that show the percentages of monitoring stations that fall within the three GLT classes as well as the number of monitoring locations for that country.
• Level 1, alternative: bar-chart with that shows for each country the percentages of monitoring stations that fall within the three GLT classes as well as the number of monitoring locations for that country.
• Level 2: map of Europe with individual monitoring locations indicating the GLT using a yellow-grey-blue colour scheme.
• Level 3: graphs for each monitoring locations, showing the individual groundwater level data points (m + sea level) over time and, in case of a significant trend, the groundwater level trend line and slope (m/y).
(The second and third level are made available via a link to the data and maps section of the website.)

Note: For monitoring stations with the confidentiality remark, locations, groundwater level data, and groundwater level trends will not be shown in maps. However, these monitoring locations are used at aggregated level assessments and visualisations.

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2.5 Data sources

The main underlying data for developing the EEA groundwater level trend indicator come from the WISE - Water Quantity (WISE-3) database. In addition, coordinates of monitoring locations are available from the WISE - Spatial Data (WISE-5). Countries that are members of the European Environment Information and Observation Network (Eionet) voluntarily report groundwater level data to the WISE-3 and WISE-5 dataflow. The time series cover 2000-2019 in the current database were used for this indicator.

The dataset for the Netherlands (NL) was based on a selection of the validation dataset of the National Hydrological model instrument (NHI). The selection was made by spatial coverage and coverage of the different geohydrological settings typical for the Netherlands. Data are available via the online data portal of the NHI (data.nhi.nu).

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2.6 Geographic coverage

EEA member countries and cooperating countries:  Austria, Belgium, Bulgaria, Cyprus, Denmark, Estonia, France, Ireland, Italy, Latvia, Lithuania, Netherlands, North Macedonia, Poland, Slovakia, Slovenia, Sweden, Switzerland.

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2.7 Time coverage

Period of analysis is 2000-2019; length of monitoring data series varies with minimum length of 5 years.

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2.8 Frequency of dissemination

Every 2 years, depending on the data availability.

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2.9 References

EC, 2009. Guidance on groundwater status and trend assessment, Common Implementation Strategy for the Water Framework Directive (200/60/EC), Technical Report - 2009 – 026. (https://circabc.europa.eu/sd/a/ff303ad4-8783-43d3-989a-55b65ca03afc/Guidance_document_N%C2%B018.pdf) accessed 22 September 2021.

EC, 2019. Communication from the Commission to the European Parliament, the European Council, the European Economic and Social Committee of the Regions;; The European Green Deal; COM/2019/640 final (https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A52019DC0640) accessed 22 September 2021.

EC, 2000, Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy (http://data.europa.eu/eli/dir/2000/60/oj( accessed 22 September 2021.

EC, 2006, Directive 2006/118/EC of the European Parliament and of the Council of 12 December 2006 on the protection of groundwater against pollution and deterioration(http://data.europa.eu/eli/dir/2006/118/oj) accessed 22 September 2021.

EEA, 2017, CLIM 002 indicator; Indicator Assessment - Mean precipitation - European Environment Agency (https://www.eea.europa.eu/data-and-maps/indicators/european-precipitation-2/assessment) accessed 22 September 2021.

EEA, 2019, CSI 018 indicator; Indicator Assessment - Use of freshwater resources in Europe - European Environment Agency (https://www.eea.europa.eu/data-and-maps/indicators/use-of-freshwater-resources-3/assessment-4) accessed 22 September 2021.

EPA, 2010, Methodology for establishing groundwater threshold values and the assessment of chemical and quantitative status of groundwater, including an assessment of pollution trends and trend reversal, Environmental Protection Agency, Hydrometric and Groundwater Section, Office of Environmental Assessment, Wexford, Ireland (https://www.epa.ie/publications/monitoring--assessment/freshwater--marine/Methodology-for-Groundwater-Chemical-&-Quantitative-Status-Methology,-TVs-and-Trends.pdf)  accessed 22 September 2021.

Galvis Rodríguez, S., E.H. Sutanudjaja, and M. Faneca Sànchez. 2017. “Memo: Update on the Groundwater Risk Indicators.” Memo. Delft, the Netherlands: Deltares.

Hendriks, D.M.D., Mens, M., Kramer, N., Borren, W., Diermanse, F., 2015. Changing e-flows resulting from land use change and altered groundwater conditions in the Regge catchment, the Netherlands,  Extended Abstract, REFORM International Conference on River and Stream Restoration “Novel Approaches to Assess and Rehabilitate Modified Rivers”

Hofste, R., S. Kuzma, S. Walker, E.H. Sutanudjaja, et. al. 2019. “Aqueduct 3.0: Updated Decision. Relevant Global Water Risk Indicators.” Technical Note. Washington, DC: World Resources Institute,  (https://www.wri.org/publication/aqueduct-30) accessed 22 September 2021.

Le Duy, N., T. V. Khanh Nguyen, D.V. Nguyen, A.T. Tran, H. T. Nguyen, I. Heidbüchel, B. Merz, H. Apel, 2021, Groundwater dynamics in the Vietnamese Mekong Delta: Trends, memory effects, and response times, Journal of Hydrology: Regional Studies, Volume 33, 2021, 100746, ISSN 2214-5818, (https://doi.org/10.1016/j.ejrh.2020.100746) accessed 22 September 2021.

Psomas, A., Bariamis, G., Rouillard, J., Stein, U., Roy, S., 2021, Comparative study on quantitative and chemical status of groundwater bodies. Study of the impacts of pressures on groundwater in Europe, Service Contract No 3415/B2020/EEA.58185. pp. 32 (Unpublished).

Spinoni, J., Naumann, G., Vogt, J.V., 2017, Pan-European seasonal trends and recent changes of drought frequency and severity, Global and Planetary Change, 148, pp. 113-130 (https://doi.org/10.1016/j.gloplacha.2016.11.013) accessed 22 September 2021.

WWF, 2018, Report: The Role of SDG15 in underpinning the achievement of the 2030 agenda global policy and advocacy, June 2018.

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2.10 Related products

Contact - info@eea.europa.eu

UN SDGs - 17 goals from https://w3.unece.org/SDG/Home

SDG 6 Ensure availability and sustainable management of water and sanitation for all.

Target 6.4 By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity and substantially reduce the number of people suffering from water scarcity.

Indicator 6.4.2 Level of water stress: freshwater withdrawal as a proportion of available freshwater resources, (%)

SDG 15 Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt the reverse land degradation and halt biodiversity loss.

Target 15.1 By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands, mountains and drylands, in line with obligations under international agreements

Indicator 15.1.2: Proportion of important sites for terrestrial and freshwater biodiversity that are covered by protected areas, by ecosystem type

Target 15.3 By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought and floods, and strive to achieve a land degradation-neutral world.

Indicator 15.3.1 Proportion of land that is degraded over total land area, (%)

SDG15 is linked with SDG6 as an ecosystem service, water availability and quality are derived from well-functioning ecosystems, in particular forest ecosystems, as are water storage and flow regulation, filtering and flood and drought protection, among others (WWF, 2018). Conserving and sustainably managing ecosystems is integral to effective land-use management and a sustainable and healthy food production and this in turn helps to reliably store and deliver water. Key functions inherent to forests relating to groundwater availability and cooling include:

  • Decreasing groundwater levels, due to climate change or poor groundwater management, is one of the factors that may lead to desertification;
  • Wetlands and other groundwater dependent ecosystems require availability and sustainable management of groundwater;
  • Trees enhance soil infiltration and, under suitable conditions, improve groundwater recharge;
  • Precipitation filtered through forested catchments delivers purified ground- and surface water.

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2.10 Uncertainties

Insignificant trends in the assessment are mostly due to the relatively short time series in the dataset. The minimum length of measurement series to be used for the calculation of the GLT indicator set at 5 years within a ten-year time window. This 5-year minimum is based on the minimum range of timeseries requested by the WFD CIS document on trends analysis of groundwater pollutants (EC, 2009). However, basing the indicator calculation on less than 10 years of the evaluation period creates uncertainties in the indicator results and may give the false impression that no groundwater level trends occur. It is recommended to improve the dataset and increase the length of the time series at the monitoring locations as much as possible.

In addition, the relatively simple approach of the Groundwater Level Trend indicator compared to other indicators may bring some uncertainties:

  • GLT indicator is more sensitive to seasonal variability than the Standardized Groundwater trend indicator. However, because of the aggregation at the annual scale, the effects of seasonal variability in the indicator calculation are minimized.
  • Like other groundwater quantity indicators, the GLT indicator focusses on one aspect of the groundwater. The GLT indicator does not explain the cause of the increase or decrease of the groundwater levels. Groundwater levels and the derived GLT indicator can be strongly influenced by location. Groundwater level is mainly influenced by local and site-specific recharge processes and by regional to site-specific saturated flow processes that are not simply spatially correlated with the long-term climate change impacts where data coverage is not sufficient. Consequently, any interpretation or analysis of the resulting GLT needs to reflect an appreciation of the hydrogeological context of the observation monitoring sites. A set of indicators might be needed for undertaking integrated groundwater assessments and explaining the reasons for changing in groundwater levels.

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Data uncertainties

As most of the data are reported in annual temporal resolution, this significantly limits the possibility to capture the monthly or seasonal variations of the groundwater levels based on the regional hydrological and hydrogeological dynamics (e.g. groundwater recharge, baseflow to rivers, infiltration of snow melt, etc.).

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The timeseries on groundwater level available in the WISE SoE water quantity have numerous breaks of one, two or even more years. Hence, the current underlying dataset does not always have sufficient length to assess a long-term phenomenon, such the groundwater level variation. Only 15% of the finally integrated stations in the analysis have more than 10 years of available annual (or reported or aggregated) groundwater level trends. This limits for the moment the expansion of temporal and spatial coverage of the indicator upscaling of the results at national, European and regional levels.

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Caution also is needed with the spatial aggregations presented. It should be noted that for all European regions, various countries are missing from totals. Especially for Southern Europe, the number of countries and available monitoring stations which is included in the assessment is limited.

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