Continental hydrosystem modelling: the concept of nested stream–aquifer interfaces

International audienceCoupled hydrological-hydrogeological models, emphasising the importance of the stream–aquifer interface, are more and more used in hydrological sciences for pluri-disciplinary studies aiming at investigating environmental is-sues. Based on an extensive literature review, stream–aquifer interfaces are described at five different scales: local [10 cm– ∼ 10 m], intermediate [∼ 10 m–∼ 1 km], watershed [10 km 2 – ∼ 1000 km 2 ], regional [10 000 km 2 –∼ 1 M km 2 ] and conti-nental scales [> 10 M km 2 ]. This led us to develop the con-cept of nested stream–aquifer interfaces, which extends the well-known vision of nested groundwater pathways towards the surface, where the mixing of low frequency processes and high frequency processes coupled with the complexity of geomorphological features and heterogeneities creates hy-drological spiralling. This conceptual framework allows the identification of a hierarchical order of the multi-scale con-trol factors of stream–aquifer hydrological exchanges, from the larger scale to the finer scale. The hyporheic corridor, which couples the river to its 3-D hyporheic zone, is then identified as the key component for scaling hydrological pro-cesses occurring at the interface. The identification of the hy-porheic corridor as the support of the hydrological processes scaling is an important step for the development of regional studies, which is one of the main concerns for water practi-tioners and resources managers. In a second part, the modelling of the stream–aquifer in-terface at various scales is investigated with the help of the conductance model. Although the usage of the temperature as a tracer of the flow is a robust method for the assess-ment of stream–aquifer exchanges at the local scale, there is a crucial need to develop innovative methodologies for as-sessing stream–aquifer exchanges at the regional scale. After formulating the conductance model at the regional and inter-mediate scales, we address this challenging issue with the de-velopment of an iterative modelling methodology, which en-sures the consistency of stream–aquifer exchanges between the intermediate and regional scales. Finally, practical recommendations are provided for the study of the interface using the innovative methodology MIM (Measurements–Interpolation–Modelling), which is graphi-cally developed, scaling in space the three pools of methods needed to fully understand stream–aquifer interfaces at vari-ous scales. In the MIM space, stream–aquifer interfaces that can be studied by a given approach are localised. The ef-ficiency of the method is demonstrated with two examples. The first one proposes an upscaling framework, structured around river reaches of ∼ 10–100 m, from the local to the wa-tershed scale. The second example highlights the usefulness of space borne data to improve the assessment of stream– aquifer exchanges at the regional and continental scales. We conclude that further developments in modelling and field measurements have to be undertaken at the regional scale to enable a proper modelling of stream–aquifer exchanges from the local to the continental scale

Assessing water and energy fluxes in a regional hydrosystem: case study of the Seine basin

While it is well accepted that climate change and growing water needs affect long-term sustainable water resources management, performing accurate simulations of water cycle and energy balance dynamics at regional scale remains a challenging task.Traditional Soil-Vegetation-Atmosphere-Transfer (SVAT) models are used for numerical surface water and energy simulations. These models, by conception, do not account for the groundwater lower boundary that permits a full hydrosystem representation. Conversely, while addressing important features such as subsurface heterogeneity and river–aquifer exchanges, groundwater models often integrate overly simplified upper boundary conditions ignoring soil heating and the impacts of vegetation processes on radiation fluxes and root-zone uptakes. In this paper, one of the first attempts to jointly model water and energy fluxes with a special focus on both surface and groundwater at the regional scale is proposed on the Seine hydrosystem (78,650 km$^{2}$), which overlays one of the main multi-aquifer systems of Europe.This study couples the SVAT model ORCHIDEE and the process-based hydrological–hydrogeological model CaWaQS, which describes water fluxes, via a one-way coupling approach from ORCHIDEE toward CaWaQS based on the blueprint published by [de Marsily et al., 1978]. An original transport library based on the resolution of the diffusion/advection transport equation was developed in order to simulate heat transfer in both 1D-river networks and pseudo-3D aquifer systems. In addition, an analytical solution is used to simulate heat transport through aquitards and streambeds. Simulated ORCHIDEE surface water and energy fluxes feed fast surface runoff and slow recharge respectively and then is used as CaWaQS forcings to compute river discharges, hydraulic heads and temperature dynamics through space and time, within each of the hydrosystem compartments. The tool makes it possible to establish a fully consistent water and energy budget over a period of 17 years. It also simulates temperature evolution in each aquifer and evaluates that river thermal regulation mostly relies by order of importance on short wave radiations (109.3 W${\cdot }$m$^{-2}$), groundwater fluxes (48.1 W${\cdot }$m$^{-2}$) and surface runoff (22.7 W${\cdot }$m$^{-2}$)

A combined experimental and numerical study of pore water pressure variations in sub -permafrost groundwater

International audienceThe past few decades have seen a rapid development and progress in research on past and current hydrologic impacts of permafrost evolution. In permafrost area, groundwater is subdivided into two zones: supra-permafrost and sub-permafrost which are separated by permafrost. Knowledge of the sub-permafrost aquifers is often lacking due to the difficulty to access those systems. The few available data show that this aquifers are generally artesian below the continuous permafrost. In the literature, there are two plausible explanations for the relatively high pore pressures in the sub-permafrost aquifer; the recharge related to the ice sheet melting and the expulsion of water related to the ice expansion. In this study, we investigated areas where ice sheets have never developed like in the Paris basin region. The ice expansion induces also soil surface uplift. Our study focuses on modifications of pore water pressure in the sub-permafrost aquifer and the soil surface motion during the permafrost development (freezing front deepening). To fill in the gaps to the field data availability, we developed an experimental approach. Experimental design was undertaken at the Laboratory M2C (Université de Caen-Basse Normandie, CNRS, France). The device consisted in a 2 m2 box insulated at all sides except on the top where a surface temperature was prescribed. The box is filled with silty sand of which hydraulics and thermal parameters are known. Soil temperatures, pore water pressure and soil motion are continuously recorded at different elevations in the sand-box. We developed a two-dimensional transient fully coupled heat and water transport model to simulate thawing and freezing processes taking into account the phase change (Latent heat effects). The balance equations are solved using of a finite difference numerical scheme. Experimental results are used to verify the implementation of the hydro-mechanical processes in our numerical simulations. Experimental and numerical approaches allowed us to verify and quantify the fact that the pressures induced by the ice volumetric expansion are translated into overpressure generated in sub-permafrost groundwater and a soil surface uplift

Numerical Assessment of Groundwater Flowpaths below a Streambed in Alluvial Plains Impacted by a Pumping Field

International audienceThe quality of the water from a riverbank well field is the result of the mixing ratios between the surface water and the local and regional groundwater. The mixing ratio is controlled by the complex processes involved in the surface water–groundwater interactions. In addition, the drawdown of the groundwater level greatly determines the water head differences between the river water and groundwater, as well as the field flowpath inside the alluvial plain, which subsequently impacts the water origin in the well. In common view, groundwater flows from both sides of the valley towards the river, and the groundwater divide is located at the middle of the river. Here, we studied the standard case of a river connected with an alluvial aquifer exploited by a linear pumping field on one riverbank, and we proposed to determine the physical parameters controlling the occurrence of groundwater flow below the river from one bank to the other (cross-riverbank flow). For this purpose, a 2D saturated–unsaturated flow numerical model is used to analyze the groundwater flowpath below a streambed. The alternative scenarios of surface water–groundwater interactions considered here are based on variable regional gradient conditions, pumping conditions, streambed clogging and the aquifer thickness to the river width ratio (aspect ratio). Parameters such as the aspect ratio and the properties of the clogging layer play a crucial role in the occurrence of this flow, and its magnitude increases with the aquifer thickness and the streambed clogging. We demonstrate that for an aspect ratio below 0.2, cross-riverbank flow is negligible. Conversely, when the aspect ratio exceeds 0.7, 20% of the well water comes from the other bank and can even exceed the river contribution when the aspect ratio reaches 0.95. In this situation, contaminant transfers from the opposite riverbank should not be neglected even at low clogging

Développements numériques au sein de la plateforme de modélisation des hydrosystèmes CaWaQS : Introduction de premières fonctionnalités de transport conservatif

La plateforme de modélisation des hydrosystèmes CaWaQS, actuellement développée au Centre de Géosciences de MINES ParisTech, permet de simuler la dynamique des différents termes du cycle de l’eau au sein d'un système. Cet outil a été conçu, et est continuellement maintenu et amélioré, au sein et en marge des travaux du programme PIREN-Seine, et ce, depuis le début des années 2000. Des développements récents ont, entre autres, permis d’intégrer à la dernière version CaWaQS2.88 de nouvelles fonctionnalités permettant la simulation du transport de soluté conservatif en régime transitoire dans les compartiments de surface, de sub-surface et de la zone non-saturée. Outre le détail technique de ces avancées, ce document fait état des conceptualisations introduites ainsi que des formulations mathématiques et algorithmes associés. Une fois finalisés, ces développements permettront de modéliser le transfert simultané d’espèces multiples dissoutes et/ou de chaleur dans l’ensemble des compartiments d’un hydrosystème, en parallèle de la simulation des écoulements des eaux. Une fois l’ensemble des vérifications numériques nécessaires réalisées, la plateforme sera alors mobilisable dans le cadre de problématiques de transfert de matière et de chaleur à diverses échelles d’espace et de temps

Roles of groundwater processes in the evolution of complex landscape of discontinuous permafrost

International audienceThe Hay River Lowland in the Northwest Territories is a 140,000 km 2 region of discontinuous and sporadic permafrost with a high density of peatlands. The landcover consists of permafrost plateaus, channel fens, and ombrotrophic flat bogs, occurring as a complex mosaic of patches. The permafrost is contained within peat-covered permafrost plateaus that rise 1-2 m above the surrounding fens and bogs. The region is experiencing a rapid warming over the past several decades, and large-scale (e.g. 50 km grids), vertical energy transfer models suggest a pole-ward shift of the discontinuous permafrost zone in the future. At the Scotty Creek research basin in the Hay River Lowland, recent field-based and remote sensing observations indicate a rapid lateral thawing of permafrost and deepening of the active layer. It is expected that the lateral transfer of subsurface energy is at least partially responsible for thawing, but the relative roles of conductive transfer and advective transfer mediated by groundwater processes is not well understood. Field observation of differential thawing of the active layer also indicates the presence of strong feedback mechanism mediated by groundwater. We will use two-and three-dimensional numerical models of subsurface water and heat transfer to examine the magnitude of subsurface heat fluxes and test the feasibility of various hypotheses regarding the lateral thawing of permafrost including: 1) the circulation of warm water around permafrost plateau "island" has a significant effect on lateral thawing, 2) variable saturation of peat affects the spatial distribution of permafrost thaw rates, 3) a small depression in permafrost plateau grows into a wetland as a result of groundwater-feedback process and eventually merge into larger, interconnected wetlands and 4) the amplitude of seasonal air temperature fluctuation affects the permafrost geometry and the pathway of groundwater flow

Experimental and numerical assessment of transient stream–aquifer exchange during disconnection

International audienceUnderstanding the state of connection processes of stream-aquifer systems is of great interest for water resources management, particularly in semi-arid regions and where groundwater is extracted in the vicinity of a river bank. Here we present a combined experimental-numerical study to explain physical processes involved in disconnected stream-aquifer systems. A stream-aquifer sand box was built to measure the infiltration rate through the stream bed during aquifer drainage. The pressures in the saturated zone of the aquifer and the infiltration rate were measured in order to quantify the fluid flow in this system. The transient transitional stage between connected and disconnected flow regimes, which was obtained experimentally, is characterised by a maximum infiltration rate across the stream bed before a decrease towards a constant value. This behaviour is analysed by means of transient numerical simulations using relevant hydrodynamic parameters. The importance of the drainage kinematics and unsaturated zone parameters for the temporal variation of the infiltration rate is demonstrated. The possible occurrence of a maximum infiltration rate value during the transitional stage is characterised into a general view of the stream-aquifer disconnection with direct implications for pumping near a stream