Influence of River Rating Curves Interpolation Methods on In-stream Water Level Assessment and Stream-aquifer Exchanges in a Regional Distributed Hydro(Geo)logical Model

The main objective of this study is to provide a realistic simulation of river stage in regional river networks in order to improve the quantification of stream-aquifer exchanges. The study focuses on the Oise basin (4 500 km2, part of the 65 000 km2-Seine basin in Northern France) where two original methodologies of rating curves estimations are proposed. The general framework is the distributed model Eau-Dyssée, which couples existing specialized models to address water resources in river basins. In particular, it simulates flow in aquifer units with a finite difference pseudo 3D model and river flow with a Muskingum model. Rating curves are used in the regional distributed hydro(geo)logical model to deduce river stage from the routed discharge, which permits to calculate the exchanges between aquifer units and rivers. The first methodology, which was already validated in the Oise basin, is based on simulating the main rivers with a 1D Saint-Venant model, from which functional stage-discharge relationships, or rating curves, are derived at a 200-m resolution and projected onto each 1-km grid-cell of the regional model. Such method can only be developed on well instrumented basins. In order to estimate river height on most basins (even those where the St Venant approach is not valid or cannot be set up due to lack of data), a second methodology is developed using data calculated with models at lower resolution (≥ 500 m): Rating curves at each center of the river network at regional scale are thus interpolated, based on a segmentation of the space compatible with the hydraulics and the regional model. This second methodology has been carried out over half the Seine basin river network, and the aim of the study is to validate it in the Oise basin with regards to the results of the first one. Assessed by the first method, average stream-aquifer exchanges are 39 mm.yr-1 for aquifer to streams fluxes and 2 mm.yr-1 for streams to aquifer fluxes, mainly due to storage in aquifer units during storm events. The stream to aquifer fluxes during high flow periods involve a longer transfer time in the aquifer units near to the river network, what corresponds to an increase of stored water in the aquifer system. In terms of spatial impact on simulated piezometric heads, the area influenced by in-stream water level fluctuations extends across 3 to 20 km around the streams, depending on the hydrogeological setting of the aquifer unit (confined/unconfined), with deviations of the simulated piezometric heads from their average ranging from a few centimeters to more than 1 m in aquifer grid-cells near the main stream. The second methodology leads to similar results offering a low computational cost opportunity for taking into account in-stream water level fluctuations in regional distributed process-based hydro(geo)logical models. It is an efficient way to improve the physics of the stream-aquifer interactions and better assess soil water content at the regional scale, with a limited computational burden owing to the pre-computation of the rating curves.