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Water fluxes at ecohydrological interfaces are a key driver for biogeochemical processing and they influence habitat properties. This chapter covers methods which are specifically suitable for the stream–groundwater interface. The point-scale approaches, however, can be applied in the general context of groundwater–surface water interactions, e.g. for lakes. Water fluxes across the stream–streambed interface are practically never homogeneous in space. Point methods are used to detect water fluxes across the stream–groundwater interface at one point or spot. Point methods comprise direct observation of water fluxes with seepage meters, methods based on Darcy's law, as well as natural and artificial tracer techniques. Reach-scale integral methods can be used when the spatial scale of interest is so large that multiple point measurements become infeasible.

In this chapter, the authors aim to uncover the organizational principles – the main drivers and controls, and their interactions and feedbacks – that determine the development and capacity of ecohydrological interfaces to transform the flow of energy, water, and matter between adjacent ecosystems. They compare the characteristics of transformation processes at different ecohydrological interfaces in freshwater ecosystems, including groundwater–surface water, groundwater–vadose zone, and benthic–pelagic interfaces, to determine common or unique features of their non-linear process dynamics. Based on a comparison of the organizational principles of different ecohydrological interfaces, the authors propose a roadmap for the development of multi-scale conceptual models of ecohydrological interface processes and their interactions that can be expanded to other types of ecohydrological interfaces.

In this chapter, the authors provide a comprehensive analysis and synthesis of the interactions between important drivers and controls of hyporheic exchange and biogeochemical cycling and how they vary across scales, integrating results from a wide range of case studies that go beyond current conceptual model frameworks. They discuss the interactions of different local-to-regional controls and drivers of hyporheic zone processes such as hydrodynamic and hydrostatic drivers of hyporheic exchange, sediment hydraulic conductivity, the role of autochthonous organic matter sources, and feedbacks between hydrological exchange and ecological processes in the streambed. The authors explore the implications of these interactions for biogeochemical cycling in the landscape context. They integrate conceptualizations of organizational principles of hyporheic exchange and biogeochemical cycling from reach to catchment scale. The authors expect that increasing awareness and embracing the landscape organizing principles of hyporheic zones will advance the future of research at groundwater–surface water interfaces.

Stream water (SW) infiltration to the subsurface and subsequent mixing with groundwater (GW) is crucial for controlling water quality in river-corridors. Mixing of solutes from SW and GW triggers biogeochemical reactions, which can attenuate contaminant influx from upstream SW or from GW that eventually enters the river in downstream regions. It is known that geological heterogeneity affects SW-GW exchange fluxes (EF), however, the combined effects of both hydrological and geological aspects (e.g., sand fraction present in the aquifer material) on EF and on SW-GW mixing in strong contrasting bimodal aquifer units remain unclear. Here, we examined this gap by combining geostatistical realizations, fully-integrated numerical flow simulations, and a mixing-cell routine to assess the major controls on EF and to evaluate how mixing develops and is affected by stream discharge events with different magnitudes and durations. Results show that subsurface heterogeneity at the river-reach scale mainly affects EF magnitudes rather than EF patterns. Yet, both EF magnitudes and SW-GW mixing increased with the introduction of subsurface heterogeneity and with the increase of average hydraulic conductivity (K) values and sand fraction in the models. The simulations further indicated a larger potential for mixing under more frequent, short events regardless of the aquifer material, however, mixing values were generally higher for sandier and heterogeneous models. Lastly, for a high K contrast between subsurface units, these effects were more pronounced. This characterization is critical for river restoration strategies and for downstream management of dam-regulated rivers. Our study elucidates the interplay between hydrological and geological controls on the development of SW-GW mixing at intermediate scales and highlights the importance of considering aquifer characteristics in future studies.

In 2018-2019, Central Europe experienced an unprecedented 2-year drought with severe impacts on society and ecosystems. In this study, we analyzed the impact of this drought on water quality by comparing long-Term (1997-2017) nitrate export with 2018-2019 export in a heterogeneous mesoscale catchment. We combined data-driven analysis with process-based modeling to analyze nitrogen retention and the underlying mechanisms in the soils and during subsurface transport. We found a drought-induced shift in concentration-discharge relationships, reflecting exceptionally low riverine nitrate concentrations during dry periods and exceptionally high concentrations during subsequent wet periods. Nitrate loads were up to 73ĝ€¯% higher compared to the long-Term load-discharge relationship. Model simulations confirmed that this increase was driven by decreased denitrification and plant uptake and subsequent flushing of accumulated nitrogen during rewetting. Fast transit times (<2 months) during wet periods in the upstream sub-catchments enabled a fast water quality response to drought. In contrast, longer transit times downstream (>20 years) inhibited a fast response but potentially contribute to a long-Term drought legacy. Overall, our study reveals that severe droughts, which are predicted to become more frequent across Europe, can reduce the nitrogen retention capacity of catchments, thereby intensifying nitrate pollution and threatening water quality.

Regional groundwater resources are crucial for water supply, maintaining environmental stream flows and the integrity of aquatic ecosystems. At the same time, however, these resources are increasingly threatened by the effects of climate change. More extreme weather conditions like exceptional droughts are expected to increase water stress by increasing water demand and decreasing water availability even in humid regions such as Germany. To identify consistent long-term trends and areas vulnerable to droughts, it is important to characterize and understand similarities and differences in groundwater dynamics across sites. Herein, we analyze groundwater head responses to climatic variability at more than 6500 groundwater wells over the last 30 years in Germany to identify response clusters. Principal component analysis (PCA) revealed that about two-thirds of the observed groundwater level variability across all wells can be explained by five typological time series. These time series represent different response patterns to climatic forcing capturing distinct dampening effects and time lags, with different weights (loadings) being assigned to each groundwater well. The subsequently identified clusters of the wells reveal clear regional structures suggesting underlying spatial controls. In the next step, groundwater drought characteristics (e.g., severity, duration) will be considered and linked to climate and landscape properties. This study may help to understand regional controls and to identify zones vulnerable to groundwater droughts where water supplies are at risk in the long term and mitigation measures are needed.

The assessment and forecasting of nutrient loss by tile drains in agricultural areas often rely on physically-based models that have adequate representations of macropores and tile drains. Macroporosity has been adequately represented in hydrological models using a dual continuum approach. However, its implementation in hydrological and solute transport models is limited to plot-scale or to one- and two-dimensional models due to the large number of parameters that are rarely available and the long computational times. The purpose of this study is to simulate a tracer test using a 3D coupled surface-subsurface model to improve the representation of the tracer concentration at the drainage discharge. A three-dimensional HydroGeoSphere model was developed and calibrated to simulate tile drainage discharge, Br mass discharge and hydraulic heads from a Br tracer test in a densely tile-drained field. The conductivity of the drain was one of the most important parameters for drain discharge and solute transport simulations. The model accurately simulated drainage discharge and Br transport to tile drains. However, most of the Br peaks and the late-time Br mass in the drain outflow were underestimated. Our simulation results indicate that explicitly representing tile drains with seepage nodes allows for a physically-based, yet computationally-efficient representation of Br transport behavior surrounding tile drains at field scale. However, we cannot confirm that the single-porosity model with immobile zone is suitable for simulating the Br peaks at the drain outlet and the late-time Br mass. Improvements to the model include the implementation of heterogeneous soil layers and the inclusion of more measured data to reduce uncertainty during calibration.

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3Dĝ€¯+ĝ€¯time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface-subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space-time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space-time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface-subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space-time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space-time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components.