In the subsurface, the faults of the Peel Boundary Fault Zone, part of the Roer Valley Rift System in the Netherlands, Germany, and Belgium, often act as fault-parallel conduits and fault-perpendicular barriers for groundwater flow. However, our understanding of the conduit-barrier architecture of these faults remains limited. This study addresses this gap by presenting a detailed overview of the spatial variation in hydraulic conductivity and anisotropy, based on 161 collected samples, resulting in 150 measurements of saturated hydraulic conductivity (Ksat) and total porosity (Ptotal), and 151 measurements of median grain size (GS) from a trench site near Uden, the Netherlands. The results indicate that Ksat values across the fault range by almost four orders of magnitude, from 0.004 to 32.1 m day-1. Within this range, the fault exhibits the lowest horizontal (0.01-3.6) and vertical (0.004-7.1) Ksat values. Compared to the fault, values in the foot-wall damage zone (horizontal: 1.4-22.6 and vertical: 0.08-18.4) are significantly higher for both orientations, whereas in the hanging-wall damage zone (horizontal: 2.0-22.7 and vertical: 1.8-9.5), all values are higher, but only the horizontal values reach statistical significance. These findings indicate that the conduit-barrier effect of faults in unconsolidated sediments extends to the near surface. Hydraulic conductivity largely correlates positively with median GS but shows little to no overall correlation with total porosity (Ptotal). This trench study also reveals that the near-surface conduit-barrier architecture and hydraulic conductivity distribution is likely to evolve over geological timescales due to ongoing hydrogeological processes and fault activity. Such insight has the potential to contribute to future approaches for protecting and restoring fault-related seepage areas known as 'wijstgronden'.
Subsidence-induced surface instabilities, such as vertical sharp steps in the topography (known as ‘drempels’ in Dutch), subsidence, and sinkholes, can severely impact infrastructure and public safety. These surface displacements features have been observed during and immediately after coal mining in the South Limburg province of the Netherlands. In this study, a numerical framework for modelling large deformation processes, namely the material point method (MPM), is applied to investigate the underlying mechanisms driving the formation of these complex surface features during coal seam excavation. The modelling of this study focuses on a case study from Heerlen, South Limburg, where a shopping centre in the ’t Loon area partially collapsed due to the development of a sinkhole. Different seam configurations and excavation procedures are tested to assess their influence on the magnitude and spatial distribution of ground deformation. The MPM simulations demonstrate a clear connection between the observed deformation patterns and the early longwall mining processes. Modelled stress and strain concentrations coincide with the observed locations of both drempels and the sinkhole at the ground surface. Furthermore, additional insights were obtained. For example, the direction of seam excavation was found to contribute significantly to the overall distribution of deformations, with larger deformations occurring near the starting position and decreasing towards the end excavation. In contrast, reducing the seam dip angle further amplified the deformations, regardless of the excavation direction, with horizontal seams producing more pronounced effects. These behaviours arise in the absence of geological heterogeneities, indicating that operational configuration alone can predispose sites to the development of surface anomalies. This set of results demonstrates that specific mining and excavation configurations can trigger distinct surface deformation features at predictable locations. The study highlights the potential of MPM to capture the complex mechanisms driving mine collapse and ground subsidence, offering both improved understanding and a means to identify vulnerable areas for post-mining geohazard assessment based on subsurface configurations.
Geological models are important for subsurface engineering and it is crucial to identify their uncertainties. However, uncertainties in their geological input can be elusive and easily overlooked. Through a data review of Paleozoic geology of the Euregion-Meuse-Rhine, uncertainties in geomodelling inputs are identified and their causes are categorised into four groups: (1) stratigraphic interpretation, (2) fault interpretation, (3) transferring data, and (4) uncertainty in legacy materials. Examining these uncertainties reveals numerous sources for them that are intertwined. The number of connected sources of uncertainty demonstrate that the uncertainty chain in geomodelling is complex, calling for further investigation into the magnitude of the identified uncertainties. The Paleozoic geology in the study region has structural complexity in which geomodelling is hampered by limited outcrops and scattered input data. We compile input and examine data inconsistencies by collecting legacy literature and maps, conducting fieldwork, and compiling a dataset of 738 boreholes. Stratigraphic profiles of new boreholes (Cottessen-01, Banholt-01, and Terziet-02) are also included and two boreholes (Kastanjelaan-02 and RWTH-01) are re-evaluated with additional palynological constraints. Differences are found between various stratigraphic profiles for the latter two boreholes among different sources and updated stratigraphic profiles are presented for them. Comparing a newly drilled borehole with an existing geological cross-section reveals a >1 km depth mismatch between stratigraphic stages. Comparing stratigraphy of the borehole dataset with different geological maps reveals various degrees of agreement. The identified inconsistencies demonstrate the necessity of validating input data before embarking on any geomodelling exercise.
Research into faunal assemblages from the Upper Miocene and Lower Pliocene at Mill-Langenboom, province of Noord-Brabant, the Netherlands, has led to the recognition of premaxillary and dentary beaks of molid fishes (Molidae, Tetraodontiformes). These ex-situ finds, originating either from the Breda Formation or Oosterhout Formation, or both, are here assigned to two extinct taxa, namely, Ranzania tenneyorum and Mola pileata. Mola pileata has previously been recorded from the Middle Miocene of Belgium, the Netherlands, and the United States of America, while R. tenneyorum was up to now known only from the Lower-Middle Miocene Calvert Formation in Virginia (US). The present record thus extends the known geographical and stratigraphical ranges of R. tenneyorum to north-west Europe and the Middle-Miocene-Lower Pliocene.
In this paper, a synthesis is given of a large multi-disciplinary project, which included physical, botanical, zoological and archaeological studies, accelerator mass spectrometer from numerous locations in and along the extensive Moervaart palaeolake (NW Belgium), south of the Maldegem-Stekene Coversand Ridge. This rich dataset enabled a detailed reconstruction of climate, vegetation development and human presence in the period from the Weichselian Lateglacial and early Holocene. In addition, this dataset was used to make spatial reconstructions of the vegetation patterns in the direct surroundings of the Moervaart palaeolake for seven time slices and artist impressions for three moments in the archaeological record. These vegetation maps and the high resolution data on climate and the abiotic landscape are compared to former human occupation patterns to give insight in these early human presences in NW Europe. The first evidence for human presence, after a long period of absence, was found from the Allered period. During this period, hunter-gatherers of the Federmesser culture were present in encampments along the northern shore of the Moervaart palaeolake which had developed during the Belling period. Both the improving climate and the availability of a fresh water source stimulated human presence. The vegetation transitioned from a tundra landscape in the Belling period to a boreal forest with birch and pine in the Allered. During the following cold period of the Younger Dryas, forests retreated and tundra vegetation redeveloped. At the same time, the Moervaart palaeolake and most of the surrounding dune ponds turned dry. Evidence of human presence in the region during the Younger Dryas period is scarce, presumably related to the colder climatic conditions and the strongly reduced availability of fresh water sources. Due to climate warming during the early Holocene, boreal forests expanded again. However, evidence of human occupation of the area remains scarce. The Preboreal forest expansion was shortly interrupted by another cold reversal, the so-called Preboreal Oscillation or 11.4 event. After this event, hunter-gatherers returned to the area, then settling preferably along the dry banks of the Kale/Durme river, a tributary of the Scheldt river, which was the only source of fresh water in the region. During the following Boreal, coniferous forests were gradually replaced by deciduous forests which had developed initially with hazel, elm and oak, but later (Atlantic) also with lime, alder and ash. Hunter-gatherer site-density was highest during the first part of the Boreal, when hazel dominated the landscape. Afterwards, site-density dropped considerably; however, it is not clear whether this reflects a marked population reduction or rather points to changing mobility in response to a changing environment.
Many stream valleys in Northwestern Europe were once peat-filled during the Holocene. Nonetheless, they are often not considered as peat valleys in restoration projects, as they are now covered by thick clastic deposits. This sediment influx is often attributed to past deforestation and intensification of agriculture on adjacent hill slopes. However, this explanation fails in lowlands such as the Netherlands. How the clastic material formed in those stream valleys is still unclear. This study aims to determine the origin of thick humic sand covers in formerly peat-rich stream valleys. Two stream valleys in the sandy southern Netherlands were studied, the Keersop and the Kleine Dommel. The formation of the humic sand covers was studied by mapping the soils and subsurface lithology, luminescence dating and a review of archaeological and historical information. Our interdisciplinary research design has revealed direct human controls on historical floodplain transformations from peat-dominated to clastic-dominated in both valleys. The results show that the humic sand covers display many similarities with other Plaggic Anthrosols in the region, which developed after fertilisation of arable lands since the Middle Ages. Both contain humic sand, traces of charcoal and ceramics, and both can contain light-coloured sand inclusions. Our datings show the soils in the Kleine Dommel valley were anthropogenically raised during the late 17th or early 18th centuries. We argue that artificial raising of stream valleys with sands was a common practice in the southern Netherlands, intended to support agriculture on the existing peaty soils by improving their bearing capacity and drainage. This identified human-induced lowland floodplain transformation from peat-dominated to clastic-dominated underlines that these past human alterations should be considered when restoring stream valley ecosystems.
Seismic monitoring is essential for understanding subsurface processes, particularly in geothermal operations where low-magnitude events can provide valuable insights into reservoir behaviour. There are two significant challenges when monitoring the seismicity in Dutch geothermal operations: (1) detecting signals from seismic events as noise levels are typically high in regions hosting geothermal operations, and (2) accurately estimating their corresponding hypocentre and uncertainty. In this study, we present a comprehensive workflow for detecting and characterising low-magnitude seismic events. Specifically, we integrated data preparation, template-matching and machine-learning-based event detection, and probabilistic hypocentre estimation. Applying this workflow to 4 months of recordings in Kwintsheul, Netherlands, we detected 65 events with coherent signals, including six weak seismic events (ML < 0.0) near a local fault and a geothermal injection well. These events suggest the presence of a recurring microseismic sequence previously unreported in the area. However, spatial uncertainties, the short monitoring period, and the limited azimuthal coverage make the nature of these events unclear. Our findings highlight the importance of improving network design and refining velocity models to reduce uncertainties in event locations and magnitudes. The proposed workflow offers a scalable solution for enhancing seismic monitoring, particularly in urban and geothermal settings.
One of the consequences of deeper drainage of peat soils is the increase in surface level subsidence. Over a period of 50 years, soil surface subsidence and subsidence at different depths in the soil profile were measured using levelling and subsidence plates at 29 sites in seven locations in the Netherlands. Over those 50 years, the surface level for these sites has dropped by an average of 0.7 cm/year. A significant relationship between the measured ditch water level and the soil surface level subsidence rate was found. The subsidence plates, which were installed at different depths, allowed us to compare the contribution of the various soil layers to the subsidence of the surface. This showed that about 2/3 of the subsidence took place in the layers, where oxygen can penetrate (<0.8 m below soil surface). In those layers, oxidation and shrinkage are the most important subsidence processes. In the layers that are always saturated, 1/3 of the ground level subsidence still occurred, which is mainly caused by consolidation and creep but with a contribution of anaerobic decomposition. Finally, the results showed that subsidence rates during the period 1970-1990 were on average higher than during the period 1990-2024. Variability over sites was large, illustrating that site-specific conditions have a large impact on soil subsidence rates. The implications of our research are that peat subsidence monitoring, in the Netherlands and elsewhere, has to (1) be long-term, (2) monitor at different depths and (3) include multiple sites.
The lack of age-indicative calcareous nannoplankton has rendered the mid Eocene stratigraphic framework of the southern North Sea Basin uncertain. Palynological analyses by this study succeed in bridging this gap. A series of wells in the border region between Belgium and the Netherlands was biostratigraphically analysed, based on dinocyst species. The Belgian middle to late Eocene lithostratigraphic framework, comprising the Maldegem and Zelzate formations and their respective members, was applied to interpret the wells, which strongly increased the existing stratigraphic detail for the Dutch wells. A well-log correlation panel shows consistent biostratigraphic ages for the different middle to late Eocene units. It shows that the Zomergem and Buisputten members of the Maldegem Formation are still of Lutetian age, whereas the Onderdijke Member of the same formation is completely of Bartonian age. Palynological assemblages indicate an intra-Bartonian sea-level fall at the boundary between the Maldegem and Zelzate formations. The lowermost few metres of the Zelzate Formation consistently comprise Bartonian dinocyst species, which had been previously interpreted as reworked, but are now considered in situ, placing the Bartonian/Priabonian transition in the basal Zelzate Formation. The newly established Bartonian/ Priabonian boundary corresponds to the start of a new sea-level rise, indicated by an increase in gamma-ray values, in a nummulite level attributed to calcareous nannoplankton NP18 biozone. Hence, the previously established hiatus between the Belgian Maldegem and Zelzate formations now falls within the (upper part of the) Bartonian.
By ratifying the Paris Climate Agreement, The Netherlands has committed to an ambitious climate policy with the Dutch government aiming to significantly reduce greenhouse gas emissions. The transformation of the energy system to achieve sustainability, security, and affordability goals requires time, a long-term policy perspective and a renewed appreciation for technical competence development. The value and understanding of the subsurface are essential to the future of sustainable energy generation. In this paper, a technical competence framework is presented, which has been developed to aid in managing and developing the technical workforce required to utilize that subsurface value. The core competences identified have been broken down into distinct levels of proficiency, including a description of proof points. This aims to support and guide the individual professional to allow carrying out a self-assessment and to establish their technical competence profile. Many of the core competences are valid across value chains (hydrocarbon exploration, production and storage, geothermal, and CO2 and H2-storage) supporting the deployment and rotation of technical staff across value chains. Within the volatile and uncertain conditions of the energy transition, a high professional standard and a high level of mobility allow for shifting focus to other business priorities and requirements rapidly; they aid to job security conditions and encourage personal development and professional growth. This paper aims to provide the Dutch professional geoscientific community recommendations on how to further improve the resilience of the skill pool. A number of bottlenecks and challenges to enhance a thriving community of energy-geoscientists are identified, though mitigation measures are (partly) in place and will offer the opportunity to further strengthen the shared ambition to support and accelerate the energy transition, to continue substituting hydrocarbon-based energy with sustainable resources and to limit the net CO2 output from the energy industry.
This study provides evidence for the presence of channel incisions at the base of Lower Miocene glauconitic sands deposited in the southern North Sea Basin. A cross-boundary correlation panel between boreholes in Belgium and the Netherlands shows that the glauconitic sands of the Lower to Middle Miocene Berchem Formation strongly vary in thickness from less than 15 m to almost 70 m. The palynological analyses of borehole samples show that the main thickness change can be attributed to the presence/absence of the middle Burdigalian sands (dinocyst biozone NSM3) in the basal section of the Berchem Formation. These sands can be interpreted as infill of channels cut in the Rupelian clays of the Boom Formation. Indeed, also along a seismic line nearby, individual channels incised up to 45 m deep into the Boom Formation can be observed at the base of the Berchem Formation. The results of this study raise the question of whether other local middle Burdigalian sediment bodies in the region may also represent channel infills. Because the base of the Berchem Formation coincides with a major hiatus spanning the late Oligocene to the earliest Miocene, the channel incisions at this base could be related to the eustatic sea-level fall that occurred at the Oligocene-Miocene transition. During a middle Burdigalian sea-level rise, the sea transgressed into the channels, widening and filling them with sediments. Compared to other transgressed paleo-valley systems, which generally comprise at least in part fluvial or estuarine sediments, those in the base of the Berchem Formation are entirely composed of shallow marine, glauconite-rich sediments. By late Burdigalian times, the channels were filled, and sediment thicknesses became much more uniform across the region.
This study assesses the feasibility of utilizing well water in historic Middelburg, Zeeland, the Netherlands, as a source of drinking or graywater for residential purposes. Wells are prevalent in century old Middelburg houses when these were dug to access otherwise scarce freshwater. These wells became obsolete as modern amenities made freshwater available in all houses. However, many of these wells remain and some still discharge water, to the extent that it has to be pumped out and wasted. Given the specific challenges faced in this delta, of increasingly dry summers and saltwater intrusion in aquifers, freshwater can become even more scarce and costly. It is essential to explore every potential freshwater source, including this neglected well water. Therefore, seven wells in Middelburg were tested for common water quality parameters over a period of 6 months, including pH, temperature, dissolved oxygen, conductivity, phosphate, lead, copper, and E. coli. The conductivity confirmed the water to be freshwater, pointing to rainwater as a source, which finds its way underground and flows on remnants of sandy tidal creek beds. Dissolved oxygen levels were low in all wells except one. E. coli was not found, but unidentified coliform bacteria were present. All other parameters tested were within a normal range for drinking water. Despite these yet unknown coliform bacteria, the water in some of the wells is still useable as graywater. As most of the water is now pumped out, the residents can use the results of this study to find useful applications for their water as water stress in the area is increasing. By investigating new freshwater sources, this study contributes to the ongoing search for solutions to mitigate the ever-growing pressures on global freshwater resources.
Earth sciences are generally considered an important science discipline and a key in the solution of present societal problems. They are also important for interpreting the development of mankind in the past. The dynamics of landscapes are strongly intertwined with occupation patterns and are essential to interpret and date stratigraphical sequences linked to archaeological sites. Archaeologists thus need to have knowledge of earth sciences. But are they really educated in this field of sciences? This article discusses the way earth sciences are part of educational programmes in archaeology of several renowned international universities and compares them to the Dutch archaeological degree programmes. Next, it discusses the role of earth scientists and the application of earth science in the Dutch archaeological commercial practice and argues that earth sciences are not used to its fullest potential. Vice versa, also earth scientists can benefit from a more integrated approach, since they use archaeological data sparsely. It is argued that the present multidisciplinary approach should be turned into a transdisciplinary approach that already starts in the educational system, which may result in more innovative research outcomes in today’s archaeological practices.
In order to constrain different drivers of subsidence in the Groningen gas field region, the integration of geomechanical simulations into a data assimilation procedure is crucial. Existing geomechanical models vary in complexity depending on their implementation of the available input data of the subsurface geometry and properties and reservoir pressure. High-complexity models are associated with many parameters to be estimated and tend to be computationally expensive, hindering their practical use in data assimilation. We develop a mechanical model that is optimised in terms of model complexity for the context of simulating surface deformation above the Groningen gas field. The reservoir discretisation and vertical elastic layering are simplified such that model details that are unlikely to be generating surface signals resolvable in geodetic data are eliminated. We demonstrate that the optimised model is ~100 times more numerically efficient than complete models. We also determine the sensitivity of subsidence to the lateral compaction resolution and the elastic layering of our efficient model, to constrain the model resolution in future data assimilation applications for Groningen.
The challenges of providing sustainable drinking water are growing due to resource mismanagement, contamination threats and rising demand, which are further intensified by climate change. This further underscores the need for building-in resilience in existing extraction points to gain flexibility against uncertain and unforeseen developments. Although invisible, groundwater is a key drinking water source globally, including in the Netherlands, where over 60% of drinking water comes from it. The Dutch regulations, limited space and competition for water require adaptive strategies that enhance sustainability in water provision. Here, we identified and categorised various groundwater and surface water extraction archetypes in the Netherlands based on land use, extraction depth and local geology, assessing their susceptibility to contamination and operational challenges. Then, we evaluated four solution concepts to enhance sustainability in drinking water supply: the Water Battery (large-scale managed aquifer recharge), Fresh/Salt extraction (mitigated coastal salinisation), Switching between extractions (balancing demands in space) and Resource City (promoting circularity in urban water supply). Practical examples are already in place in the Netherlands as the Epe Water Battery shows successful infiltration and storage of groundwater to meet local demands and avoid undesirable low groundwater levels. We also explore the legal and operational challenges, emphasising stakeholder collaboration, proactive policies and the need for strategic investments in water quality improvement for a resilient, sustainable water supply in the face of climate change.
The geomorphic development of major rivers and smaller tributaries in response to climatic and environmental changes has been studied intensively in the past. However, impacts on confluence regions, with tributaries, have rarely been investigated. We aim to explore the similarities and differences in fluvial development over the last glacial to interglacial transition in the confluence area of the Meuse and the Loobeek in the southern Netherlands. We established five coring transects to investigate the fluvial architecture and sedimentary environments of the systems. Pollen analyses and radiocarbon dating enabled to establish the regional and local vegetation and the palaeogeographic evolution. The fluvial responses of the large Meuse and small Loobeek are strongly different and depend on differences in discharge, sediment supply and vegetation development. The fluvial response to the early Lateglacial warming is rapid in the small tributary and more delayed in the large-scale system. Smaller or shorter climate changes (during the Younger Dryas) are not registered in the channel pattern of the small-scale system. An anastomosing or diffuse drainage pattern and peat formation characterise the brook system during most of the Holocene, in contrast to the meandering Meuse system.
Luminescence dating methods are widely used to date coastal sediments, while luminescence tracing methods are an upcoming approach to reconstruct coastal sediment pathways. Both methods rely on subaqueous resetting (bleaching) of luminescence signals and would benefit from quantification of this process in the natural coastal environment. We describe the set-up and outcomes of an in situ subaqueous bleaching experiment for luminescence signals of K-feldspar grains in the Dutch Wadden Sea. We deployed a full-day bleaching field experiment with irradiated feldspar samples tethered to a pole at various positions within and above the water column to quantify (1) the bleaching potential, that is, the light intensity and spectrum as a function of time, depth and tidal stage, and (2) the bleaching efficiency, that is, the degree of bleaching of infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals measured at 150, 225 and 290 degrees C after a full day of light exposure above and below water. Our bleaching-potential results show that the strongest subaqueous light attenuation took place during low tide when sediment concentrations are the highest. We also observed stronger attenuation of the ultraviolet part of the spectrum compared to other parts of the spectrum. Our bleaching-efficiency results show that bleaching reduces with depth, that pIRIR signals bleach slower than IRSL signals underwater and that bleaching efficiency reduces with pIRIR measurement temperature. None of the investigated signals were fully reset after 13.5 hours of light exposure, even for subaerially exposed samples. Our work provides the first quantitative data on pIRIR bleaching in a natural subaqueous environment, which is relevant for K-feldspar-based luminescence dating and tracing applications.
Past hydrogeological processes and human impacts may exert substantial memory effects on today's groundwater systems. Thorough characterization of such long-term processes is required for scientists and policymakers to predict the hydrogeological impacts of land management options. Especially in data-scarce areas, historical data are essential to unravel long-term hydrogeological processes, which could not be identified by short-term fieldwork or model simulations alone. However, historical data are often overlooked or only used as background information in most hydrogeological studies. We show that the combination of historical reports and quantitative data yields major insights in the hydrogeological system of Cura & ccedil;ao, a small semi-arid Caribbean island. Reconstructing the island's groundwater conditions over the past 500 years revealed that deforestation and excessive abstraction has had a detrimental effect on the island's groundwater reserves. Historical notes and data revealed major signs of seawater intrusion, especially during abstraction peaks in the island's industrial era. Intrusion effects are still observed locally on the island today, but additional groundwater recharge by waste water has caused freshening elsewhere. We hypothesize that the observed aquifer replenishment locally enhances submarine groundwater discharge, flushing accumulated nutrients and pollutants towards Cura & ccedil;ao's fringing coral reefs. We expect that this study's insights motivate more hydrogeologists to use historical reports and data in future studies.
Monitoring groundwater levels and soil moisture content (SMC) is crucial for managing water resources and assessing risks, but can be challenging, especially over large acreages. Recent advances in geophysical methods provide new opportunities for accurate groundwater assessment. Seismic wave speed data, sensitive to changes in pore water pressure, can be used in a passive monitoring approach, while electrical conductivity data can be used for monitoring SMC. Combining seismic and electromagnetic induction (EMI)-based monitoring techniques enhances our understanding of groundwater dynamics. Seismic methods enable wide spatial coverage with moderate depth resolution, whereas EMI offers high-resolution, rapid data acquisition, particularly effective for shallow subsurface monitoring. Integrating these approaches can leverage the strengths of each, yielding comprehensive, high-resolution insights into dynamic subsurface hydrological processes. Integrating these approaches allows for improved groundwater monitoring, aiding in better understanding and managing droughts in regions like the Netherlands.
Groundwater is a vital resource for various water users in the Netherlands. However, due to a changing climate, increasing water demand and changes in the water system, the country is increasingly exposed to groundwater droughts. Water managers use various indicators and statistics to identify groundwater droughts. These indicators characterise the drought for example in terms of intensity, duration and probability of occurrence. Often, these indicators require information on long-term average groundwater conditions and extreme situations that can occur over long periods. However, the availability of long-term groundwater observations of more than ten years in length is limited. Particularly, extreme groundwater drought events are ill-described and subject to large uncertainty in their characterisation. This study explores a novel method for obtaining long-term phreatic groundwater levels by combining a data-driven time series model using transfer function-noise modelling with detrended historical meteorological time series representing the current climate. The method is applied to an area in the Netherlands to generate groundwater levels for the period 1910-2022. Our results reveal differences in the characterisation of groundwater droughts when the extended groundwater time series are compared with observations of a limited duration (eight years). Using the 2018 summer drought event as an example, we find that the probability of this groundwater drought occurring is approximately once every twelve years, based on the eight-year observation period. However, this probability reduces to a once every 24-year event when using historically generated groundwater time series to characterise the groundwater drought. We conclude that characterising droughts with the extended groundwater time series based on historical meteorological data can provide a more comprehensive insight into the intensity and frequency of groundwater droughts, as well as the probability of occurrence of current groundwater levels. Hence, the proposed method supports water managers in establishing return period-based criteria for measures, such as deciding when to implement irrigation bans.
