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

Abstract. Coupled 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 issues. 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 km2–~1000 km2], regional [10 000 km2–~1 M km2] and continental scales [>10 M km2]. This led us to develop the concept 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 hydrological spiralling. This conceptual framework allows the identification of a hierarchical order of the multi-scale control 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 processes occurring at the interface. The identification of the hyporheic 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 practitioners and resources managers. In a second part, the modelling of the stream–aquifer interface 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 assessment of stream–aquifer exchanges at the local scale, there is a crucial need to develop innovative methodologies for assessing stream–aquifer exchanges at the regional scale. After formulating the conductance model at the regional and intermediate scales, we address this challenging issue with the development of an iterative modelling methodology, which ensures 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 graphically developed, scaling in space the three pools of methods needed to fully understand stream–aquifer interfaces at various scales. In the MIM space, stream–aquifer interfaces that can be studied by a given approach are localised. The efficiency 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 watershed 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.