Groundwater is a drought resilient source of water supply for many water users globally. Managing these highly-used groundwater stores is complicated by the episodic nature of droughts and by our limited understanding of water systems' response to extreme events. Models are useful tools to simulate a range of prepared drought interventions, however, we need to ensure robust representation of surface water and groundwater storage, users of both resources, and associated management interventions for drought resilience. A robust modelling approach is therefore essential for decision-making in groundwater management. In this study, we present a Socio-Hydrological Water Resource (SHOWER) model for drought management in groundwater-rich regions. We evaluate SHOWER using a response-based and a data-based model evaluation in Great Britain which considers the modelling uncertainty, dynamic impact of management and modelling setups available. In the response-based evaluation, we first examined the model consistency with our understanding of the system functioning and evaluated the influence of modelled management scenarios in normal and droughts conditions on discharge and groundwater levels. Secondly, in the data-based evaluation we tested the accuracy of heavily influenced discharge and groundwater level simulations in three catchments representative of typical hydrogeological conditions and water management practices in Great Britain. Results from the response-based method show consistent simulations for all model setups. We identified which parameters were influential to model output at which times. Integrated water management interventions have significant impact on flows and groundwater beyond parameter uncertainty and show leverage to reduce droughts by minimising shortages in water demand. The data-based analysis shows that calibration can be focused on either source-specific or combined model outputs using a "best overall" calibration approach that captures groundwater levels and low flows. The source-specific calibrations result in the highest and narrowest KGE ranges for discharge and groundwater (KGE: 0.75-0.84 and 0.62-0.95 respectively) with larger ranges using a "best overall" approach (KGE: 0.55-0.79 and 0.27-0.91). With the modular and open-access structure of SHOWER we aim to provide a useful new tool for groundwater managers to explore management interventions further, increasing drought resilience strategies using a robust modelling approach.

Large-sample hydrological datasets containing data for tens to thousands of catchments are invaluable for hydrological process understanding and modelling. CAMELS (Catchment Attributes and MEteorology for Large-sample Studies) datasets provide hydro-meteorological timeseries, catchment attributes and catchment boundaries. Here, we present the second version of CAMELS-GB. CAMELS-GB v2 collates millions of observations from across Great Britain at hourly to monthly timescales, including quality-controlled daily river flows, catchment boundaries, and catchment characteristics from the UK National River Flow Archive. The new features include (1) extended daily hydro-meteorological timeseries from 1970-2022 including meteorological timeseries from new observed climate datasets, (2) hourly precipitation, river flow and level timeseries, (3) groundwater level timeseries and attributes for 55 groundwater wells, and (4) new catchment attributes characterising changing land cover, peak flows and human influences. These data are provided for 671 catchments across Great Britain spanning a diverse range of geophysical characteristics and human influences. CAMELS-GB v2 represents a step change for environmental and modelling analyses across Great Britain, particularly for the characterisation of sub-daily hydrological processes, and is made available as an open dataset (Coxon et al., 2025; 10.5285/9a46d428-958f-4ac1-86eb-94eee70c0955).

The 2022 European drought has underscored critical deficiencies in European water management. This paper explores these shortcomings and suggests a way forward for European drought risk management. In particular, we focus on four key aspects: the increasing drought risk, its spatial and temporal impacts, current management approaches, and how these differ across Europe. We base our findings on a continent-wide survey of water managers involved in responding to the 2022 drought. The survey collected 481 responses from 30 European countries and contained 19 questions on perceived sectorial impacts and the drought risk management practices implemented by the respondents' organizations. Information from the survey was supported with climate-related data on drought severity as quantified by the Standardised Precipitation Evapotranspiration Index, to offer a comprehensive overview of how extreme historical droughts are managed across Europe. Our findings reveal a consensus on the growing risk of drought, driven by droughts' rising frequency and intensity. While the 2022 event affected most of the continent, our findings show significant regional disparities in management capacity. In many countries - particularly those with agriculture-dominated economies - drought responses remain short-term and reactive, often leading to potentially maladaptive practices. Despite these challenges, we also observe a positive shift, with organizations showing increased awareness and preparedness. Hence, the lessons learnt from the 2022 event may provide an ideal opportunity to mainstream European-wide drought risk management. To seize this opportunity, we advocate for a European Drought Directive, to harmonize and enforce risk management policies across the continent. This directive should promote a systemic, integrated, and long-term risk management perspective. It should also set clear guidelines at the national and sub-national level, and for cross-boundary drought collaboration. This study and its companion paper, "From Crisis to Capacity: Institutional Preparedness and Response During the 2022 European Drought", result from work carried out by the Drought in the Anthropocene network, an initiative of the International Association of Hydrological Sciences (IAHS).

Groundwater is a vital component of the hydrologic cycle and also the largest human and ecosystem accessible freshwater storage, which plays an important role in many hydrological processes. However, in groundwater-dominated catchments where inter-catchment groundwater flow through subsurface flow pathways, most hydrological models lack explicit representations of these transboundary surface-subsurface interactions, resulting poor performance in hydrological predictions. Additional complexity introduced by intense groundwater abstractions and management schemes are also poorly represented in current hydrological models, which hinders accurate hydrological simulations. Therefore, developing integrated modelling frameworks for simulating the interactions between surface water, groundwater and human influences is needed for accurate hydrological predictions in these regions. DECIPHeR is a flexible hydrological modelling framework, which has demonstrated its good performance across a diverse range of catchments in Great Britain. However, in groundwater-dominated catchments, enhancements are needed in representing surface-subsurface water interactions for better model performance. This study integrates a national-scale groundwater model into DECIPHeR. We will utilize observational hydro-meteorological data to calibrate and validate the coupled model across 475 catchments. Additionally, a large sample of groundwater level data (over 3000 sites) in England will be used to further evaluate the model. Initial tests show that the coupled model outperforms DECIPHeR in Chalk catchments and also performs well (KGE>0.6) in other geology. The coupled models enable the assessment of surface-groundwater impacts, facilitating the potential quantification of human-water interactions, i.e. groundwater abstractions, on hydrological simulations. This analysis aims to support effective water supply and demand management strategies across Great Britain by providing insights into the influence of surface-groundwater interactions on the hydrological system.

Preparing for drought conditions is complicated by the episodic nature of droughts and by our limited understanding of water systems’ response to extreme events. In this, models are useful tools to simulate a range of plausible to low likelihood drought conditions. Water managers may use these simulations to make plans and consider consequences for both normal and extreme drought events. However, critical in this is the representation of water system resilience to drought conditions and simulated management decisions to in/decrease drought resilience. Decision-making in groundwater management could herein benefit from a robust modelling approach that considers the complexity and uncertainty in water availability, dynamic impact of management and modelling setups available.In this study, we have converted a lumped conceptual socio-hydrological model to an operational tool to support groundwater management in Great Britain by applying a response-based and a data-based model evaluation. In the response-based evaluation, we first examined the model consistency with our understanding of the system functioning, and the influence of modelled management scenarios on model predictions. In the data-based evaluation, we tested the accuracy of heavily influenced discharge and groundwater level predictions in three catchments representative of typical hydrogeological conditions and water management practices in Great Britain.Results show consistent simulations across catchments and identified pointers for influential model parameters in drought conditions. Modelled water management interventions have varying influence on simulated model output. Most effective drought management scenarios have (elements of) integrated water storage use, which minimises shortages in water demand. The data-based analysis shows that calibration can be focused on either low flows or groundwater storage, with reasonable results for both model outputs. We provide a source-specific and ‘best overall’ calibration approach that capture groundwater levels and low flows, which also indicates how model parameters (dis)agree with open-source data and our model perception of the modelled water system.

Uncertainty and sensitivity analysis are becoming an integral part of mathematical modelling of earth and environmental systems. Uncertainty analysis aims at quantifying uncertainty in model outputs, which helps to avoid spurious precision and increase the trustworthiness of model-informed decisions. Sensitivity analysis aims at identifying the key sources of output uncertainty, which helps to set priorities for uncertainty reduction and model improvement. In this presentation, we draw on a range of recent studies and projects to discuss the status of uncertainty and sensitivity analysis, focusing in particular on ‘global’ approaches, whereby uncertainties and sensitivities are quantified across the entire space of plausible variability of model inputs. We highlight some of the challenges and untapped potential of these methodologies, including: (1) innovative ways to use global sensitivity analysis to test the ‘internal consistency’ of models and therefore support their diagnostic evaluation; (2) challenges and opportunities to promote the uptake of these methodologies to increasingly complex models, chains of models, and models used in industry; (3) the limits of uncertainty and sensitivity analysis when dealing with epistemic, poorly bounded or unquantifiable sources of uncertainties.

Freshwater resources are increasingly under threat from climate change and increasing water demand. Catchment-scale hydrological models generate hydrological projections that underpin the management and sustainability of future water resources. Yet, water systems are increasingly interconnected across catchment boundaries through nationally strategic water supply schemes that aim to ensure a reliable supply of water in a changing climate. In this presentation, we draw on a range of studies from across Great Britain to discuss the challenges and complexities of hydrological modelling for future water resources management from catchment to national scales. We focus on interconnected water systems including catchments impacted by (1) inter-catchment groundwater flows and (2) water transfers via reservoirs, abstractions and wastewater treatment plants. For the human-impacted catchments, we identify where and when representing human interactions are important for robust streamflow projections. As water systems become more interconnected in space and time, we highlight the need to move beyond the catchment scale for future water resources management.

As the adverse impacts of hydrological extremes increase in many regions of the world, a better understanding of the drivers of changes in risk and impacts is essential for effective flood and drought risk management and climate adaptation. However, there is currently a lack of comprehensive, empirical data about the processes, interactions, and feedbacks in complex human-water systems leading to flood and drought impacts. Here we present a benchmark dataset containing socio-hydrological data of paired events, i.e. two floods or two droughts that occurred in the same area. The 45 paired events occurred in 42 different study areas and cover a wide range of socio-economic and hydro-climatic conditions. The dataset is unique in covering both floods and droughts, in the number of cases assessed and in the quantity of socio-hydrological data. The benchmark dataset comprises (1) detailed review-style reports about the events and key processes between the two events of a pair; (2) the key data table containing variables that assess the indicators which characterize management shortcomings, hazard, exposure, vulnerability, and impacts of all events; and (3) a table of the indicators of change that indicate the differences between the first and second event of a pair. The advantages of the dataset are that it enables comparative analyses across all the paired events based on the indicators of change and allows for detailed context- and location-specific assessments based on the extensive data and reports of the individual study areas. The dataset can be used by the scientific community for exploratory data analyses, e.g. focused on causal links between risk management; changes in hazard, exposure and vulnerability; and flood or drought impacts. The data can also be used for the development, calibration, and validation of socio-hydrological models. The dataset is available to the public through the GFZ Data Services (Kreibich et al., 2023,https://doi.org/10.5880/GFZ.4.4.2023.001).

<p>Decision-making in groundwater management could benefit from a robust modelling approach that considers the complexity and uncertainty in water availability, dynamic impact of management and modelling setups available. Modelling groundwater is a complex matter on its own given the heterogeneous aquifers, delayed climate signal in groundwater recharge and dynamic influence of water abstractions. Due to this complexity, decision-making models are often simplified to address only the main impacts on the hydrological cycle. Whilst this simplification is necessary, it is important to examine model process controls and uncertainty in parameters of a simplified model setup. </p> <p>In this study, we have converted a lumped conceptual socio-hydrological model to an operational tool for supporting decision-making by (1) evaluating and (2) calibrating the model. First, we applied global sensitivity to examine the “consistency” and “leverage” of the model. A model is considered consistent when modelled process controls match our system understanding. Leverage is observed when modelled strategies have adequate influence on modelling output regardless of parameter uncertainty. Results show that even with large uncertainty in parameter values, consistency is achieved for all hydrological variables. Input parameters defining management strategies are found to have significant leverage, as varying them induce noticeable changes in simulation outputs regardless of the physical conditions or uncertainty in parameters. When looking at hydrological extremes, this impact was amplified. </p> <p>Second, the model was calibrated for a range of catchments in the UK. The eleven model parameters were constrained using statistical criteria to identify an optimum parameter range. Model outputs were compared to observations (discharge and groundwater level time series using both similarity and signature-based evaluation criteria. Additionally, we sourced open access UK datasets to validate our data-based parameter ranges with local information. In general, calibrated model outputs represent surface water and groundwater features well and in particular, baseflow generation is well-represented. This encourages model applications for examining regional/national policies aiming to protect groundwater-fed streams. Exploratory model runs can also be used to facilitate discussions on new/altered management strategies and may spark further detailed modelling once a suitable strategy is identified. </p>

Human influences can both intensify or mitigate hydrological droughts significantly altering their severity, duration, and frequency via non-linear and dynamic feedbacks. Despite their large influence, current understanding of when, where and to what degree, human-water interactions modify hydrological drought is lacking. One of the key reasons for this is the scarce availability of quantitative human water use data as they are typically considered commercially sensitive and hard to obtain. Consequently, we often rely on static, low-resolution indicators of human water use (such as global water use databases) or qualitative information on human water use, when in reality human-water interactions are highly place-specific and non-stationary over time due to changes in water management and policies.In this study, we will disentangle human influences on hydrological droughts using observational hydro-meteorological and groundwater data and a unique dataset of spatially explicit, time-varying abstractions and discharges for a large sample of catchments across England. Building on recent work to quantify and detect human influences, we will use a suite of hydrological signatures to characterise deviations in droughts and low flows from a large sample of benchmark (i.e. near-natural) catchments. We will link these deviations to different characteristics of the abstractions data (e.g. seasonal catchment averages, abstraction purpose) and to key water management schemes (e.g. low flow alleviation schemes). In doing so, we will advance our current understanding of how humans influence hydrological droughts and how we can improve the collection of human-water use data for future environmental analyses.

Afforestation, as one of the major drivers of land cover change, has the potential to provide a wide range of ecosystem services. Aside from carbon sequestration, afforestation can improve hydrological regulation by increasing soil water storage capacity and reducing surface water runoff. However, afforested areas are rarely studied over time scales appropriate to determine when changes in soil hydrological processes occur as the planted (mixed) forests establish and grow. This study investigates the seasonal soil moisture and temperature dynamics, as well as the event‐based responses to precipitation and dry periods, for a mature and a juvenile forest ecosystem over a 5‐year time period. Generally, soil moisture was higher in the juvenile forest than in the mature forest, suggesting a lower physiological water demand. Following the 2018 drought, soil moisture dynamics in the growing juvenile plantation began to match those of the mature forest, owing to canopy development and possibly also to internal resilience mechanisms of the young forest to these external hot weather perturbations. Soil temperature dynamics in the juvenile plantation followed air temperature patterns closely, indicating lower thermal regulation capacity compared to the mature forest. While our findings show that an aggrading juvenile plantation achieves mature forest shallow soil moisture storage dynamics at an early stage, well before physiological maturity, this was not the case for soil temperature. Our results shed light on long‐term trends of seasonal and event‐based responses of soil moisture and temperatures in different‐aged forest systems, which can be used to inform future assessments of hydrological and ecosystem responses to disturbances and forest management.

In recent years, drought impacts have been perceived as more severe and frequent than those of past events throughout Europe. Due to the heterogeneity of Europe’s hydro- climatological situation as well as the multiple Nations on the continent, drought events and their impacts vary with respect to location, sector, extent, duration and scale. In order to understand recent effects of drought and their possible drivers, national representatives distributed a uniform questionnaire to water management related stakeholders at different scales of 28 contributing countries. The survey focused on collecting information on stakeholders’ perceptions of drought, impacts on water resources and beyond, water availability and current drought management strategies at national and regional scales. The survey results were compared with the actual drought hazard information registered by the European Drought Observatory (EDO) for 2018 and 2019. The final results of the study highlight the diversity among national drought event perception and the value of implemented drought management strategies. Only few countries practise drought management, an absence of drought management is mostly attributed to lacking of resources, but also lacking political will for implementation and lacking political advice. Supported by the national representatives’ perspectives, the study concludes with an urgent need to further reduce drought impacts by constructing and implementing a European macro-level drought governance approach, such as a directive, which would strengthen national drought management and lessen harm to human and natural potentials.

Balancing drinking water supply and environmental water needs requires careful use of water resources. Some water sources are more resilient to climate extremes than others. For example, groundwater resources can buffer climate extremes, as groundwater storage in aquifers is slowly released, complementing water supply from surface sources during periods of water scarcity, heatwaves and (extreme) drought events. However, this resilience declines when groundwater storage is compromised due to periodic or sustained overuse with severe consequences for groundwater-dependant ecosystems. Sustainable groundwater management needs a robust decision-making approach, that looks beyond historical drought events and prepare for a possible combination of extreme conditions in a future climate, including not only climate uncertainty but also a changing water anthropogenic demand and environmental water needs. In this study we present a bottom-up approach to support robust decision-making to improve climate resilience of drinking water supply systems, considering both surface water and groundwater use. We consider uncertainties related to model parameters, changing water demand and future climate conditions, as we apply this method to a system-level representation of a water management region in South England. Results show under which climate conditions current water management strategies are confidently meeting drinking water supply with a margin for (seasonally) increased water demand. Future extreme conditions reveal increasing competing interests with environmental water needs and possible shortages in drinking water supply. Mapping the associated (un)certainty of short-term and long-term management strategies shows the value of robust decision-making to sustainable water use in complex water resource systems.

Risk management has reduced vulnerability to floods and droughts globally 1 , 2 , yet their impacts are still increasing 3 . An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data 4 , 5 . On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change 3 .

Abstract. Drought events and their impacts vary spatially and temporally due to diverse pedo-climatic and hydrologic conditions, as well as variations in exposure and vulnerability, such as demographics and response actions. While hazardous severity and frequency of past drought events have been studied in detail, little is known about the effect of drought management strategies on the actual impacts, and how the hazard is perceived by relevant stakeholders for inducing action. In a continental study, we characterised and assessed the impacts and the perceptions of two recent drought events (2018 and 2019) in Europe and examined the relationship between management strategies and drought perception, hazard and impacts. The study was based on a pan-European survey involving national representatives from 28 countries and relevant stakeholders responding to a standard questionnaire. The survey focused on collecting information on stakeholders’ perceptions of drought, impacts on water resources and beyond, water availability and current drought management strategies at national and regional scales. The survey results were compared with the actual drought hazard information registered by the European Drought Observatory (EDO) for 2018 and 2019. The results highlighted high diversity in drought perceptions across different countries and in values of implemented drought management strategies to alleviate impacts by increasing national and sub-national awareness and resilience. The study concludes with an urgent need to further reduce drought impacts by constructing and implementing a European macro-level drought governance approach, such as a directive, which would strengthen national drought management and lessen harm to human and natural potentials.

In recent years, drought impacts have been more severe and frequent than past impacts throughout Europe. Due to the heterogeneity of Europe’s hydro- climatological situation as well as the multiple nations on the continent, drought events and their impacts vary with respect to location, sector, extent, duration and scale. In order to understand recent effects of drought and their possible drivers, national representatives distributed a uniform questionnaire to water management stakeholders of 28 contributing countries. Here, we focus on obtaining information on stakeholders’ drought perception,impacts, and current management strategies on a national and sub-national scale. With the survey, we analyse how strong the relationship between perceptions and actual hazard information is. Actual drought hazard information from the European Drought Observatory for the years 2018 and 2019 is compared with the questionnaire’s results. The results of the study highlight the diversity among national drought perceptions and the value of already existing drought management strategies. An absence of coordinated drought management is mostly attributed to a lack of resources and macro- governmental guidance. Supported by the national perspectives, possible macro-governmental pathways to increase national and sub-national awareness and resilience are discussed. The results support the need for national drought policies, which could be pushed forward with international drought management directives.

Increasing meteorological drought frequency and rising water demand drive groundwater exploitation beyond sustainable limits. In heavily-stressed aquifers mitigation strategies, such as Managed Aquifer Recharge (MAR), are needed to restore depleted groundwater storage. MAR is also designed to overcome short dry periods. However, wider impacts of MAR as a drought mitigation strategy remain to be quantified. The objective of this study is to assess impacts of MAR in heavily-stressed aquifers using a case study of the Central Valley in California (USA). The novelty of this study lies in its analytical approach based on long-term observational data of precipitation, groundwater levels, and MAR operations. The impact of MAR operations is assessed regionally and for different temporal scales. Results show spatially-coherent clusters of groundwater level time series in the Central Valley representing three main patterns that manifest themselves in different groundwater drought characteristics and long-term trends. The first regional pattern shows lengthened groundwater droughts and declining groundwater levels over time, indicating effects of over abstraction in aquifer sections without MAR. The second regional pattern shows reduced groundwater drought duration and magnitude related to periodically rising groundwater levels, showing short-term MAR impacts. The third regional pattern shows alleviated groundwater droughts and groundwater levels show a long-term rise, representing long-term MAR impacts. Mitigated groundwater droughts and long-term rise in groundwater levels reveal the value of long-term MAR operations and their contribution toward sustainable groundwater management. Increased institutional support is recommended to ensure longevity of MAR and thereby amplify its success as regional drought mitigation strategy in heavily-stressed aquifers.

Managing water-human systems in times of water shortage and droughts is key to avoid overexploitation and reduce drought impacts. Drought policies are designed to structure water management response and avoid crisis management, aiming to sustain both environmental and anthropogenic water demand. However, the impact of drought policies on the hydrological cycle is rarely assessed. We developed a socio-hydrological model, simulating feedbacks between water availability and managed water use for 3 decades. Thereby, we aim to assess the impact of drought policies on both surface water and groundwater droughts. We tested this model in an idealised catchment driven by English climate data, representing English water resource management practices and drought policies. The model environment includes a surface water storage (reservoir), a range of hydrogeological conditions for the groundwater module, and an option to import surface water or groundwater that could all be used to satisfy anthropogenic and environmental water demand. Modelled scenarios represent four aspects of drought policies: 1) increased water supply, 2) restricted water use, 3) conjunctive water use, and 4) maintained environmental flow requirements that were evaluated in separate and combined scenarios. Results show that hydrological droughts are mitigated in scenarios applying conjunctive use, particularly in low groundwater storage systems. In high groundwater storage systems, maintaining environmental flows reduces hydrological droughts most, particularly for surface water droughts. Scenarios that gradually increase water supply or restrict water use have opposite impacts on droughts and these scenarios are in balance when combined according to the policies in the resources management plans. Most combined scenarios reduce the severity and occurrence of hydrological droughts, although the dependency on importing water increases, satisfying a third of the total anthropogenic water demand. The increased dependency on imported water shows the considerable pressure on water resources and the delicate balance of water-human systems during droughts that calls for short-term and long-term sustainability targets within drought policies.