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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.

Many countries fund catchment observatories and networks to provide observational data, test models and hypotheses, discover new insights, catalyse the development of new technologies and enhance interdisciplinary collaboration. These catchment networks provide a wealth of observational data, yet synthesising information across catchment observatories to produce process-based understanding is challenging. To generalise findings from place-based studies, we need greater synthesis across catchment networks and thus careful consideration of the design and topology of catchment observatories and monitoring networks.In this paper, we collate information from 80 catchment observatories/networks and conduct 21 questionnaires with project leads with the aim of reviewing the strengths and weaknesses of catchment observatories to provide recommendations that can inform future catchment observatory and network design. The catchment observatories encompass a wide range of flow regimes, science questions and spatial/temporal scales with 25, 33 and 22 observatories from the UK, Europe, and North America respectively. Most catchment observatories in the monitoring catalogue are concentrated in upland catchment systems monitoring flashy flow regimes, with very few focused on lowland systems and no catchment observatories focused on urban catchments. The choice of catchment observatory location was focused upon logistics and catchment characteristics, with logistics and the day-to-day running of the observatory highlighted as the aspect catchment observatory programme managers found most difficult. Many interviewees noted that the design of the observatory was a key phase in planning and an aspect they would have done differently.Finally, we recommend key design guidelines for future catchment observatory and networks. This includes the need for a scoping and planning phase, community co-designed, digital infrastructure that enables FAIR data provision, and flexible and extensible catchment topology. Critically, knowledge transfer needs to be built in from the beginning of catchment observatories to enable transferability of new insights and understanding across linked catchment networks to tackle grand challenges within hydrology.

With the help of a physically based recharge-groundwater flow model and robust detrended fluctuation analysis (r-DFAn), the effect of local (catchment-scale) forcing on groundwater levels’ scaling behavior in a riparian aquifer in Wallingford, UK, is investigated. The local forcings investigated in this study include the rainfall’s temporal scaling behavior (which is simulated by changing rainfall’s intermittency parameter in a β-lognormal multiplicative random cascade model), the aquifer’s physical parameters (saturated hydraulic conductivity, specific yield, the empirical coefficients of the water retention curve, and the river stage’s scaling behavior). Groundwater level’s scaling behaviour was found to be most sensitive to rainfall’s fractal behaviour. Additionally, there is preliminary evidence suggesting that changes to the rainfall’s local scaling behaviour (i.e., change to the series’ scaling that induces crossovers) affects the groundwater’s and the recharge’s local scaling behaviour.

Drought forecasting and early warning systems for water resource extremes are increasingly important tools in water resource management in Europe where increased population density and climate change are expected to place greater pressures on water supply. In this context, the North Atlantic Oscillation (NAO) is often used to indicate future water resource behaviours (including droughts) over Europe, given its dominant control on winter rainfall totals in the North Atlantic region. Recent hydroclimate research has focused on the role of multiannual periodicities in the NAO in driving low frequency behaviours in some water resources, suggesting that notable improvements to lead-times in forecasting may be possible by incorporating these multiannual relationships. However, the importance of multiannual NAO periodicities for driving water resource behaviour, and the feasibility of this relationship for indicating future droughts, has yet to be assessed in the context of known non-stationarities that are internal to the NAO and its influence on European meteorological processes. Here we quantify the time–frequency relationship between the NAO and a large dataset of water resources records to identify key non-stationarities that have dominated multiannual behaviour of water resource extremes over recent decades. The most dominant of these is a 7.5-year periodicity in water resource extremes since approximately 1970 but which has been diminishing since 2005. Furthermore, we show that the non-stationary relationship between the NAO and European rainfall is clearly expressed at multiannual periodicities in the water resource records assessed. These multiannual behaviours are found to have modulated historical water resource anomalies to an extent that is comparable to the projected effects of a worst-case climate change scenario. Furthermore, there is limited systematic understanding in existing atmospheric research for non-stationarities in these periodic behaviours which poses considerable implications to existing water resource forecasting and projection systems, as well as the use of these periodic behaviours as an indicator of future water resource drought. Natural Environment Research Council (NERC): NE/M009009/1 and NE/L010070/1

Water resource management (WRM) practices, such as groundwater and surface water abstractions and effluent discharges, may impact baseflow. Here the CAMELS-GB large-sample hydrology dataset is used to assess the impacts of such practices on Baseflow Index (BFI) using statistical models of 429 catchments from Great Britain. Two complementary modelling schemes, multiple linear regression (LR) and machine learning (random forests, RF), are used to investigate the relationship between BFI and two sets of covariates (natural covariates only and a combined set of natural and WRM covariates). The LR and RF models show good agreement between explanatory covariates. In all models, the extent of fractured aquifers, clay soils, non-aquifers, and crop cover in catchments, catchment topography, and aridity are significant or important natural covariates in explaining BFI. When WRM terms are included, groundwater abstraction is significant or the most important WRM covariate in both modelling schemes, and effluent discharge to rivers is also identified as significant or influential, although natural covariates still provide the main explanatory power of the models. Surface water abstraction is a significant covariate in the LR model but of only minor importance in the RF model. Reservoir storage covariates are not significant or are unimportant in both the LR and RF models for this large-sample analysis. Inclusion of WRM terms improves the performance of some models in specific catchments. The LR models of high BFI catchments with relatively high levels of groundwater abstraction show the greatest improvements, and there is some evidence of improvement in LR models of catchments with moderate to high effluent discharges. However, there is no evidence that the inclusion of the WRM covariates improves the performance of LR models for catchments with high surface water abstraction or that they improve the performance of the RF models. These observations are discussed within a conceptual framework for baseflow generation that incorporates WRM practices. A wide range of schemes and measures are used to manage water resources in the UK. These include conjunctive-use and low-flow alleviation schemes and hands-off flow measures. Systematic information on such schemes is currently unavailable in CAMELS-GB, and their specific effects on BFI cannot be constrained by the current study. Given the significance or importance of WRM terms in the models, it is recommended that information on WRM, particularly groundwater abstraction, should be included where possible in future large-sample hydrological datasets and in the analysis and prediction of BFI and other measures of baseflow.

There is a scarcity of long-term groundwater hydrographs from sub-Saharan Africa to investigate groundwater sustainability, processes and controls. This paper presents an analysis of 21 hydrographs from semi-arid South Africa. Hydrographs from 1980 to 2000 were converted to standardised groundwater level indices and rationalised into four types (C1–C4) using hierarchical cluster analysis. Mean hydrographs for each type were cross-correlated with standardised precipitation and streamflow indices. Relationships with the El Niño–Southern Oscillation (ENSO) were also investigated. The four hydrograph types show a transition of autocorrelation over increasing timescales and increasingly subdued responses to rainfall. Type C1 strongly relates to rainfall, responding in most years, whereas C4 notably responds to only a single extreme event in 2000 and has limited relationship with rainfall. Types C2, C3 and C4 have stronger statistical relationships with standardised streamflow than standardised rainfall. C3 and C4 changes are significantly (p

Effective management of groundwater resources during drought is essential. How is groundwater currently managed during droughts, and in the face of environmental change, what should be the future priorities? Four themes are explored, from the perspective of groundwater management in England (UK): (1) integration of drought definitions; (2) enhanced fundamental monitoring; (3) integrated modelling of groundwater in the water cycle; and (4) better information sharing. Whilst these themes are considered in the context of England, globally, they are relevant wherever groundwater is affected by drought.

An understanding of multi-annual behaviour in streamflow allows for better estimation of the risks associated with hydrological extremes. This can enable improved preparedness for streamflow-dependant services, such as freshwater ecology, drinking water supply and agriculture. Recently, efforts have focused on detecting relationships between long-term hydrological behaviour and oscillatory climate systems (such as the North Atlantic Oscillation – NAO). For instance, the approximate 7 year periodicity of the NAO has been detected in groundwater-level records in the North Atlantic region, providing potential improvements to the preparedness for future water resource extremes due to their repetitive, periodic nature. However, the extent to which these 7-year, NAO-like signals are propagated to streamflow, and the catchment processes that modulate this propagation, are currently unknown. Here, we show statistically significant evidence that these 7-year periodicities are present in streamflow (and associated catchment rainfall), by applying multi-resolution analysis to a large data set of streamflow and associated catchment rainfall across the UK. Our results provide new evidence for spatial patterns of NAO periodicities in UK rainfall, with areas of greatest NAO signal found in southwest England, south Wales, Northern Ireland and central Scotland, and show that NAO-like periodicities account for a greater proportion of streamflow variability in these areas. Furthermore, we find that catchments with greater subsurface pathway contribution, as characterised by the baseflow index (BFI), generally show increased NAO-like signal strength and that subsurface response times (as characterised by groundwater response time – GRT), of between 4 and 8 years, show a greater signal presence. Our results provide a foundation of understanding for the screening and use of streamflow teleconnections for improving the practice and policy of long-term streamflow resource management.

Groundwater-dominated catchments are often critical for nationally-important water resources. Many conceptual rainfall-runoff models used for the simulation of river flows tend to degrade in their model performance in groundwater-dominated catchments as they are rarely designed to simulate spatial groundwater behaviours or interactions with surface waters. Intercatchment groundwater flow is one such neglected variable. Efforts have been made to incorporate this process into existing models, but there is a need for greater emphasis on improving our perceptual models of groundwater-surface water interactions prior to any model edits.In this study, national meteorological, hydrological, hydrogeological, geological and artificial influence (characterising abstractions and return flows) datasets are used to develop a perceptual model of intercatchment groundwater flow and how it varies spatially and temporally across the River Thames. We characterise the water balance, presence of gaining/losing river reaches and intra-annual dynamics in 80 subcatchments of the River Thames in the UK, taking advantage of its wealth of data, densely gauged river network and geological variability.We show the prevalence of non-conservative river reaches across the study area, with heterogeneity both between, and within, geological units giving rise to a complex distribution of recharge and discharge points along the river network. We identify where non-conservative reaches can be attributed to intercatchment groundwater flow, and where other processes (e.g. human abstractions and discharge uncertainty) are likely the cause. Escarpments of Chalk and Jurassic Limestone show evidence of intercatchment groundwater flow both from headwater to downstream reaches, and out-of-catchment via springlines. We found temporal as well as spatial variability across the study area, with more seasonality and variability in river catchments on Jurassic Limestone outcrops and less on Chalk and Lower Greensand outcrops. Our results show the need for a degree of local investigation and hydrogeological perceptualisation within regional analysis, which we show to be achievable given relatively simple geological interpretation and data requirements. We then discuss the inclusion of external flow fluxes within existing models to enable calibration improvements in groundwater-dominated catchments, and, importantly, the characterisation of these fluxes given the temporal and spatial variability of intercatchment groundwater flow that our perceptual model has shown.

Drought forecasting and early warning systems for water resource extremes are increasingly important tools in water resource management, particularly in Europe where increased population density and climate change are expected to place greater pressures on water supply. In this context, the North Atlantic Oscillation (NAO) ais often used to indicate future water resource behaviours (including droughts) over Europe, given its dominant control on winter rainfall totals in the North Atlantic region. Recent hydroclimate research has focused on the role of multiannual periodicities in the NAO in driving low frequency behaviours in some water resources, suggesting that notable improvements to lead-times in forecasting may be possible by incorporating these multiannual relationships. However, the importance of multiannual NAO periodicities for driving water resource behaviour, and the feasibility of this relationship for indicating future droughts, has yet to be assessed in the context of known non-stationarities that are internal to the NAO and its influence on European meteorological processes. Here we quantify the time-frequency relationship between the NAO and a large dataset of water resources records to identify key non-stationarities that have dominated multiannual behaviour of water resource extremes over recent decades. The most dominant of these is a 7.5-year periodicity in water resource extremes since approximately 1970 but which has been diminishing since 2005. Furthermore, we show that the non-stationary relationship between the NAO and European rainfall is clearly expressed at multiannual periodicities in the water resource records assessed. These multiannual behaviours are found to have modulated historical water resource anomalies to an extent that is comparable to the projected effects of a worst-case climate change scenario. Furthermore, there is limited systematic understanding in existing atmospheric research for non-stationaries in these periodic behaviours which poses considerable implications to existing water resource forecasting and projection systems, as well as the use of these periodic behaviours as an indicator of future water resource drought.

Managing water–human systems during water shortages or droughts is key to avoid the overexploitation of water resources and, in particular, groundwater. Groundwater is a crucial water resource during droughts as it sustains both environmental and anthropogenic water demand. Drought management is often guided by drought policies, to avoid crisis management, and actively introduced management strategies. However, the impact of drought management strategies on hydrological droughts is rarely assessed. In this study, we present a newly developed socio-hydrological model, simulating the relation between water availability and managed water use over 3 decades. Thereby, we aim to assess the impact of drought policies on both baseflow and groundwater droughts. We tested this model in an idealised virtual catchment based on climate data, water resource management practices and drought policies in England. The model includes surface water storage (reservoir), groundwater storage for a range of hydrogeological conditions and optional imported surface water or groundwater. These modelled water sources can all be used to satisfy anthropogenic and environmental water demand. We tested the following four aspects of drought management strategies: (1) increased water supply, (2) restricted water demand, (3) conjunctive water use and (4) maintained environmental flow requirements by restricting groundwater abstractions. These four strategies were evaluated in separate and combined scenarios. Results show mitigated droughts for both baseflow and groundwater droughts in scenarios applying conjunctive use, particularly in systems with small groundwater storage. In systems with large groundwater storage, maintaining environmental flows reduces hydrological droughts most. Scenarios increasing water supply or restricting water demand have an opposing effect on hydrological droughts, although these scenarios are in balance when combined at the same time. Most combined scenarios reduce the severity and occurrence of hydrological droughts, given an incremental dependency on imported water that satisfies up to a third of the total anthropogenic water demand. The necessity for importing water shows the considerable pressure on water resources, and the delicate balance of water–human systems during droughts calls for short-term and long-term sustainability targets within drought policies.

Food webs in groundwater ecosystems are dominated by only a few top-level consumers, mainly crustaceans. These obligate groundwater dwellers—or stygobites—clearly interact with groundwater ...

Managing water-human systems in times of water shortage and droughts is key to avoid overexploitation of water resources, particularly for groundwater, which is a crucial water resource during droughts sustaining both environmental and anthropogenic water demand. Drought management is often guided by drought policies to avoid crisis management and to actively introduce management strategies during droughts. However, the impact of drought management strategies on hydrological droughts is rarely assessed. In this study, we present a newly developed socio-hydrological model, simulating feedbacks between water availability and managed water use over three 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 based on climate data, water resource management practices, and drought policies in England. The model includes surface water storage (reservoir), groundwater storage for a range of hydrogeological conditions and optional imported surface water or groundwater. These modelled water sources can all be used to satisfy anthropogenic and environmental water demand. We tested four aspects of drought management strategies: 1) increased water supply, 2) restricted water demand, 3) conjunctive water use, and 4) maintained environmental flow requirements by restricting groundwater abstractions. These four strategies were evaluated in separate and combined scenarios. Results show mitigated droughts for both streamflow and groundwater droughts in scenarios applying conjunctive use, particularly in low groundwater storage systems. In high groundwater storage systems, maintaining environmental flows reduces hydrological droughts most. Scenarios increasing or restricting water demand have an opposing effect on droughts, although these scenarios are in balance when combined at the same time. Most combined scenarios reduce the severity and occurrence of hydrological droughts given an incremental dependency on imported water that satisfies up to a third of the total anthropogenic water demand. The necessity for importing 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.

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.

Water resource forecasting generally centres on understanding hydrological variability over coming months or years, so that water managers can prepare for extremes such as droughts or floods (Chang & Guo, 2020; Hao et al., 2018). Some forecasting systems seek to project further into the future to allow long‐term planning of infrastructure and resilience to extremes and climate change (Svensson et al., 2015). These systems can rely directly or indirectly on outputs from Global Climate Models (GCMs; such as gridded reanalysis datasets) to forecast hydrological conditions (Bhatt & Mall, 2015; Ionita & Nagavciuc, 2020). In the North Atlantic region, in particular Western Europe, the North Atlantic Oscillation (NAO) is used as an indicator for hydrometeorological conditions given its leading control on winter rainfall totals (Hurrell & Deser, 2010; Scaife et al., 2008, 2014). A dipole of pressure anomalies over the North Atlantic, the NAO's positive phase (greater than average pressure gradient; NAO+) results in wetter conditions in northwest Europe with dryer conditions in southwest Europe (Rust et al., 2018; Trigo et al., 2004). Its negative phase (weaker than average pressure gradient; NAO−) results in the inverse effect on rainfall (Folland et al., 2015; and as shown by the correlation coefficients in Figure 1). Given this relationship, and, considering the role of winter rainfall variability in groundwater drought development (e.g., reduced winter recharge) and generation of late winter/early spring floods, the NAO offers a potential explanatory variable when understanding the behaviour of some hydrological extremes. image

There is a no lack of significant open questions in the field of hydrology. How will hydrological connectivity between freshwater bodies be altered by future human alterations to the hydrological cycle? Where does water go when it rains? Or what is the future space–time variability of flood and drought events? However, the answers to these questions will vary with location due to the specific and often poorly understood local boundary conditions and system properties that control the functional behaviour of a catchment or any other hydrologic control volume. We suggest that an open, shared and evolving perceptual model of a region's hydrology is critical to tailor our science questions, as it would be for any other study domain from the plot to the continental scale. In this opinion piece, we begin to discuss the elements of and point out some knowledge gaps in the perceptual model of the terrestrial water cycle of Great Britain. We discuss six major knowledge gaps and propose four key ways to reduce them. While the specific knowledge gaps in our perceptual model do not necessarily transfer to other places, we believe that the development of such perceptual models should be at the core of the debate for all hydrologic communities, and we encourage others to have a similar debate for their hydrologic domain.

An understanding of multi-annual behaviour in streamflow allows for better estimation of the risks associated with hydrological extremes. This is can enable improved preparedness for streamflow-dependant services such as freshwater ecology, drinking water supply and agriculture. Recently, efforts have focused on detecting relationships between long-term hydrological behaviour and oscillatory climate systems (such as the NAO). For instance, the approximate 7-year periodicity of the NAO has been detected in groundwater level records in the North Atlantic region, providing a degree of forecasting for future water resource extremes due to their repeating, periodic nature. However, the extent to which these 7-year NAO-like signals are propagated to streamflow, and the catchment processes that modulate this propagation, are currently unknown. Here, we show statistically significant evidence that these 7-year periodicities are present in streamflow (and associated catchment rainfall), by applying multi-resolution analysis to a large dataset of streamflow and associated catchment rainfall across the UK. Our results provide new evidence for spatial patterns of NAO periodicities in UK rainfall with areas of greatest NAO signal found in south west England, South Wales, Northern Ireland and central Scotland, and that NAO-like periodicities account for a greater proportion of streamflow variability in these areas. Furthermore, we show that subsurface pathway contribution, as characterised by the Baseflow Index (BFI), and the response times of subsurface pathways, as characterised by Groundwater response Time (GRT), are influential factors for streamflow sensitivity to these NAO-like cycles. Our results provide critical process understanding for the screening and use of streamflow teleconnections for the improving the practice and policy of long-term streamflow resource management.

Study region: The study region is Great Britain (GB), a small non-continental island landmass in North West Europe Study focus: Data for Total Dissolved Solids (TDS) from groundwater samples can be used to characterise regional-scale variations in the quality of deep groundwater systems. Combined with information about typical well-depths, TDS data can be used to identify the presence of currently undeveloped fresh or brackish groundwater at depth that may require protection. This study considers the distribution of TDS with depth relative to sea level in the main GB aquifers and selected other key hydrogeological units, and demonstrates how useful insights can be obtained from data-led analyses of depth variations in groundwater chemistry if the regional context of hydrogeological systems is taken into account. New hydrogeological insights: In GB, TDS varies over about five orders of magnitude, up to about 330,000 mg/L, with a general increase in mineralisation with depth. Overall, there is a transition from fresh 10,000 mg/L groundwater at about 700 m. Given that the 95 %tile depth of water wells is about 200 m, it is evident that there is currently undeveloped fresh groundwater at depth across large parts of the study area that may require protection, although it is inferred that TDS is not the only factor limiting exploitation and use of these deeper resources. As in this study, previous data-led analyses of fresh groundwater at depth have typically analysed TDS as depth below surface. However, if TDS data is analysed relative to sea level and in the context of regional hydrogeological information or models, additional insights can be gained on the distribution and controls on fresh groundwater at depth. Projecting TDS data into a 3D hydrogeological model of the study area shows that fresh groundwater at depth exhibits spatial coherence and is generally associated with relatively deep sedimentary basins overlying older, less permeable basement. Keywords: Groundwater, Total dissolved solids (TDS), GB

Groundwater systems are susceptible to droughts. However, the relationship between driving meteorological droughts and resulting groundwater droughts is particularly complex due to spatially and temporally varying climate drivers and spatially heterogeneous catchment and aquifer characteristics, as well as the potential effects of longer-term groundwater overexploitation and of groundwater abstraction and management interventions during episodes of meteorological drought. Consequently, many previous studies of the propagation of meteorological drought to groundwater systems have typically been geographically limited in scope, focussing on characterising aquifer units within a catchment or basin, and/or temporally limited to specific episodes of drought. Based on a new European-wide dataset consisting of groundwater level data from over 6,000 sites, here we describe the results of a multidecadal analysis of the expression of major episodes of meteorological drought at the continental scale in groundwater systems, independent of local hydrogeological setting.In this study, raw groundwater level time series are modelled using an impulse response function of precipitation to obtain monthly groundwater levels that are then standardised. Sites with long-term trends in groundwater level are identified and usually inferred to be associated with overexploitation or other anthropogenic influences. Cluster analysis of the modelled standardised hydrographs is used to identify spatially coherent ‘type’ groundwater hydrographs. These type hydrographs can be characterised by differences in the autocorrelation of the underlying groundwater hydrographs, but may also reflective continental-scale variations in the driving meteorology. Finally, episodes of groundwater drought are extracted from the type groundwater hydrographs and compared with the driving meteorological droughts. The data provides evidence for the coherent response of groundwater systems to droughts across large areas of Europe depending on driving meteorology and the ‘memory’ of the groundwater system, and drought events such as in 2011-12, 2015 and 2017-18 showed spatial coherence across different European regions.

River flows are the result of dynamically changing, interacting and non-linear processes of surface, near subsurface and often deeper groundwater flow from climatic drivers. Conceptual rainfall-runoff models, whilst providing advantages in computational efficiency and more minimal data requirements, often struggle to simulate contributions from groundwaters, resulting in poor model calibration. Improving predictions of river flows in these catchments is, however, critical to water resources planning and management, particularly in the UK where groundwater contributes 30% of public water supply in England. In order to improve model predictions in groundwater-dominated catchments, we conduct a detailed analysis of available observational data to better understand groundwater-surface water interactions and processes on a regional (aquifer) and local (river reach) scale, over geologically variable areas.National meteorological, hydrological, hydrogeological, geological and artificial influence (characterising abstractions and return flows) datasets are used to develop a conceptualisation of the groundwater processes occurring in 99 subcatchments of the River Thames in the UK. We use these data to characterise the water balance, intercatchment groundwater flows, gaining/losing river reaches and hydrograph dynamics of these subcatchments, and investigate how dominant groundwater processes vary spatially and temporally. The River Thames has been selected as our case study owing to its wealth of data, densely gauged river network and geological variability.We show that intercatchment groundwater flow is needed to ‘close’ the water balance in many catchments located on aquifer outcrops and find evidence of river-groundwater level flow thresholds. Importantly, we find that seasonality is a key control on the accurate representation of groundwater-surface water interaction processes and that the spatial and temporal variability of those processes varies greatly for different geologies across the Thames basin. We also demonstrate the importance of human influences to understand some of these spatial processes. We then identify the physical processes that existing conceptual rainfall-runoff models are likely missing, and what may be required to enable model calibration improvements in groundwater-dominated catchments.

Over the last decade or so many studies of the hydrologic characteristics of basins have been driven by the desire to develop models that enable prediction of particular signatures, such as baseflow and Base Flow Index (BFI), in ungauged basins (PUB). These studies typically focus on understanding how readily available mapped or remotely sensed data can be used to infer hydrologic signals. However, in the specific case of baseflow, there is a recognition that we still have a poor understanding of the relative influences of underlying hydrological processes at appropriate scales, particularly in anthropogenically impacted catchments. New opportunities are being offered to better understand relationships between BFI and various controls on baseflow through the production of large sample catchment datasets. Here we present the results of an analysis of one such large-sample dataset, CAMELS-GB, investigating the relative importance of different hydrogeological controls on baseflow, including factors such as: climatology; hydrogeology; geophysical catchment characteristics, e.g. soil characteristics and land cover; and, anthropogenic influences, e.g. discharge from reservoirs and from sewage treatment works (STWs), abstraction, and mains leakage.CAMELS-GB consists of daily hydrometerorological time series for the period 1970-2015 and landscape, catchment and hydrogeological attributes for 671 catchments in Great Britain. Machine learning approaches, including random forest algorithms, are used to investigate the influence of catchment characteristics on BFI and to inform the selection of hydrologically reasonable parameters to quantify relationships using linear regression models. We describe how the regression models can be used to investigate and characterise the sensitivity of estimates of BFI to: i.) the underlying hydrogeological mapping; ii.) the spatial support scale of the analysis; and iii.) anthropogenic influences.

It is widely acknowledged that waterbodies are becoming increasingly affected by a wide range of drivers of change arising from human activity. To illustrate how this can be quantified a linked modelling approach was applied in the Thames river basin in southern UK. Changes to river flows, water temperature, river and reservoir quality were predicted under three contrasting future “storylines”; one an extension of present day rates of economic development, the others representing more extreme and less sustainable visions. Modelling revealed that lower baseflow conditions will arise under all storylines. For the less extreme storyline river water quality is likely to deteriorate but reservoir quality will improve slightly. The two more extreme futures could not be supported by current management strategies to meet water demand. To satisfy these scenarios, transfer of river water from outside the Thames river basin would be necessary. Consequently, some improvement over present day water quality in the river may be seen, and for most indicators conditions would be better than in the less extreme storyline. However, because phosphorus concentrations will rise, the invoked changes in water demand management would not be of a form suitable to prevent a marked deterioration in reservoir water quality.

The ability to predict future variability of groundwater resources in time and space is of critical importance to drought management. Periodic control on groundwater levels from oscillatory climatic systems (such as the North Atlantic Oscillation) offers a potentially valuable source of longer term forecasting capability. While some studies have found evidence of the influence of such climatic oscillations within groundwater records, there is little information on how periodic signals propagate between a climatic system and a groundwater resource. This paper develops a conceptual model of this relationship for groundwater resources in the UK and Europe, based on a review of current research. The studies reviewed here reveal key spatial and temporal signal modulations between climatic oscillations, precipitation, groundwater recharge and groundwater discharge. Generally positive correlations are found between the NAO (as a dominant influence) and precipitation in northern Europe indicating a strong control on water available for groundwater recharge. These periodic signals in precipitation are transformed by the unsaturated and saturated zones, such that signals are damped and lagged. This modulation has been identified to varying degrees, and is dependent on the shape, storage and transmissivity of an aquifer system. This goes part way towards explaining the differences in periodic signal strength found across many groundwater systems in current research. So that an understanding of these relationships can be used by water managers in building resilience to drought, several research gaps have been identified. Among these are improved quantification of spatial groundwater sensitivity to periodic control, and better identification of the hydrogeological controls on signal lagging and damping. Principally, research needs to move towards developing improved predictive capability for the use of periodic climate oscillations as indicators of longer term groundwater variability.

Groundwater ecosystems comprising micro-organisms and metazoans provide an important contribution to global biodiversity. Their complexity depends on geology, which determines the physical habitat available, and the chemical conditions within it. Despite this, methods of classifying groundwater habitats using geological data are not well established, and researchers have called for higher resolution habitat frameworks. A novel habitat typology for England and Wales (UK) is proposed, which distinguishes 11 geological habitats (geo-habitats) on hydrogeological principles and maps their distribution. Hydrogeological and hydrochemical data are used to determine the characteristics of each geo-habitat, and demonstrate their differences. Using these abiotic parameters, a new method to determine abiotic habitat quality is then developed. The geo-habitats had significantly different characteristics validating the classification system. Karstic and porous habitats were generally of higher quality than fractured habitats. All geo-habitats were highly heterogeneous, containing both high quality habitat patches that are likely to be suitable for fauna, and areas of low quality that may limit faunal distributions. Overall, 70 % of England and Wales are covered by lower quality fractured habitats, with only 13 % covered by higher quality habitats. The main areas of high quality habitats occur in central England as north-south trending belts, possibly facilitating dispersal along this axis. They are separated by low quality geo-habitats that may prevent east-west dispersal of fauna. In south-west England and in Wales suitable geo-habitats occur as small isolated patches. Overall, this paper provides a new national-scale typology that is adaptable for studies in other geographic areas.

Understanding the impact of climate change on borehole yields from fractured aquifers is essential for future water resources planning and management. Although variation in hydraulic conductivity with depth (VKD) in fractured aquifers is a well-known phenomenon, the relative significance of climate change and VKD on borehole yield estimates is poorly understood. We hypothesize that VKD exerts a significant additional control on borehole yields under climate change that has not been considered in yield assessments to date. We developed a simple two-layered radial groundwater flow model of an idealised pumping borehole in the fractured Chalk aquifer of south-east England, and applied 11 VKD profiles based on a simple conceptual representation of variability in hydraulic conductivity with depth in the Chalk. For each time step, the transmissivity is calculated by integrating the hydraulic conductivity VKD profile over the saturated depth calculated at the previous time step. For each VKD and resulting transmissivity, we applied 20 climate scenarios and six constant pumping rates for the period 1962 – 2014. We then estimated borehole yields based on the derived lowest pumping water levels during key drought years (e.g. 1976). We show that the hydraulic properties of the aquifer are more significant (p < 0.001) than changes in climate (p > 0.1) in controlling lowest pumping groundwater levels when abstraction rates are < 9000 m3/day, and that both are significant when abstraction ≥ 9000 m3/day. Hydraulic conductivity is as significant a control as climate on borehole yields, although responses are non-linear associated with whether pumping water level-pumping rate curves intersect key yield constraints (e.g. pump intake depth, major inflow horizons). It is recommended that variations in hydraulic conductivity with depth are taken into consideration in future assessments of borehole yields under climate change. The approach presented is generic and can be applied across different aquifers where vertical heterogeneity is present and affects transmissivity.

Predicting the next major drought is of paramount interest to water managers globally. Estimating the onset of groundwater drought is of particular importance, as groundwater resources are often assumed to be more resilient when surface water resources begin to fail. A potential source of long-term forecasting is offered by possible periodic controls on groundwater level via teleconnections with oscillatory ocean–atmosphere systems. However, relationships between large-scale climate systems and regional to local-scale rainfall, evapotranspiration (ET) and groundwater are often complex and non-linear so that the influence of long-term climate cycles on groundwater drought remains poorly understood. Furthermore, it is currently unknown whether the absolute contribution of multi-annual climate variability to total groundwater storage is significant. This study assesses the extent to which multi-annual variability in groundwater can be used to indicate the timing of groundwater droughts in the UK. Continuous wavelet transforms show how repeating teleconnection-driven 7-year and 16–32-year cycles in the majority of groundwater sites from all the UK's major aquifers can systematically control the recurrence of groundwater drought; and we provide evidence that these periodic modes are driven by teleconnections. Wavelet reconstructions demonstrate that multi-annual periodicities of the North Atlantic Oscillation, known to drive North Atlantic meteorology, comprise up to 40 % of the total groundwater storage variability. Furthermore, the majority of UK recorded droughts in recent history coincide with a minimum phase in the 7-year NAO-driven cycles in groundwater level, providing insight into drought occurrences on a multi-annual timescale. Long-range groundwater drought forecasts via climate teleconnections present transformational opportunities to drought prediction and its management across the North Atlantic region.

Benchmarking model performance across large samples of catchments is useful to guide future model development. Given uncertainties in the observational data we use to drive and evaluate hydrological models, and uncertainties in the structure and parameterisation of models we use to produce hydrological simulations and predictions, it is essential that model evaluation is undertaken within an uncertainty analysis framework. Here, we benchmark the capability of multiple, lumped hydrological models across Great Britain, by focusing on daily flow and peak flow simulation. Four hydrological model structures from the Framework for Understanding Structural Errors (FUSE) were applied to over 1100 catchments. Model performance was then evaluated using a standard performance metric for daily flows, and more novel performance metrics for peak flows considering parameter uncertainty. Our results show that simple, lumped hydrological models were able to produce adequate simulations across most of Great Britain, with median Nash–Sutcliffe efficiency scores of 0.72–0.78 across all catchments. All four models showed a similar spatial pattern of performance, producing better simulations in the wetter catchments to the west, and poor model performance in Scotland and southeast England. Poor model performance was often linked to the catchment water balance, with models unable to capture the catchment hydrology where the water balance did not close. Overall, performance was similar between model structures, but different models performed better for different catchment characteristics and for assessing daily or peak flows, demonstrating the value of using an ensemble of model structures. This research demonstrates what conceptual lumped models can achieve as a performance benchmark, as well as providing interesting insights into where and why these simple models may fail. The large number of river catchments included in this study makes it an appropriate benchmark for any future developments of a national model of Great Britain.

Here we present the first empirical evidence for changes in groundwater drought associated with anthropogenic warming in the absence of long-term changes in precipitation. Analysing standardised indices of monthly groundwater levels, precipitation and temperature, using two unique groundwater level data sets from the Chalk aquifer, UK, for the period 1891 to 2015, we show that precipitation deficits are the main control on groundwater drought formation and propagation. However, long-term changes in groundwater drought are shown to be associated with anthropogenic warming over the study period. These include increases in the frequency and intensity of individual groundwater drought months, and increases in the frequency, magnitude and intensity of episodes of groundwater drought, as well as an increasing tendency for both longer episodes of groundwater drought and for an increase in droughts of less than 1 year in duration. We also identify a transition from a coincidence of episodes of groundwater drought with precipitation droughts at the end of the 19th century, to an increasing coincidence with both precipitation droughts and with hot periods in the early 21st century. In the absence of long-term changes in precipitation deficits, we infer that the changing nature of groundwater droughts is due to changes in evapotranspiration (ET) associated with anthropogenic warming. We note that although the water tables are relatively deep at the two study sites, a thick capillary fringe of at least 30 m in the Chalk means that ET should not be limited by precipitation at either site. ET may be supported by groundwater through major episodes of groundwater drought and, hence, long-term changes in ET associated with anthropogenic warming may drive long-term changes in groundwater drought phenomena in the Chalk aquifer. Given the extent of shallow groundwater globally, anthropogenic warming may widely effect changes to groundwater drought characteristics in temperate environments.

Multi-decadal groundwater level records, which provide information about long-term variability and trends, are relatively rare. Whilst a number of studies have sought to reconstruct river flow records, there have been few attempts to reconstruct groundwater level time-series over a number of decades. Using long rainfall and temperature records, we developed and applied a methodology to do this using a lumped conceptual model. We applied the model to six sites in the UK, in four different aquifers: Chalk, limestone, sandstone and Greensand. Acceptable models of observed monthly groundwater levels were generated at four of the sites, with maximum Nash–Sutcliffe Efficiency scores of between 0.84 and 0.93 over the calibration and evaluation periods, respectively. These four models were then used to reconstruct the monthly groundwater level time-series over approximately 60 years back to 1910. Uncertainty in the simulated levels associated with model parameters was assessed using the Generalized Likelihood Uncertainty Estimation method. Known historical droughts and wet period in the UK are clearly identifiable in the reconstructed levels, which were compared using the Standardized Groundwater Level Index. Such reconstructed records provide additional information with which to improve estimates of the frequency, severity and duration of groundwater level extremes and their spatial coherence, which for example is important for the assessment of the yield of boreholes during drought periods

Here we present the first empirical evidence for changes in groundwater droughts associated with anthropogenic warming in the absence of significant long-term trends in precipitation. Analysing standardised indices of monthly groundwater levels, precipitation and temperature, using two unique groundwater level data sets from the Chalk aquifer, UK for the period 1891 to 2015, we describe an increase in both the frequency and intensity of groundwater drought months, and an increase in frequency, duration and intensity of episodes of groundwater drought since 1891 associated with anthropogenic warming. We also identify a transition from coincidence of episodes of groundwater drought with precipitation droughts at the end of the 19th century, to an increasing coincidence with both precipitation droughts and with hot periods in the early 21st century. In the absence either of long-term changes in precipitation deficits during episodes of groundwater drought or long-term changes in the occurrence and intensity of consecutive dry winters, we infer that the changing nature of groundwater droughts is due to changes in evapotranspiration (ET) associated with anthropogenic warming. We note that although the water tables are relatively deep at the two study sites, a thick capillary fringe of at least 30 m in the Chalk means that ET should not be limited by precipitation at either site; that ET may be supported by groundwater through major episodes of groundwater drought; and, hence, long-term changes in ET associated with anthropogenic warming may drive long-term changes in groundwater drought phenomena in the Chalk aquifer. Given the extent of shallow groundwater globally, this phenomenon may be widespread in temperate environments.

An empirical (geo)statistical modelling scheme is developed to address the challenges of modelling the severity and distribution of groundwater droughts given their spatially and temporally heterogeneous nature and given typically highly irregular groundwater level observations in space and time. The scheme is tested using GWL measurements from 948 observation boreholes across the Chalk aquifer (UK) to estimate monthly groundwater drought status from 1960 to 2013. For each borehole, monthly GWLs are simulated using empirical mixed models where the fixed effects are based on applying an impulse response function to the local monthly precipitation. Modelled GWLs are standardised using the Standardised Groundwater Index (SGI) and the monthly SGI values interpolated across the aquifer to produce spatially distributed monthly maps of SGI drought status for 54 years for the Chalk. The mixed models include fewer parameters than comparable lumped parameter groundwater models while explaining a similar proportion (more than 65%) of GWL variation. In addition, the empirical modelling approach enables confidence bounds on the predicted GWLs and SGI values to be estimated without the need for prior information about catchment or aquifer parameters. The results of the modelling scheme are illustrated for three major episodes of multi-annual drought (1975–1976; 1988–1992; 2011–2012). They agree with previous documented analyses of the groundwater droughts while providing for the first time a systematic, spatially coherent characterisation of the events. The scheme is amenable to use in near real time to provide short term forecasts of groundwater drought status if suitable forecasts of the driving meteorology are available.

There have been many published studies aiming to identify temporal changes in river flow time-series, most of which use monotonic trend tests such as the Mann–Kendall test. Although robust to both the distribution of the data and incomplete records, these tests have important limitations and provide no information as to whether a change in variability mirrors a change in magnitude. This study develops a new method for detecting periods of change in a river flow time-series using Temporally Shifting Variograms, TSV, based on applying variograms to moving windows in a time-series and comparing these to the long-term average variogram, which characterises the temporal dependence structure in the river flow time-series. Variogram properties in each moving window can also be related to potential meteorological drivers. The method is applied to 94 UK catchments which were chosen to have minimal anthropogenic influences and good quality data between 1980 and 2012 inclusive. Each of the four variogram parameters (Range, Sill and two measures of semi-variance) characterise different aspects of change in the river flow regime, and have a different relationship with the precipitation characteristics. Three variogram parameters (the Sill and the two measures of semi-variance) are related to variability (either day-to-day or over the time-series) and have the largest correlations with indicators describing the magnitude and variability of precipitation. The fourth (the Range) is dependent on the relationship between the river flow on successive days and is most correlated with the length of wet and dry periods. Two prominent periods of change were identified: 1995 to 2001 and 2004 to 2012. The first period of change is attributed to an increase in the magnitude of rainfall whilst the second period is attributed to an increase in variability in the rainfall. The study demonstrates that variograms have considerable potential for application in the detection and attribution of temporal variability and change in hydrological systems.

The English Lowlands is a relatively dry, densely populated region in the south-east of the UK in which water is used intensively. Consequently, parts of the region are water-stressed and face growing water resource pressures. The region is heavily dependent on groundwater and particularly vulnerable to long, multi-annual droughts primarily associated with dry winters. Despite this vulnerability, the atmospheric drivers of multi-annual droughts in the region are poorly understood, an obstacle to developing appropriate drought management strategies, including monitoring and early warning systems. To advance our understanding, we assess known key climate drivers in the winter half-year (October–March) and their likely relationships with multi-annual droughts in the region. We characterise historic multi-annual drought episodes back to 1910 for the English Lowlands using various meteorological and hydrological data sets. Multi-annual droughts are identified using a gridded precipitation series for the entire region, and refined using the Standardized Precipitation Index (SPI), Standardized Streamflow Index (SSI) and Standardized Groundwater level Index (SGI) applied to regional-scale river flow and groundwater time series. We explore linkages between a range of potential climatic driving factors and precipitation, river flow and groundwater level indicators in the English Lowlands for the winter half-year. The drivers or forcings include El Niño–Southern Oscillation (ENSO), the North Atlantic tripole sea surface temperature (SST) pattern, the Quasi-Biennial Oscillation (QBO), solar and volcanic forcing and the Atlantic Multi-decadal Oscillation (AMO). As expected, no single driver convincingly explains the occurrence of any multi-annual drought in the historical record. However, we demonstrate, for the first time, an association between La Niña episodes and winter rainfall deficits in some major multi-annual drought episodes in the English Lowlands. We also show significant (albeit relatively weak) links between ENSO and drought indicators applied to river flow and groundwater levels. We also show that some of the other drivers listed above are likely to influence English Lowlands rainfall. We conclude by signposting a direction for this future research effort.

We examine the evidence for climate-change impacts on groundwater levels provided by studies of the historical observational record, and future climate-change impact modelling. To date no evidence has been found for systematic changes in groundwater drought frequency or intensity in the UK, but some evidence of multi-annual to decadal coherence of groundwater levels and large-scale climate indices has been found, which should be considered when trying to identify any trends. We analyse trends in long groundwater level time-series monitored in seven observation boreholes in the Chalk aquifer, and identify statistically significant declines at four of these sites, but do not attempt to attribute these to a change in a stimulus. The evidence for the impacts of future climate change on UK groundwater recharge and levels is limited. The number of studies that have been undertaken is small and different approaches have been adopted to quantify impacts. Furthermore, these studies have generally focused on relatively small regions and reported local findings. Consequently, it has been difficult to compare them between locations. We undertake some additional analysis of the probabilistic outputs of the one recent impact study that has produced coherent multi-site projections of changes in groundwater levels. These results suggest reductions in annual and average summer levels, and increases in average winter levels, by the 2050s under a high greenhouse gas emissions scenario, at most of the sites modelled, when expressed by the median of the ensemble of simulations. It is concluded, however, that local hydrogeological conditions can be an important control on the simulated response to a future climate projection.

This paper describes the development of the first operational seasonal hydrological forecasting service for the UK, the Hydrological Outlook UK (HOUK). Since June 2013, this service has delivered monthly forecasts of streamflow and groundwater levels, with an emphasis on forecasting hydrological conditions over the next three months, accompanied by outlooks over longer time horizons. This system is based on three complementary approaches combined to produce the outlooks: (i) national-scale modelling of streamflow and groundwater levels based on dynamic seasonal rainfall forecasts, (ii) catchment-scale modelling where streamflow and groundwater level models are driven by historical meteorological forcings (i.e. the Ensemble Streamflow Prediction, ESP, approach), and (iii) a catchment-scale statistical method based on persistence and historical analogues. This paper provides the background to the Hydrological Outlook, describes the various component methods in detail and then considers the impact and usefulness of the product. As an example of a multi-method, operational seasonal hydrological forecasting system, it is hoped that this overview provides useful information and context for other forecasting initiatives around the world.

Shale gas is considered by many to have the potential to provide the UK with greater energy security, economic growth and jobs. However, development of a shale gas industry is highly contentious due to environmental concerns including the risk of groundwater pollution. Evidence suggests that the vertical separation between exploited shale units and aquifers is an important factor in the risk to groundwater from shale gas exploitation. A methodology is presented to assess the vertical separation between different pairs of aquifers and shales that are present across England and Wales. The application of the method is then demonstrated for two of these pairs-the Cretaceous Chalk Group aquifer and the Upper Jurassic Kimmeridge Clay Formation, and the Triassic sandstone aquifer and the Carboniferous Bowland Shale Formation. Challenges in defining what might be considered criteria for 'safe separation' between a shale gas formation and an overlying aquifer are discussed, in particular with respect to uncertainties in geological properties, aquifer extents and determination of socially acceptable risk levels. Modelled vertical separations suggest that the risk of aquifer contamination from shale exploration will vary greatly between shale-aquifer pairs and between regions and this will need to be considered carefully as part of the risk assessment and management for any shale gas development.Le gaz de schiste est considéré par beaucoup comme ayant le potentiel de procurer au Royaume Uni une plus grande sécurité énergétique, une croissance économique et des emplois. Cependant, le développement de l’industrie du gaz de schiste est fortement contesté en raison des problèmes environnementaux, qui comprennent le risque de pollution des eaux souterraines. Des données probantes suggèrent que l’isolation verticale entre les unités de schistes exploitées et les aquifères est un facteur important vis-à-vis du risque de contamination des eaux souterraines associé à l’exploitation du gaz de schiste. Une méthodologie est proposée pour évaluer la séparation verticale entre différents couples aquifères—schistes présents en Angleterre et Pays de Galle. L’application de la méthode est ensuite décrite pour deux de ces couples, d’une part l’aquifère du Groupe de la Craie du Crétacé et la formation des Argiles du Kimméridgien (Jurassique supérieur), et l’aquifère des Grés du Trias et la formation carbonifère des schistes du Bowland. Les enjeux de la définition de ce qui pourrait être considéré comme critère d’une « isolation sécuritaire » entre une formation à gaz de schiste et l’aquifère sus-jacent sont discutées, en particulier au regard des incertitudes sur les propriétés géologiques, l’extension des aquifères et la définition des niveaux de risques socialement acceptables. Les isolations verticales modélisées suggèrent que le risque de contamination d’un aquifère lié à une exploration du schiste variera grandement d’un couple schiste–aquifère à l’autre et d’une région à l’autre et que cela devra être examiné avec soin dans le cadre de l’évaluation du risque et de la gestion de tout développement du gaz de schiste.Muchos consideran que el shale gas tiene el potencial de proporcionar al Reino Unido una mayor seguridad energética, crecimiento económico y empleos. Sin embargo, el desarrollo de una industria del shale gas es muy polémico debido a las preocupaciones ambientales, incluido el riesgo de contaminación del agua subterránea. La evidencia sugiere que la separación vertical entre las unidades de shale explotadas y los acuíferos es un factor importante en el riesgo para las aguas subterráneas en la explotación del shale. Se presenta una metodología para evaluar la separación vertical entre los diferentes pares de acuíferos y los shales que están presentes en Inglaterra y Gales. La aplicación del método se demuestra para dos de estos pares—el acuífero Chalk Group del Cretácico y la Kimmeridge Clay Formation del Jurásico Superior, y el acuífero de arenisca del Triásico y la Bowland Shale Formation del Carbonífero. Se discuten los desafíos para definir los que podrían considerarse criterios para una “separación segura” entre una formación de shale gas y un acuífero superpuesto, en particular con respecto a incertidumbres en las propiedades geológicas, extensiones de acuíferos y la determinación de niveles de riesgo socialmente aceptables. Las separaciones verticales modeladas sugieren que el riesgo de contaminación del acuífero por la exploración del shale variará mucho entre los pares de shale–acuífero y entre regiones, y esto deberá considerarse cuidadosamente como parte de la evaluación y gestión de riesgos para cualquier desarrollo de shale gas.许多人认为页岩气具有给英国提供较大能源安全、经济增长和工作的潜力。然而,由于环境方面的关切包括地下水污染的风险,页岩气行业的发展争议很大。证据表明,开采过的页岩单元和含水层之间的垂直分离是页岩气开采对地下水产生的风险中一个重要的因素。这里论述了评价了英格兰和威尔士现有的不同对含水层和页岩之间垂直分离的方法。然后通过展示了其中的两对—白垩纪白垩组含水层和上侏罗纪Kimmeridge粘土层、三叠纪砂岩含水层和石炭纪Bowland页岩层介绍了该方法的应用情况。论述了在定义什么可能是页岩气层和上覆地层之间“安全分离”的标准中的挑战,特别是有关地质特性的不确定性、含水层范围和社会接受风险水平的确定等。模拟的垂直分离表明,由于页岩勘探导致的含水层污染风险每对之间、不同地区之间差异很大,在任何页岩气开发中这需要作为风险评价和管理的一部分认真给予考虑。.O gás de xisto é considerado um potencial fornecedor de maior segurança energética, crescimento econômico e empregos ao Reino Unido. Porém, o desenvolvimento de uma indústria de gás de xisto é altamente controverso devido a preocupações ambientais, como o risco de poluição das águas subterrâneas. Evidências sugerem que a separação vertical entre unidades de xisto exploradas e aquíferos é um fator importante de risco para as águas subterrâneas na exploração de gás de xisto. Uma metodologia é apresentada para avaliar a separação vertical entre diferentes pares de aquíferos e blocos de xisto presentes em toda a Inglaterra e País de Gales. A aplicação do método é feita em dois pares—o Aquífero do Grupo Cretáceo Chalk com a Formação Argilosa do Jurássico Superior Kimmeridge, e o Aquífero de Arenito Triássico com a Formação Xistosa do Carbonífero Bowland. Os desafios para definir o que pode ser considerado critérios de “separação segura” entre uma formação de gás de xisto e um aquífero sobrejacente são discutidos, em particular no que diz respeito a incertezas em propriedades geológicas, extensões de aquíferos e determinação de níveis de risco socialmente aceitáveis. As separações verticais modeladas sugerem que o risco de contaminação do aquífero por exploração de xisto variará muito entre os pares de xisto–aquífero e suas regiões, devendo assim serem consideradas cuidadosamente como parte da avaliação e gerenciamento de risco para qualquer desenvolvimento de gás de xisto.

In order to determine objectively the fractal behaviour of a time series, and to facilitate potential future attempts to assess model performance by incorporating fractal behaviour, a multi-order robust detrended fluctuation analysis (r-DFAn) procedure is developed herein. The r-DFAn procedure allows for robust and automated quantification of mono-fractal behaviour. The fractal behaviour is quantified with three parts: a global scaling exponent, crossovers, and local scaling exponents. The robustness of the r-DFAn procedure is established by the systematic use of robust regression, piecewise linear regression, Analysis of Covariance (ANCOVA) and Multiple Comparison Procedure to determine statistically significant scaling exponents and optimum crossover locations. The MATLAB code implementing the r-DFAn procedure has also been open sourced to enable reproducible results. r-DFAn will be illustrated on a synthetic signal after which is used to analyse high-resolution hydrologic data; although the r-DFAn procedure is not limited to hydrological or geophysical time series. The hydrological data are 4 year-long datasets (January 2012 to January 2016) of 1-min groundwater level, river stage, groundwater and river temperature, and 15-min precipitation and air temperature, at Wallingford, UK. The datasets are analysed in both time and fractal domains. The study area is a shallow riparian aquifer in hydraulic connection to River Thames, which traverses the site. The unusually high resolution datasets, along with the responsive nature of the aquifer, enable detailed examination of the various data and their interconnections in both time- and fractal-domains.

Controls on the spatiotemporal extent of groundwater flooding are poorly understood, despite the long duration of groundwater flood events and distinct social and economic impacts. We developed a novel approach using statistical analysis of groundwater level hydrographs and impulse response functions (IRFs) and applied it to the 2013/14 Chalk groundwater flooding in the English Lowlands. We proposed a standardised index of groundwater flooding which we calculated for monthly groundwater levels for 26 boreholes in the Chalk. We grouped these standardised series using k-means cluster analysis and cross-correlated the cluster centroids with the Standardised Precipitation Index (SPI) accumulated over time intervals between 1 and 60 months. This analysis reveals two spatially coherent groups of standardised hydrographs which responded to precipitation over different timescales. We estimated IRF models of the groundwater level response to effective precipitation for three boreholes in each group. The IRF models corroborate the SPI analysis showing different response functions between the groups. We applied identical effective precipitation inputs to each of the IRF models and observed differences between the hydrographs from each group. It is suggested this is due to the hydrogeological properties of the Chalk and of overlying relatively low permeability superficial deposits (recent unconsolidated sediments overlying the bedrock, such as clays and tills), which are extensive over one of the groups. The overarching controls on groundwater flood response are concluded to be a complex combination of antecedent conditions, rainfall and catchment hydrogeological properties. These controls should be taken into consideration when anticipating and managing future groundwater flood events. The approach presented is generic and parsimonious and can be easily applied where sufficient groundwater level and rainfall data are available.

Controls on the basin-scale distribution of hydraulic conductivity of superficial deposits are assessed in the context of hydrological setting and basin evolution and are investigated using a case study from the Thames Basin, UK. A conceptual model of superficial deposits across the Thames Basin is used to define six lithostratigraphic classes of superficial deposits: pre-Anglian Clay-with-Flint deposits; pre-Anglian River Terrace Deposits associated with the ancestral River Thames and its tributaries; Tills formed during the Anglian glaciations; glacio-fluvial sand and gravel deposits formed during the Anglian; post-Anglian River Terrace Deposits associated with the modern-day River Thames and tributaries; post-Anglian alluvium associated with the modern-day River Thames and tributaries. Hydraulic conductivity of the superficial deposits has been estimated from grain-size distribution data, originally collected for mineral resource assessments, using the Kozeny–Carman method. Based on 6411 samples from 1416 boreholes, estimated hydraulic conductivity ranges from 0.2 to 5942 m day –1 , median and mean hydraulic conductivities are 1.67 and 26.72 m day –1 respectively, and the overall distribution of hydraulic conductivity values has a strong positive skew An apparent reduction in mean hydraulic conductivity with increasing age of the deposit is observed, particularly for the River Terrace Deposits. A reduction in maximum hydraulic conductivity at depths >10 m is also observed and the relationship between hydraulic conductivity and depth is controlled by the type of superficial deposit. At the catchment to basin scale, variation in hydraulic conductivity with depth may be explained with reference to both the deposit types and the age of the deposits. Where hydraulic conductivity is found to be intimately linked to the Quaternary evolution of the basin, through contrasts in age and deposit type, permeability variations at the basin scale may be constrained by applying a suitably refined conceptual model of the superficial deposits.

This article presents an innovative framework for analysing environmental governance challenges by focusing on their Drivers, Responses and Impacts (DRI). It builds on and modifies the widely applied Drivers, Pressures, States, Impacts and Responses (DPSIR) model. It suggests, firstly and most importantly, that the various temporal and spatial scales at which Drivers, Responses and Impacts operate should be included in the DRI conceptual framework. Secondly, the framework focuses on Drivers, Impacts and Responses in order to provide a parsimonious account of a drought system that can be informed by a range of social science, humanities and science data. 'Pressures' are therefore considered as a sub-category of 'Drivers'. 'States' are a sub-category of 'Impacts'. Thirdly, and most fundamentally in order to facilitate cross-disciplinary research of droughts, the DRI framework defines each of its elements, 'Drivers', 'Pressures', 'States', 'Impacts' and 'Responses' as capable of being shaped by both linked natural and social factors. This is different from existing DPSIR models which often see 'Responses' and 'Impacts' as located mainly in the social world, while 'States' are considered to be states within the natural environment only. The article illustrates this argument through an application of the DRI framework to the 1976 and 2003-6 droughts. The article also starts to address how - in cross-disciplinary research that encompasses physical and social sciences - claims about relationships between Drivers as well as Impacts of and Responses to drought over time can be methodologically justified. While the DRI framework has been inductively developed out of research on droughts we argue that it can be applied to a range of environmental governance challenges.

Informal and formal likelihood methods can be used to quantify uncertainty in modelled predictions of groundwater levels (GWLs). Informal methods use a relatively subjective criterion to identify sets of plausible or behavioural parameters of the GWL models. In contrast, formal methods specify a statistical model for the residuals or errors of the GWL model. The formal uncertainty estimates are only reliable when the assumptions of the statistical model are appropriate. We apply the formal approach to historical reconstructions of GWL hydrographs from four UK boreholes. We test whether a model which assumes Gaussian and independent errors is sufficient to represent the residuals or whether a model which includes temporal autocorrelation and a general non-Gaussian distribution is required. Groundwater level hydrographs are often observed at irregular time intervals so we use geostatistical methods to quantify the temporal autocorrelation rather than more standard time series methods such as autoregressive models. According to the Akaike Information Criterion, the more general statistical model better represents the residuals of the GWL model. However, no substantial difference between the accuracy of the GWL predictions and the estimates of their uncertainty is observed when the two statistical models are compared. When the general model is applied, significant temporal correlation over periods ranging from 3 to 20 months is evident for the different boreholes. When the GWL model parameters are sampled using a Markov Chain Monte Carlo approach the distributions based on the general statistical model differ from those of the Gaussian model, particularly for the boreholes with the most autocorrelation. These results suggest that the independent Gaussian model of residuals is sufficient to estimate the uncertainty of a GWL prediction on a single date. However, if realistically autocorrelated simulations of GWL hydrographs for multiple dates are required or if the distributions of the GWL model parameters are of interest, then the more general statistical model should be used.

Groundwater-adapted species (known as stygobites) provide an important contribution to biodiversity. Groundwater ecosystems are some of the oldest on earth, and contain many endemic species adapted to live in an environment with no light and limited resources. The controls on stygobite distributions are not yet fully resolved because of the complex interaction between many processes operating at different scales. Many of these processes are geological or hydrogeological in nature and therefore more detailed geological and hydrogeological studies could provide improved understanding of stygobite distributions. Hydrogeologists can assist ecologists by providing expertise on both general geological characteristics of sampling sites, and how groundwater at sampling sites relates to the wider aquifer setting. Geological input would be especially useful in stygobite dispersal studies because dispersal depends upon habitat continuity associated with geological dispersal corridors, and is limited where rocks that do not provide a suitable habitat form geological barriers. Stygobite studies are of benefit to hydrogeology because stygobite distributions can provide information on ground¬water-surface water interaction and aquifer connectivity over a range of spatio-temporal scales. Future studies using DNA analysis of stygobites may provide much more detailed information on hydraulic connectivity within and between aquifers. There is also potential for the development of stygobites as indicators of groundwater quality. The biogeochemical function of stygobites is of interest to both hydrogeologists and ecologists. Studies have demonstrated that stygobites graze biofilms and bacteria but their role in biogeochemical cycles is still not fully understood. Ecosystem services provided by groundwater fauna depend upon their abundance and biomass. Future studies using hydrogeological data (e.g. borehole packer techniques) may provide an improved understanding of where in aquifers stygobites live and how many there are, which would be an important step towards assessing the significance of their role in biogeochemical cycling of nutrients and carbon.

Groundwater flooding has moved up the policy-makers' agenda as a result of the United Kingdom experiencing extensive groundwater flooding in winter 2000/2001. However, there is a lack of appropriate methods and data to support groundwater flood risk assessment. The implications for flood risk assessment of groundwater flooding are outlined using a study of the Chalk aquifer underlying the Pang and Lambourn catchments in Berkshire, UK. Groundwater flooding in the Chalk results from the water table reaching the land surface and producing long-duration surface flows (weeks to months), causing significant disruption to transport infrastructure and households. By analyzing existing data with a farmers' survey, it was found that groundwater flooding consists of a combination of intermittent stream discharge and anomalous springflow. This work shows that there is a significant challenge involved in drawing together data and understanding of groundwater flooding, which includes vital local knowledge, reasonable risk assessment procedures and deterministic modelling.

This paper reviews the potential impacts of climate change on nitrate concentrations in groundwater of the UK using a Source–Pathway–Receptor framework. Changes in temperature, precipitation quantity and distribution, and atmospheric carbon dioxide concentrations will affect the agricultural nitrate source term through changes in both soil processes and agricultural productivity. Non-agricultural source terms, such as urban areas and atmospheric deposition, are also expected to be affected. The implications for the rate of nitrate leaching to groundwater as a result of these changes are not yet fully understood but predictions suggest that leaching rate may increase under future climate scenarios. Climate change will affect the hydrological cycle with changes to recharge, groundwater levels and resources and flow processes. These changes will impact on concentrations of nitrate in abstracted water and other receptors, such as surface water and groundwater-fed wetlands. The implications for nitrate leaching to groundwater as a result of climate changes are not yet well enough understood to be able to make useful predictions without more site-specific data. The few studies which address the whole cycle show likely changes in nitrate leaching ranging from limited increases to a possible doubling of aquifer concentrations by 2100. These changes may be masked by nitrate reductions from improved agricultural practices, but a range of adaption measures need to be identified. Future impact may also be driven by economic responses to climate change.

The role of lithology in influencing basin form and function is explored empirically by investigating correlations between a range of catchment variables, where the spatial unit of analysis is not surface catchments but lithologically coherent groundwater units. Using the Thames basin, UK, as a case study, nine groundwater units have been identified. Values for 11 hydrological and geomorphological variables, including rainfall, drainage density, Baseflow Index, aquifer porosity, storage coefficient and log-hydraulic conductivity, aquifer and drainage elevation, river incision, and hypsometric integral have been estimated for each of the groundwater units in the basin, and Pearson correlation coefficients calculated for all pairs of variables. Seven of the correlation coefficients are found to be significant at a confidence level of > 99%. Negative correlations between drainage density and log aquifer hydraulic conductivity, and between drainage density and river incision, and positive correlations between log-hydraulic conductivity and river incision, log-hydraulic conductivity and Baseflow Index, and between Baseflow Index and river incision are inferred to have consistent causal explanations. For example, incision of rivers into aquifers leads to relative increases in hydraulic gradients in the vicinity of rivers which, in turn, promotes the development of secondary porosity increasing both aquifer hydraulic conductivity and, hence, Baseflow Index. The implication of this interpretation is that the geomorphological evolution of basins is intimately linked to the evolution of hydraulic conductivity of the underlying aquifers. This is consistent with, and supports the notion of a coupled complexly evolving surface water-groundwater system.