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Human actions compromise the many life-supporting functions provided by the freshwater cycle. Yet, scientific understanding of anthropogenic freshwater change and its long-term evolution is limited. Using a multi-model ensemble of global hydrological models, we estimate how, over a 145-year industrial period, streamflow and soil moisture have deviated from pre-industrial baseline conditions (defined by 5th–95th percentiles, at 0.5° grid level and monthly time step). We find increased frequency of local deviations on ~45% of land area, mainly in regions under heavy direct or indirect human pressures. To estimate humanity’s aggregate impact on the freshwater cycle, we present the evolution of deviation occurrence at regional to global scales. Currently, local streamflow and soil moisture deviations occur on 18.2% and 15.8% of global land area, respectively, which is 8.0 and 4.7 percentage points beyond the ~3 percentage point wide pre-industrial variability envelope. Finally, we discuss applying our approach to define a planetary boundary for freshwater change. Our results indicate a substantial shift from stable pre-industrial streamflow and soil moisture conditions to persistently increasing change and a transgression of the freshwater change planetary boundary, calling for urgent actions to reduce human disturbance of the freshwater cycle.

Groundwater resources do not exist in isolation but are deeply connected with social and ecological systems. As humans continue to modify the land surface, drive climate change, and place greater pressures on global freshwater resources, it is increasingly necessary to assess global groundwater resources through their relationships to these coupled systems. While several global classifications of physical groundwater systems exist, there is no data-driven global typology based on groundwater interactions with connected social and ecological systems. Though physical attributes remain hydrogeologically important, a more expansive systems-oriented classification is needed for policy development, applied research, and to develop the next generation of global hydrological models.We fill this gap by producing a spatially-explicit, moderate resolution (5 arcminute) global map of groundwater system archetypes based on groundwater interactions with social and ecological systems. These include interactions with streamflow, ecosystems, climate, agriculture, the economy, and water governance and management, all underpinned by existing global data. Archetypes, each with a unique set of interaction strengths and combinations, form a finite set of characteristic “fingerprints” that represent the dominant modes of interactions between groundwater and connected social and ecological systems. We find all WHYMAP large aquifer systems of the world are characterized by multiple social-ecological archetypes, suggesting that differentiated, context-appropriate approaches are necessary within large aquifers that are often assumed as uniform in global assessments and initiatives.We derive archetypes using multiple clustering algorithms and assign archetype membership based on majority agreement across clustering methods after cluster reclassification to create comparable maps. This multiple-method approach renders the archetypes more robust and less contingent on a single clustering algorithm while simultaneously enabling greater representation of archetype uncertainty.We additionally provide an outlook on sustainable development opportunities and challenges for each archetype. We summarize data sets that represent notable social-ecological outcomes related to the UN Sustainable Development Goals (SDGs), including: crop yield gaps (SDG 2), remotely sensed groundwater storage trends (SDG 6), economic inequality (SDG 10), human modification of terrestrial systems (SDG 15), and likelihood for hydropolitical interaction (SDG 16), among others. This work provides a number of useful contributions. First, the combination of archetyping (i.e., system characterization) and archetype-specific SDG outlook analysis provides a robust, data-driven overview of the role of groundwater in the global sustainability discourse. Secondly, the archetypes identify social-ecological system similarities across the globe, which may support interregional cooperation and networking, coordinated investment and interventions. Thirdly, as we harness the rapid growth in global data that document groundwater system interactions as the basis for our analysis, we simultaneously provide a synthesis and snapshot of the pertinent global data space. This snapshot can be used to identify the need for further data collection, especially on socio-economic interactions that remain underrepresented in global data. And finally, the archetypes raise awareness, build capacity, and shift mental models about the emerging perspective that it is necessary to conceptualize groundwater as a socially and ecologically connected resource.

The increasing number and quality of numerical groundwater models worldwide represent a great source of knowledge for local, regional, and international scientists as well as water managers and decision makers. This development is facilitated by recent advancements in computational tools and access to open software. At the same time, scientific journals stress the importance of sharing model codes and data upon publishing, setting a new publishing standard. Altogether, these developments in the groundwater modelling field create a richer and more dynamic environment, fostering model reproducibility. Such an environment calls for a global, integrated, and standardized database of groundwater models to help members of the groundwater modelling community to search, deposit, and analyse groundwater model information. Unfortunately, despite attempts in the past, such a database is not yet constructed and made available to the public. This is why multiple universities and institutes from different countries came together to create the Groundwater Model Portal (GroMoPo), where groundwater model information can be collected and shared easily. The process of building GroMoPo started by collecting information about individual groundwater models via an online form where various information was compiled by researchers from the institutes involved in the project. Apart from simple information such as names of model developers, year of model development, and country of origin, we also collected information on model implementation (e.g. software used, time and area covered, calibration and validation data availability). The collected data is stored at the Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI) HydroShare environment. The collected data are freely accessible via a web portal, which allows the user to query and visualize groundwater model information and to contribute new models. This web portal also allows new users to submit groundwater model information and explore previously collected data. Furthermore, we plan to keep GroMoPo updated in the future as an ongoing service, with CUAHSI’s help, for the hydrological community. We collected information from more than 500 groundwater models in our first phase. This number might appear large, but we estimate that it captures only a few percent of published peer-reviewed articles that include a groundwater model. Therefore, we wish to invite the groundwater modelling community to contribute to and use GroMoPo, expanding our group even further to ensure that more data is collected and shared in the future. With such community involvement, we hope to facilitate meta-analysis and comparative studies, enable broader sensitivity and uncertainty analysis, avoid duplication or replication in groundwater modelling efforts, increase the visibility of existing models and associated publications, and create a teaching tool for aspiring groundwater modelers.

Protected areas are a key tool for conserving biodiversity, sustaining ecosystem services and improving human well-being. Global initiatives that aim to expand and connect protected areas generally focus on controlling ‘above ground’ impacts such as land use, overlooking the potential for human actions in adjacent areas to affect protected areas through groundwater flow. Here, we assess the potential footprint of these impacts by mapping groundwatersheds. We find that over five in six protected areas globally (85%) have groundwatersheds that are underprotected. Half of all protected areas have a groundwatershed with a spatial footprint that lies predominantly (i.e., at least 50%) outside of the protected area’s boundary. These findings highlight a widespread potential risk to protected areas from activities affecting groundwater within their groundwatersheds, underscoring the need for groundwatershed-based protection measures. Delineating groundwatersheds can catalyze needed discussions about protected area connectivity and robustness, and investments in groundwatershed conservation and management can help protect groundwater-dependent ecosystems from external threats.

Groundwater resources are connected with social, economic, ecological, and Earth systems. We introduce the framing of groundwater-connected systems to better represent the nature and complexity of these connections in data collection, scientific investigations, governance and management approaches, and groundwater education. Groundwater-connected systems are social, economic, ecological, or Earth systems that interact with groundwater, such as irrigated agriculture, groundwater-dependent ecosystems, and cultural relationships to groundwater expressions such as springs and rivers. Groundwater-connected systems form social-ecological systems with complex behaviours such as feedbacks, non-linear processes, multiple stable system states, and path dependency. These complex behaviours are only visible through this integrated system framing and are not endogenous properties of physical groundwater systems. The framing is syncretic as it aims to provide a common conceptual foundation for the growing disciplines of socio-hydrogeology, eco-hydrogeology, groundwater governance, and hydro-social groundwater analysis. The framing also facilitates greater alignment between the groundwater sustainability discourse and emerging sustainability concepts and principles. Aligning with these concepts and principles presents groundwater sustainability as more than a physical state to be reached that additionally must integrate place-based and multi-faceted goals, values, justice, knowledge systems, governance and management to maintain groundwater’s social, ecological, and Earth system functions. The groundwater-connected system framing can underpin a broad, methodologically pluralistic, and community-driven new wave of data collection and analysis, research, governance, management, and education. These developments, together, can invigorate efforts to foster sustainable groundwater futures in the complex systems groundwater is embedded within.

Groundwater is essential for maintaining healthy ecosystems and securing human access to freshwater. Here we show that current estimates of global groundwater accessibility by ecosystems and humans are highly uncertain. To quantify this uncertainty, we define three categories of accessibility and investigate four global groundwater models. Averaged across these models, we estimate that 23% [most deviating model: 71%] of the land area contains groundwater accessible to ecosystems and humans, 57% [29%] is accessible to humans only, and 20% [0.01%] is costly to access or inaccessible. We find that the uncertainty in estimating water table depth severely affects our ability to assess groundwater's crucial role in ecosystem health, global water supplies associated to food security, and human health, with possible implications for achieving multiple Sustainable Development Goals. To reduce this uncertainty, we outline three pathways towards (1) better global datasets, (2) alternative strategies for model evaluation, and (3) greater cooperation with regional experts.

Water is a critical resource, but ensuring its availability faces challenges from climate extremes and human intervention. In this Review, we evaluate the current and historical evolution of water resources, considering surface water and groundwater as a single, interconnected resource. Total water storage trends have varied across regions over the past century. Satellite data from the Gravity Recovery and Climate Experiment (GRACE) show declining, stable and rising trends in total water storage over the past two decades in various regions globally. Groundwater monitoring provides longer-term context over the past century, showing rising water storage in northwest India, central Pakistan and the northwest United States, and declining water storage in the US High Plains and Central Valley. Climate variability causes some changes in water storage, but human intervention, particularly irrigation, is a major driver. Water-resource resilience can be increased by diversifying management strategies. These approaches include green solutions, such as forest and wetland preservation, and grey solutions, such as increasing supplies (desalination, wastewater reuse), enhancing storage in surface reservoirs and depleted aquifers, and transporting water. A diverse portfolio of these solutions, in tandem with managing groundwater and surface water as a single resource, can address human and ecosystem needs while building a resilient water system.

In the version of this article originally published, reference 9 was incorrectly cited in the last sentence of the second paragraph under ‘Introduction’ and in the first sentence of the second paragraph under the ‘Water scarcity’ subsection. Scanlon et al. (Environ. Res. Lett. https://doi.org/ 10.1088/1748-9326/ac3bfc, 2022) was incorrectly cited in the last sentence under ‘Drivers of water-resource variability’ but is now replaced with reference 38, and Figure 3 was wrongly stated to be adapted from reference 19 instead of reference 36. Reference 40 was mistakenly cited in the last sentence of the second paragraph under the ‘Increasing water access and supplies’ subsection, and reference 37 was inadvertently duplicated in the reference list. References 28 (now reading ‘Winter, T. C., Harvey, J. W., Franke, O. L. and Alley, W. M. Ground Water and Surface Water: A Single Resource. Circular 1139 (United States Geological Survey, 1998)’) and 94 (now reading ‘Scanlon, B. R., Reedy, R. C., Faunt, C. C., Pool, D. and Uhlman, K. Enhancing drought resilience with conjunctive use and managed aquifer recharge in California and Arizona. Environ. Res. Lett. 11, 035013 (2016)’) initially referred to incorrect sources. Lastly, the name of author Hannes Müller Schmied was incorrectly spelled Hannes Mueller Schmied, and an affiliation for him was missing: Senckenberg Leibniz Biodiversity and Climate Research Centre (SBiK-F), Frankfurt am Main, Germany. The errors have been corrected in the HTML and PDF versions of the article.

openaire: EC/HE/819202/EU//SOS.aquaterra Funding Information: This research has been supported by the Canada First Research Excellence Fund (grant no. C150-2017-8). Funding Information: The authors acknowledge various funds that made this research possible. Chinchu Mohan received funding from the Canada First Research Excellence Fund (CFRE); Matti Kummu received funding from the Academy of Finland funded project WATVUL (grant no. 317320), the Academy of Finland funded project TREFORM (grant no. 339834), and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 819202). Vili Virkki received funding from the Aalto University School of Engineering Doctoral Program and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 819202). Sonja C. Jähnig acknowledges funding through the Leibniz Association for the project Freshwater Megafauna Futures. Miina Porkka received funding from European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 819202). Lan Wang-Erlandsson was supported by the European Research Council through the “Earth Resilience in the Anthropocene” project (grant no. ERC-2016-ADG 743080) and by the IKEA Foundation. Publisher Copyright: © Copyright: The freshwater ecosystems around the world are degrading, such that maintaining environmental flow Environmental flow (EF): "The quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems."- Arthington et al. (2018). (EF) in river networks is critical to their preservation. The relationship between streamflow alterations (subsequent EF violationsEF violations are deviations in streamflow beyond the upper and lower boundaries of environmental flow envelopes (EFEs). The EFEs establish an envelope for acceptable EF deviations based on pre-industrial (1801-1860) stream discharge (see Sect. 2.2 for more details)) and the freshwater biodiversity response is well established at the scale of stream reaches or small basins (g 1/4

Groundwater pumping can reduce streamflow by reducing groundwater discharge and/or inducing streamflow infiltration, which together are referred to as streamflow depletion. Recently, analytical depletion functions (ADFs) have been suggested as rapid and accurate tools for streamflow depletion assessment, but their performance has only been tested in a few hydrogeological settings. To evaluate whether they will be useful tools for other regions with contrasting stream network and hydrogeological characteristics, we compared ADFs to calibrated MODFLOW models in BX Creek and Peace Region with distinct hydrogeological settings (interior plateaus & highlands, and boreal plains, respectively) and spatial scales (165 km2 and 1952 km2, respectively) in British Columbia, Canada. Results showed that ADFs can accurately identify streams most affected by pumping for 100% and 83% of wells in the BX Creek and Peace Region, respectively, and had small prediction errors compared with MODFLOW. Specifically, the mean absolute error of predicted depletion ranged from 2% to 14% of the highest simulated pumping rate over the study period of 30 years, with improved accuracy during the pumping season. We also found contrasting responses of ADF performance to hydrostratigraphic properties such as hydraulic conductivity, aquifer thickness, streambed conductance, and well depth across two domains, indicating that different drivers control ADF accuracy in different hydrogeological settings. Therefore, we conclude that ADFs are useful tools for conjunctive water management, but a good understanding of local hydrogeological conditions is needed to address the potential uncertainty of ADFs for decision-making.

Groundwater sustainability is challenged by the difference between legal and scientific understanding of groundwater as well as the lack of focused attention to regulatory design in the literature on groundwater institutions, governance and management. The purpose of this paper is to use groundwater science to direct the necessary elements of regulatory design for the unique characteristics of groundwater. Using plain and interdisciplinary language that could be applied in any jurisdiction or region, the article describes seven groundwater characteristics as processes, functions, qualities, physical sustainability, scale, information and data, and physical state. Using the characteristics of groundwater embeds the scientific understanding of groundwater into regulatory design and enables the expression of new values, such as Indigenous rights to water. Applying these characteristics to a case study of new groundwater regulation in a sub-national jurisdiction in the global north - British Columbia, Canada - highlights the failure of regulatory design even in a well-resourced jurisdiction where environmental regulation is the norm. Groundwater in British Columbia is extremely heterogeneous in quality and function with low observation density and undefined sustainability goals where regulations are applied on uniformly. Looking forward, three recommendations can be drawn using the characteristics of groundwater to improve regulatory design in British Columbia: defining sustainability goals and ecological thresholds; regionalizing and prioritizing; and long-term planning. This science-forward and interdisciplinary approach has implications for legally pluralistic states with customary water entitlements. It also provides practitioners with an interdisciplinary language that can be useful for assessing current and future regulatory design.

Groundwater allocation in British Columbia is facing a number of important challenges as groundwater is licensed under the Water Sustainability Act and potentially included in modern treaties. These challenges include acknowledging the importance of groundwater in supporting environmental flow needs and human water use, the uncertainty and irrelevance of annual recharge estimates, and the under-appreciated importance of aquifer drainage, while tackling cumulative impacts in watersheds using adaptive management with clear sustainability goals. We summarize these challenges and then suggest ways forward so that we can more robustly, holistically and sustainably allocate groundwater resources. Some of the proposed ways forward are based on sound science and are already being implemented partially or in some regions. Not implementing these ways forward risks permanent over-allocation of groundwater resources that would impact stream ecology, endanger rural livelihoods and challenge reconciliation with First Nations.

Groundwater systems are commonly but variously defined as renewable or non-renewable based on natural fluxes of recharge or on estimates of aquifer storage and groundwater residence time. However, the principle of capture challenges simple definitions: groundwater pumping can alter the rates of groundwater recharge and/or discharge over a characteristic groundwater response time related to the hydraulic properties of the subsurface, the length scales of the system and the relative position of the abstraction wells to the hydraulic boundaries. If pumping exceeds the possible capture, continued depletion or the recovery time will then be more strongly determined by the relative rates of pumping and recharge. Hence, we argue here that a groundwater system cannot be renewable or non-renewable in and of itself, but only with reference to how the groundwater is being used. We propose a new definition explicitly focusing on use: renewable groundwater use allows for dynamically stable re-equilibrium of groundwater levels and quality on human timescales (∼50-100 years). The definition combines both ‘flux’ and ‘storage’ based perspectives on renewable use. Further, we show how a matrix of combinations of (1) the ratio of pumping to possible capture along with (2) the response or recovery timescales implicit in this definition, leads to a useful four-quadrant framework for groundwater management. The quadrants, illustrated using case studies from aquifers around the world, are a practical tool for quantitatively assessing the physical limits to the sustainability of a pumped groundwater system alongside requisite environmental and social aspects and potential pathways towards greater sustainability.

Society and ecosystems are deeply connected to, and through, hydrological processes. Significant research efforts have revealed the breadth of ways humans have become dominant drivers of the global water cycle, however less attention has been placed on how hydrological change will affect social and ecological systems at the global scale. Understanding both directions of this coupled social-ecological system are critical to achieving sustainable freshwater futures in complex, multi-objective decision making environments. Here, we identify the global hotspots for social and ecological impacts from freshwater stress and freshwater storage loss.We applied the concept of hotspot mapping, from the field of conservation biogeography, to integrated socio- and eco-hydrological considerations for the first time at the global scale. We identified 168 basins for global prioritisation that are most vulnerable to suffer social and ecological impacts from freshwater stress and storage loss. These basins encompass over 1.5 billion people, 17% of global food crop production, 13% of global gross domestic product, and hundreds of internationally significant wetlands (Ramsar sites). The impacts that can be realised in these basins include transgressed environmental flows, increased drought frequency, decreased ecological resilience, threatened water, economic, and food security through reduced freshwater availability, and increased risk of wells running dry which may exacerbate existing economic inequalities. Regions and nations home to hotspot basins include: Argentina, northeastern Brazil, southwestern USA, Northern, Eastern, and Southern Africa, the Middle East and Arabian Peninsula, the Caucasus, West Asia, northern India, Nepal, Pakistan, Southeast Asia, and northern China. The 168 hotspot basins present an initial set of regions to prioritise in global sustainability initiatives that link water, ecosystems, and society, such as the Sustainable Development Goals. Furthermore, the hotspots represent the multiple epicentres where management of trade-offs between social, economic, and ecological water uses is most crucial, and thus represent the regions where implementation of integrated water resources management (IWRM) practises becomes most critical. To this end, we compared IWRM implementation levels to our global vulnerability results. While no direct relationship was found between IWRM implementation and social-ecological vulnerability to freshwater stress and storage loss, we observed, among hotspot basins, that IWRM implementation is lower in transboundary basins than in non-transboundary basins, suggesting that greater multilateralism and cooperation are needed. We identified hotspot basins by integrating global socio-hydrological and eco-hydrological datasets, remote sensing observations of freshwater storage trends, freshwater use, and streamflow datasets into a basin-scale social-ecological vulnerability analysis. This presentation reports the findings from Huggins et al. (in press, Nature Communications), and the hotspot basin results are available online for use by policy and research communities.Freshwater stress and storage loss are only two of many important aspects of freshwater with broad social-ecological resilience implications. Developing a network of similar analyses based on other processes and attributes, such as intra- and inter-annual storage variability, and water quality considerations, will support a more comprehensive understanding of the social-ecological impacts of global hydrological change.

Groundwater pumping can cause reductions in streamflow (“streamflow depletion”) that must be quantified for conjunctive management of groundwater and surface water resources. However, streamflow depletion cannot be measured directly and is challenging to estimate because pumping impacts are masked by streamflow variability due to other factors. Here, we conduct a management-focused review of analytical, numerical, and statistical models for estimating streamflow depletion and highlight promising emerging approaches. Analytical models are easy to implement, but include many assumptions about the stream and aquifer. Numerical models are widely used for streamflow depletion assessment and can represent many processes affecting streamflow, but have high data, expertise, and computational needs. Statistical approaches are a historically underutilized tool due to difficulty in attributing causality, but emerging causal inference techniques merit future research and development. We propose that streamflow depletion-related management questions can be divided into three broad categories (attribution, impacts, and mitigation) that influence which methodology is most appropriate. We then develop decision criteria for method selection based on suitability for local conditions and the management goal, actionability with current or obtainable data and resources, transparency with respect to process and uncertainties, and reproducibility.

Field geoscience has made important scientific advances but has not consistently considered the impact of these geoscience results on communities where the fieldwork is conducted. A reconciliation-based approach calls for critical thought about who defines, participates in, owns, and uses geoscience research, particularly in light of unresolved aboriginal rights and title claims and treaty rights throughout all of Canada. Geothermal research in the Canadian Cordillera has typically focused on hot spring systems and predicting maximum temperatures at depth, estimating fluid circulation depths, and investigating the distribution of hot spring systems and their relation to major geological features that often control thermal fluid flow. Detailed fieldwork to develop local and regional conceptual models of these systems has rarely been conducted and to our best knowledge, never in partnership with a First Nation. The scope of this project was working collaboratively with Xa’xtsa First Nation to conduct detailed structural, hydrologic and hydrogeologic fieldwork to develop local and regional conceptual models of Sloquet Hot Springs, on unceded St'at'imc territory. To motivate our research and provide a successful example of a reconciliation-based approach to field geoscience, we review how resource regulation, research, relationships, and reconciliation interact in British Columbia and consider our community partnership relative to Wong et al (2020)’s 10 Calls for Action for Natural Scientists. Well drilling, testing and monitoring revealed numerous soft zones in the subsurface as well as high transmissivity suggesting bedrock in the area has significant permeability. The annual flux calculated for Sloquet Hot Springs suggests a regional flow contribution from nearby watersheds. Although surface and subsurface observations did not identify the primary fault that conveys high-temperature fluids, the potential locations of buried fault structures are hypothesized based on zones with observably high temperatures and flow along Sloquet Creek. These results and interpretations are synthesized into a conceptual model of a localized hydrogeothermal system with local and regional groundwater flow along permeable pathways in the subsurface and mixing with cooler water before discharging in some of the springs.

Our ability to fully and reliably observe and simulate the terrestrial hydrologic cycle is limited, and in-depth experimental studies cover only a tiny fraction of our landscape. On medieval maps, unexplored regions were shown as images of dragons—displaying a fear of the unknown. With time, cartographers dared to leave such areas blank, thus inviting explorations of what lay beyond the edge of current knowledge. In hydrology, we are still in a phase where maps of variables more likely contain hydrologic dragons than blank areas, which would acknowledge a lack of knowledge. In which regions is our ability to extrapolate well developed, and where is it poor? Where are available data sets informative, and where are they just poor approximations of likely system properties? How do we best identify and acknowledge these gaps to better understand and reduce the uncertainty in characterizing hydrologic systems? The accumulation of knowledge has been postulated as a fundamental mark of scientific advancement. In hydrology, we lack an effective strategy for knowledge accumulation as a community, and insufficiently focus on highlighting knowledge gaps where they exist. We propose two strategies to rectify these deficiencies. Firstly, the use of open and shared perceptual models to develop, debate, and test hypotheses. Secondly, improved knowledge accumulation in hydrology through a stronger focus on knowledge extraction and integration from available peer-reviewed articles. The latter should include metadata to tag journal articles complemented by a common hydro-meteorological database that would enable searching, organizing and analyzing previous studies in a hydrologically meaningful manner. This article is categorized under: Engineering Water > Planning Water Science of Water > Hydrological Processes Science of Water > Methods.

Groundwater pumping can cause streamflow depletion by reducing groundwater discharge to streams and/or inducing surface water infiltration. Analytical and numerical models are two standard methods used to predict streamflow depletion. Numerical models require extensive data and efforts to develop robust estimates, while analytical models are easy to implement with low data and experience requirements but are limited by numerous simplifying assumptions. We have pioneered a novel approach that balances the shortcomings of analytical and numerical models: analytical depletion functions (ADFs), which include empirical functions expanding the applicability of analytical models for real-world settings. In this paper, we outline the workflow of ADFs and synthesize results showing that the accuracy of ADFs compared against a variety of numerical models from simplified, archetypal models to sophisticated, calibrated models in both steady-state and transient conditions over diverse hydrogeological landscapes, stream networks, and spatial scales. Like analytical models, ADFs are rapidly and easily implemented and have low data requirements but have significant advantages of better agreement with numerical models and better representation of complex stream geometries. Relative to numerical models, ADFs have limited ability to explore non-pumping related impacts and incorporate subsurface heterogeneity. In conclusion, ADFs can be used as a stand-alone tool or part of decision-support tools as preliminary screening of potential groundwater pumping impacts when issuing new and existing water licenses while ensuring streamflow meets environmental flow needs.

Humans and ecosystems are deeply connected to, and through, the hydrological cycle. However, impacts of hydrological change on social and ecological systems are infrequently evaluated together at the global scale. Here, we focus on the potential for social and ecological impacts from freshwater stress and storage loss. We find basins with existing freshwater stress are drying (losing storage) disproportionately, exacerbating the challenges facing the water stressed versus non-stressed basins of the world. We map the global gradient in social-ecological vulnerability to freshwater stress and storage loss and identify hotspot basins for prioritization (n = 168). These most-vulnerable basins encompass over 1.5 billion people, 17% of global food crop production, 13% of global gross domestic product, and hundreds of significant wetlands. There are thus substantial social and ecological benefits to reducing vulnerability in hotspot basins, which can be achieved through hydro-diplomacy, social adaptive capacity building, and integrated water resources management practices., This work identifies the world’s most vulnerable basins to social and ecological impacts from freshwater stress and storage loss: a set of 168 hotspot basins for global prioritization that encompass 1.5 billion people, 17% of global food crops, 13% of global GDP, and hundreds of significant wetlands.

Analysis of karst spring recession hydrographs is essential for determining hydraulic parameters, geometric characteristics and transfer mechanisms that describe the dynamic nature of karst aquifer systems. The extraction and separation of different fast and slow flow components constituting karst spring recession hydrograph typically involve manual and subjective procedures. This subjectivity introduces bias, while manual procedures can introduce errors to the derived parameters representing the system. To provide an alternative recession extraction procedure that is automated, fully objective and easy to apply, we modified traditional streamflow extraction methods to identify components relevant for karst spring recession analysis. Mangin’s karst-specific recession analysis model was fitted to individual extracted recession segments to determine matrix and conduit recession parameters. We introduced different parameters optimisation approaches of the Mangin’s model to increase degree of freedom thereby allowing for more parameters interaction. The modified recession extraction and parameters optimisation approaches were tested on 3 karst springs in different climate conditions. The results show that the modified extraction methods are capable of distinguishing different recession components and derived parameters reasonably represent the analysed karst systems. We recorded an average KGE > 0.7 among all recession events simulated by recession parameters derived from all combinations of recession extraction methods and parameters optimisation approaches. While there are variability among parameters estimated by different combinations of extraction methods and optimisation approaches, we find even much higher variability among individual recession events. We provide suggestions to reduce the uncertainty among individual recession events and to create a more robust analysis by using multiple pairs of recession extraction method and parameters optimisation approach.

International audience; Groundwater pollution threatens human and ecosystem health in many regions around the globe. Fast flow to the groundwater through focused recharge is known to transmit short-lived pollutants into carbonate aquifers, endangering the quality of groundwaters where one quarter of the world’s population lives. However, the large-scale impact of such focused recharge on groundwater quality remains poorly understood. Here, we apply a continental-scale model to quantify the risk of groundwater contamination by degradable pollutants through focused recharge in the carbonate rock regions of Europe, North Africa, and the Middle East. We show that focused recharge is the primary reason for widespread rapid transport of contaminants to the groundwater. Where it occurs, the concentration of pollutants in groundwater recharge that have not yet degraded increases from

Human actions and climate change have drastically altered river flows across the world, resulting in adverse effects on riverine ecosystems. Environmental flows (EFs) have emerged as a prominent tool for safeguarding riverine ecosystems. However, at the global scale, the assessment of EFs is associated with significant uncertainty. Here, we present a novel method to determine EFs by Environmental Flow Envelopes (EFE), which is an envelope of variability bounded by discharge limits within which riverine ecosystems are not seriously compromised. The EFE is defined globally in approximately 4,400 sub–basins at monthly time resolution, considering also the methodological uncertainties related with global EF studies. In addition to a lower bound of discharge, the EFE introduces an upper bound of discharge, identifying areas where streamflow has increased substantially. Further, instead of only showing whether EFs are violated, as commonly done, we quantify, for the first time, the frequency, severity, and trends of EFE violations, which can be considered as potential threats to riverine ecosystems. We use pre–industrial (1801–1860) quasi-natural discharge and a suite of hydrological EFR methods and global hydrological models to estimate EFE, applying data from the ISIMIP 2b ensemble. We then compare the EFEs to recent past (1976–2005) discharge to assess the violations of the EFE. We found that the EFE violations most commonly manifest themselves by insufficient streamflow during the low flow season, with less violations during intermediate flow season, and only few violations during high flow season. These violations are widespread: discharge in half of the sub–basins of the world has violated the EFE during more than 5 % of the months between 1976 and 2005. The trends in EFE violations have mainly been increasing during the past decades and will likely remain problematic with projected increases in anthropogenic water use and hydroclimatic changes. Indications of excessive streamflow through EFE upper bound violations are relatively scarce and spatially distributed, although signs of increasing trends can be identified and potentially attributed to climate change. While the EFE provides a quick and globally robust way of determining environmental flow allocations at the sub–basin scale, local fine–tuning is necessary for practical applications and further research on the coupling between quantitative discharge and riverine ecosystem responses is required.

The benefits of harnessing rivers into human use should not come with a disproportionate expense on the Earth system. Especially, freshwater ecosystems suffer greatly from direct and indirect human impacts, such as excessive water withdrawals and climate change, which are expected to only increase in the near future. Here, we aim for quantifying the extent and degree of considerable flow alterations that threaten the well-being of freshwater ecosystems, across the world.At the global scale, the ecological status of river systems is often assessed using global hydrological models (GHMs) and hydrological environmental flow (EF) methods. These suffer from substantial uncertainties: 1) the GHMs parameterised with variable climate forcings may give highly dispersed discharge estimates and 2) individual hydrological EF methods capture ecosystem water needs poorly. We tackle these sources of uncertainty by introducing a novel methodology: environmental flow envelopes (EFEs). The EFE is an envelope of safe discharge variability between a lower and an upper bound, defined at the sub-basin scale in monthly time resolution. It is based on pre-industrial (1801-1860) discharge and a large ensemble of EF methods, GHMs, and climate forcings, using ISI-MIP2b data. Using the EFE, we can simultaneously assess the frequency and severity of ecosystem-threatening flow alterations.Comparing post-industrial (1976-2005) discharge to the EFEs, discharge in 32.7% of the total 3860 sub-basins, covering 28.4% of the global landmass, violates the EFE during more than 10% of all months across four GHMs. These violations are considered as severe threats to freshwater ecosystems. The most impacted regions include areas with high anthropogenic pressure, such as the Middle East, India, Eastern Asia, and Middle America. The violations clearly concentrate on the EFE lower bound during low or intermediate flow seasons. Discharge in 61.4% of sub-basins violates the EFE during more than 10% of low flow season months, average violation being 47.5% below the safe limit denoted by EFE lower bound. Indications of significantly increased flows by violations of the EFE upper bound are fewer and further apart, as well as lower bound violations during high flow season.Although fractional discharge allocations alone cannot fully capture the ecosystem water needs, this study is a step towards less uncertainty in global EF assessments. The introduced method provides a novel, globally robust way of estimating ecosystem water needs at the sub-basin scale. The results of this study underline the importance of the low flow season, during which EFE violations are the most prevalent. While only preliminary evidence of significantly increased flows emerges in relatively few areas, the EFE upper bound would benefit from further research. The EFE methodology can be used for exploring macro-regional areas where anthropogenic flow alteration threatens freshwater ecosystems the most. However, case-specific studies incorporating factors beyond quantitative flow only are required for practical implications.

Green water - soil moisture, evaporation, and precipitation over land - is fundamental to safeguard Earth system functioning. Nonlinear green-water driven changes in climate, ecosystems, biogeochemistry, and hydrology are becoming increasingly evident and widespread. Yet, considerations of continental to planetary scale green-water dynamics are yet to be assessed and incorporated in management and governance. Here, we propose a green water planetary boundary (PB) - as part of the planetary boundary framework that demarcates a global “safe-operating space” for humanity - for assessing green-water related changes that can affect the capacity of the Earth system to remain in Holocene-like conditions. We consider green-water related processes associated with all scales: spatially distributed units, regions or biomes, and the Earth system as a whole. The proposed green water PB variable is selected through expert elicitation based on a set of transparent evaluation criteria that consider both scientific and governability aspects. Finally, we clarify the appropriate use of a green water PB, outline remaining challenges, and propose a research agenda for future navigation and quantitative assessments of the biophysical Earth system scale boundaries of green water changes.

Groundwater age and mean residence times have been invoked as measures of groundwater sustainability, with the idea that old or "fossil" groundwater is non-renewable. This idea appears to come from the link between groundwater age and background recharge rates, which are also of questionable use in assessing the sustainability of groundwater withdrawals. The use of groundwater age to assess renewability is further complicated by its relationship with flow system geometry. Young groundwaters near recharge areas are not inherently more renewable than older groundwaters down gradient. Similarly, there is no reason to preferentially use groundwater from smaller aquifers, which will have smaller mean residence times than larger aquifers for the same recharge rate. In some cases, groundwater ages may provide some information where groundwater recharge rates were much higher in the past and systems are no longer being recharged. However, there are few examples where the relationship between depletion and changes in recharge over long time periods has been rigorously explored. Groundwater age measurements can provide insights into the functioning of groundwater flow systems and calibration targets for numerical models and we advocate for their continued use, but they are not a metric of sustainable development. Simple metrics to assess groundwater sustainability remain elusive and a more holistic approach is warranted to maintain water levels and environmental flows.

Continental- to global-scale hydrologic and land surface models increasingly include representations of the groundwater system. Such large-scale models are essential for examining, communicating, and understanding the dynamic interactions between the Earth System above and below the land surface as well as the opportunities and limits of groundwater resources. We argue that both large-scale and regional-scale groundwater models have utility, strengths and limitations so continued modeling at both scales is essential and mutually beneficial. A crucial quest is how to evaluate the realism, capabilities and performance of large-scale groundwater models given their modeling purpose of addressing large-scale science or sustainability questions as well as limitations in data availability and commensurability. Evaluation should identify if, when or where large-scale models achieve their purpose or where opportunities for improvements exists so that such models better achieve their purpose. We suggest that reproducing the spatio-temporal details of regional-scale models and matching local data is not a relevant goal. Instead, it is important to decide on reasonable model expectations regarding when a large scale model is performing “well enough” in the context of its specific purpose. The decision of reasonable expectations is necessarily subjective even if the evaluation criteria is quantitative. Our objective is to provide recommendations for improving the evaluation of groundwater representation in continental- to global-scale models. We describe current modeling strategies and evaluation practices, and subsequently discuss the value of three evaluation strategies: 1) comparing model outputs with available observations of groundwater levels or other state or flux variables (observation-based evaluation); 2) comparing several models with each other with or without reference to actual observations (model-based evaluation); and 3) comparing model behavior with expert expectations of hydrologic behaviors in particular regions or at particular times (expert-based evaluation). Based on evolving practices in model evaluation as well as innovations in observations, machine learning and expert elicitation, we argue that combining observation-, model-, and expert-based model evaluation approaches, while accounting for commensurability issues, may significantly improve the realism of groundwater representation in large-scale models. Thus advancing our ability for quantification, understanding, and prediction of crucial Earth science and sustainability problems. We encourage greater community-level communication and cooperation on this quest, including among global hydrology and land surface modelers, local to regional hydrogeologists, and hydrologists focused on model development and evaluation.

The freshwater ecosystems around the world are degrading, such that maintaining environmental flow (EF) in river networks is critical to their preservation. The relationship between streamflow alterations and, respectively, EF violations, and freshwater biodiversity is well established at the scale of stream reaches or small basins (~

(IF 6.44; Q1); International audience; Karst aquifers provide drinking water for 10% of the world’s population, support agriculture, groundwater-dependent activities, and ecosystems. These aquifers are characterised by complex groundwater-flow systems, hence, they are extremely vulnerable and protecting them requires an in-depth understanding of the systems. Poor data accessibility has limited advances in karst research and realistic representation of karst processes in large-scale hydrological studies. In this study, we present World Karst Spring hydrograph (WoKaS) database, a community-wide effort to improve data accessibility. WoKaS is the first global karst springs discharge database with over 400 spring observations collected from articles, hydrological databases and researchers. The dataset’s coverage compares to the global distribution of carbonate rocks with some bias towards the latitudes of more developed countries. WoKaS database will ensure easy access to a large-sample of good quality datasets suitable for a wide range of applications: comparative studies, trend analysis and model evaluation. This database will largely contribute to research advancement in karst hydrology, supports karst groundwater management, and promotes international and interdisciplinary collaborations.