Groundwater Stress in Europe—Assessing Uncertainties in Future Drought Risk
Groundwater is a critical freshwater resource that sustains human well-being and ecosystem functioning across Europe. However, many regions have experienced declining groundwater levels due to decreasing groundwater recharge (GWR) or increasing groundwater abstractions (GWAs). These changes can lead to groundwater-related stress, threatening water supplies and ecosystems.
Existing groundwater stress indicators estimate stress during a given period but do not address how stress changes or show the uncertainty of future stress. This article proposes a novel indicator of future groundwater stress (GWSI) due to changes in GWR and GWA, and thus the alteration of long-term mean annual groundwater discharge (GWD). Groundwater stress is defined as any alteration in GWD since ecosystems are adapted to an equilibrium state. Focusing on decreasing GWD, we quantified the future GWSI in Europe by integrating scenarios of GWR and GWA in 2070–2099.
GWR was evaluated using an ISIMIP2b multi-model ensemble (MME) of eight global hydrological models driven by the output of four global climate models under two greenhouse gas emission scenarios (RCP2.6 and RCP8.5). GWA scenarios for irrigation, domestic, and manufacturing sectors were combined with the GWR projections to generate an ensemble of GWSIs, simplified into three groundwater stress scenarios (high, intermediate, low).
Projected GWSIs vary significantly among the scenarios. For the high-stress scenario, 58% of Europe’s land area is projected to experience a GWD decrease of at least 25% under RCP8.5 compared to 38% under RCP2.6, while the respective values are 26 and 1% for the intermediate-stress scenario. Groundwater demand management alone might not prevent GWD declines under the high-stress and intermediate scenarios, particularly under RCP8.5. Therefore, climate change mitigation might be imperative for reducing the decline of GWD, especially in Eastern and Southeastern Europe, where changes in GWR are projected to be the primary cause of declining GWD.
Under RCP2.6, reductions in GWAs by 25–75% might balance a GWD decline in parts of Spain and Italy where GWAs are high, even in the high-stress scenario. In line with the precautionary principle, we recommend adapting to the high-stress scenario to minimize harm to the beneficiaries of groundwater.
Groundwater Sustains Europe’s Water Supply and Ecosystems
Groundwater serves as a major source for drinking water supply, agricultural irrigation, and manufacturing in Europe. In 2017, 24.5% of total freshwater abstractions occurred from groundwater, and it constitutes the source of 25 and 65% of the water abstractions for irrigation and public water supply, respectively.
Concerns about groundwater resources have arisen in recent years not only in regions well-known for groundwater over-extraction (mainly for irrigation), such as Southern Spain and Greece, but also from observed declining groundwater levels in Germany, the Netherlands, Belgium, Luxembourg, Denmark, and Poland. Simultaneously, increasing climate variability and drought frequency due to climate change is expected to lead to more reliance on groundwater resources.
Quantifying Groundwater Resources and Uses
Renewable groundwater resources can be approximated by using the long-term average diffuse GWR, while human water use, including groundwater abstractions (GWAs) and return flows, affects groundwater resources. However, quantifying the recent historical situation of groundwater resources is already uncertain, and potential future changes will introduce even greater levels of uncertainty.
It is standard practice to assess future climate change impacts and their uncertainty by multi-model ensembles (MMEs), where several hydrological models are driven by the bias-adjusted output of global climate models (GCMs). Projected changes in global GWR differ strongly, even for specific future greenhouse gas emissions, and are mainly due to uncertain precipitation patterns and evapotranspiration responses from vegetation to increasing atmospheric carbon dioxide levels.
Societal water demands, and thus GWAs and return flows, are also changing due to climate change and socio-demographic factors, adding further uncertainty to assessments of future groundwater resources. As temperatures rise and potential evapotranspiration increases, a greater need for agricultural irrigation may occur unless precipitation rises. Domestic water abstractions per capita, however, decreased in Europe between 2000 and 2008, while GWA patterns within the manufacturing industry exhibit significant heterogeneity across various subsectors.
Assessing Future Groundwater Stress
The increasing reliance on groundwater resources due to climate change, as well as the uncertainties of future climate and societal developments, underscores the need for groundwater management that sustains groundwater for both human and ecosystem needs. Groundwater-dependent ecosystems are adapted to long-term natural conditions, thus any alteration of hydrological fluxes, such as GWR and GWAs, disrupt the dynamic equilibrium of the system.
Decreasing groundwater levels lead to decreasing GWD to surface water bodies, threatening environmental flows in rivers and streams and other groundwater-dependent ecosystems such as wetlands, springs, and marine ecosystems. They may also negatively affect the water supply to vegetation such as forests and impact the groundwater ecosystem itself, a system that provides a unique habitat for groundwater fauna.
To assess the extent to which natural groundwater resources have been altered and put under stress by human water use, groundwater stress indicators can be quantified. However, existing indicators have limitations in interpreting future changes and presenting the full range of uncertainty.
A Novel Indicator for Future Groundwater Stress
This study proposes a novel indicator of future groundwater stress (GWSI) that quantifies stress in terms of future changes in GWD to groundwater-dependent ecosystems. Groundwater stress is defined as any alteration in GWD since ecosystems are adapted to an equilibrium state. Focusing on decreasing GWD, which is generally more harmful than increasing GWD, we quantified the future GWSI in Europe by integrating scenarios of GWR and GWA in 2070–2099.
The results reveal that projected GWSIs vary significantly among the scenarios. For the high-stress scenario, 58% of Europe’s land area is projected to experience a GWD decrease of at least 25% under RCP8.5, compared to 38% under RCP2.6. For the intermediate-stress scenario, the respective values are 26 and 1%.
These findings indicate that groundwater demand management alone might not prevent GWD declines under the high-stress and intermediate scenarios, particularly under RCP8.5. Climate change mitigation might thus be imperative for reducing the decline of GWD, especially in Eastern and Southeastern Europe, where changes in GWR are projected to be the primary cause of declining GWD.
Under RCP2.6, reductions in GWAs by 25–75% might balance a GWD decline in parts of Spain and Italy where GWAs are high, even in the high-stress scenario. In line with the precautionary principle, we recommend adapting to the high-stress scenario to minimize harm to the beneficiaries of groundwater.
Communicating Groundwater Stress Uncertainties
Quantifying the extent of future groundwater stress is challenging due to the high uncertainties in projecting GWR and GWA. The variability among hydrological models and climate change scenarios in projecting future GWR changes, and thus GWD, is very high.
To address this, the study utilized a multi-model ensemble for GWR changes and scenarios for GWAs to encompass an uncertainty range. However, the entire uncertainty range remains difficult to incorporate, as current model projections may underestimate the potential severity of future occurrences.
In line with the precautionary principle, policymakers should implement proactive regulatory measures to avert unnecessary harm before the risks materialize. The findings contrast with the current assessment of the European Water Framework Directive, indicating high risks for groundwater resources, especially in Southeastern Europe.
To support decision-making, we recommend communicating a diverse range of potential scenarios to foster transparency regarding uncertainties and enable the identification of effective strategies for reducing groundwater stress. This should involve a participatory process, allowing scientists and stakeholders to discuss the implications for adaptation strategies tailored to regional needs and stakeholder demands.