Toward Stable Groundwater–Surface Water Coupling in Landscape Evolution Models

Integrated landscape-evolution models require groundwater models that are computationally efficient, groundwater component that remains stable over multidecadal simulations, and strong coupling with surface hydraulics and sediment transport. In CLiDE, which is built on CAESAR–Lisflood, the backward-Euler groundwater update is simple, but as grid resolution or hydraulic diffusivity increases, it becomes highly restrictive due to the diffusion-type Courant–Friedrichs–Lewy (CFL) stability constraint. We present a redesign of CLiDE’s groundwater module that provides two complementary pathways: a behavior-preserving optimized explicit solver and a fully implicit formulation based on backward-Euler time integration. The implicit approach uses a Picard iteration to address the nonlinearity of unconfined transmissivity and the sparse symmetric positive-definite systems with a preconditioned conjugate-gradient solver. We benchmark both solvers across 25 years in fully coupled hydro-geomorphic experiments at the 104 km² Orgeval catchment in north-central France using hourly and daily groundwater coupling intervals. The implicit solver achieves a water mass balance at the catchment scale within 0.1% while remaining unconditionally stable at daily time steps and achieving solutions comparable to the hourly implicit solution. Groundwater head diagnostics are typically within 0.01 m of each other. The consistency in outlet hydrographs, inundation patterns, and long-term sediment-export behavior indicates that daily implicit coupling, in this case, can be selected based on process time scales, and not on numerical stability. Moreover, the optimized explicit solver accelerates the legacy scheme by 1.3 to 1.6 times refinements to specific algorithms, with no change in numerical outputs. Collectively, these advances enhance CLiDE's capability for additional fully coupled, long-duration simulations and suggest a preference between efficiency-oriented explicit updates and robustness-oriented implicit integration.