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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.

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.

In 2018–2019, large parts of Europe experienced an unprecedented multi-year drought with severe impacts on society and ecosystems. This study is among the first to analyze its impact on water quality by comparing long-term (1997–2017) nitrate export with 2018–2019 export in an intensively-monitored mesoscale catchment in Germany. We combined data-driven analysis of concentration-discharge and load-discharge relationships with process-based modelling to analyze the catchment nitrogen retention capacity and the underlying mechanisms of retention in the soils and during subsurface transport. Within the multi-year drought, we found a shift in the concentration-discharge relationship at the catchment outlet, reflecting exceptionally low riverine nitrate concentrations during dry periods and exceptionally high concentrations during subsequent wet periods. Nitrate loads during the multi-year drought were up to 70% higher than expected from the long-term relationship between discharge and loads. Model simulations suggested that this increase was driven by a decrease in denitrification and plant uptake in exceptionally dry soils and subsequent flushing of accumulated nitrogen during rewetting via fast, shallow flow paths. As a consequence, the overall capacity of the catchment to retain nitrogen was reduced, which was confirmed by model results for nitrate in the soil leachates. This observation was most evident in the upstream sub-catchments, which have relatively short transit times during wet periods (20 years) inhibit a fast response of riverine water quality to drought conditions, which might result in a long-term drought legacy becoming visible in the future. Overall, our study reveals that a severe multi-year drought threatens water quality by intensifying nitrate pollution. This is crucial knowledge for water quality management in the face of climate change, as such droughts are predicted to become more frequent and prolonged across Europe.

From 2018 to 2020, large parts of Central Europe experienced an unprecedented multi-year summer drought with severe impacts on society and ecosystems. While strong reductions of water quantity were reported, our study is one of the first to analyze its impacts on water quality by looking at nitrate export dynamics in a nested mesoscale catchment in Germany with heterogeneous landscape settings. We used concentration-discharge (CQ) relationships to analyze catchment functioning in terms of nitrate retention and release, and the mechanistic process based mHM-SAS model (Nguyen et al., 2021) to simulate the underlying belowground nitrogen (N) fluxes and associated hydrologic transit times. For the three drought years, we found an amplification of the seasonality in nitrate export with lower concentrations in summer and higher concentration in winter, compared to normal conditions. Compared to the long-term average behavior, the catchment exhibited a disproportionally high annual load export relative to the discharge available for its transport. We argue that this loss in nitrate retention capacity was driven by a complex mixture of changes in N cycling and heterogeneous transit times among different contributing areas. During dry periods, long transit times and sufficient subsurface denitrification likely caused the low in-stream nitrate concentrations in the upper, more mountainous catchment, while reduced soil denitrification and plant uptake resulted in an accumulation of N in the soils. During the following wet periods, this accumulated N was rapidly exported to the stream (median transit times < 2 month) causing a steep increase in nitrate concentrations and load export. In the downstream (lower) sub-catchment, long median transit times (> 20 years) prevent such an immediate export of the accumulated soil-N to the stream. Our modeling analysis, however, suggests that the build-up of soil N-stores and the lack of fast, shallow flow path may exacerbate N legaci

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface-subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio-temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local-to-regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies.

Microplastic (MP) particles are commonly found in freshwater environments such as rivers and lakes, negatively affecting aquatic organisms and potentially causing water quality issues. Understanding the transport and fate of MP particles in these environments is a key prerequisite to mitigate the problem. For standing water bodies (lakes, ponds) the terminal settling velocity (TSV) is a key parameter, which determines particle residence times and exposure times of organisms to MP in lakes. Here we systematically investigate the effects of the physical parameters density, volume, shape and roundness, surface roughness and hydrophobicity and lake water temperature on the TSV of a large number of particles with regular and irregular shapes (equivalent diameters: 0.5–2.5 mm) and different polymer densities using computational fluid dynamics (CFD) simulations. Simulation results are compared to laboratory settling experiments and used to evaluate existing, semi-empirical relationships to estimate TSV. The semi-empirical relationships were generally found to be in reasonable agreement with the CFD simulations (R2 > 0.92). Deviations were attributed to simplifications in their descriptions of particle shapes. Overall the CFD simulations also matched the TSVs from the experiments quite well, (R2 > 0.82), but experimental TSVs were generally slower than model TSVs with the largest differences for the irregular particles made from biodegradable polymers. The deviations of up to 58% were found to be related to the attachment of air bubbles on irregularities in the particle surfaces caused by the hydrophobicity of the MP particles. Overall, density was the most decisive parameter for TSV with increases in TSV of up to 400% followed by volume (200%), water temperature (47%) and particle roundness (45%). Our simulation results provide a frame of reference for an improved evaluation of the relative effects of different particle characteristics on their TSV in lakes. This will in t