Title. Convergence Properties of Iteratively Coupled Surface-Subsurface Models, Valentina Schüller, mathematics, Lund University.

Abstract. Water management projects such as stream bed re-naturalization affect the water table, which is relevant, e.g., for flood prediction. These physical processes can be modeled using coupled surface-subsurface flow models. They solve some form of the Richards and shallow water equations. A typical setup couples these two nonlinear partial differential equations in a partitioned approach via boundary conditions. Full interaction between the subsolvers is ensured by an iterative coupling procedure. This can be accelerated using relaxation.

We have applied linear analysis techniques to study an idealized, linear, 1D-0D version of a surface-subsurface model. These result in explicit expressions for the convergence factor and an optimal relaxation parameter, depending on material and discretization parameters. We test our analysis results numerically for fully nonlinear 2D-1D experiments based on existing benchmark problems. As we will show in this talk, the linear analysis can explain fast convergence of iterations observed in practice for different materials and test cases, even though we are not able to capture various nonlinear effects.

This is joint work with Philipp Birken and Andreas Dedner.