A Low-Dimensional Integrated Subsurface Hydrological Model Coupled with 2-D Overland Flow: Application to a Restored Fluvial Hydrosystem (upper Rhine River – France)

An integrated hydrological model that couples a low-dimensional (2-D) subsurface model to 1-D river flow and 2-D overland flow is presented and applied to a real-world system. The 2-D subsurface equation stems from a careful integration a 3-D Richards equation along a local direction normal to the bottom of the aquifer. A Normally Integrated Model (NIM) is first compared with the integrated model CATHY (CATchment HYdrology) in regard to a synthetic test case with complex geometry, heterogeneous hydraulic conductivity field of the subsurface compartment, and various interactions between the surface and the subsurface. This exercise reveals the gain in computation time associated with the use of a low-dimensional model (by a factor of 6). NIM is then applied to simulate the hydrodynamics of an actual restored hydrosystem, namely, Rohrschollen Island of the Upper Rhine River, with the aim of assessing the ability to address highly transient hydrological conditions and the efficiency of the restoration strategy. The calibration and validation steps associated with this exercise show that the coupled model describes the hydraulic head evolution and water mass balance induced by artificial injections of surface water in a channel dug for the purpose of reconnecting the Rhine to an old and clogged channel. The low-dimensional coupled model of this study is simpler than the state-of-the-art integrated models employing the solution of the 3-D Richards equation. It is, however, suited to investigate fine-scale problems in relation to water resource management and hydraulic developments, with reasonable computational costs and a physically-based approach to model flow processes.