Capacity-limited infiltration in unsaturated slopes with root uptake: a semi-implicit 1D Richards model with Reduced-Order HM Storage

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4. Capacity-limited infiltration in unsaturated slopes with root uptake: a semi-implicit 1D Richards model with Reduced-Order HM Storage

Rainfall infiltration alters matric suction and shear strength in partially saturated slopes, necessitating rapid and robust tools to simulate short-term transients during intense storms. This study proposes a 1D, slope-aligned model that solves a Gardner-type Richards equation with a depth-dependent root-uptake sink. A reduced-order, storage-only hydro-mechanical (HM) coupling is adopted: a lumped suction-based stiffness augments hydraulic storage and updates void ratio/porosity, thereby modulating hydraulic conductivity K_S and the Gardner capillary-scaling parameter a. This retains first-order skeleton compressibility at negligible cost, deliberately avoiding a fully coupled mechanical equilibrium, and is suited to early-warning systems and extensive parametric studies. Time integration employs a semi-implicit Crank-Nicolson (CN) scheme with an explicit half-step predictor, which controls local transients without requiring nonlinear iterations. In the simulated case, rainfall is applied in six-hour blocks; the testbed consists of a 2 m homogeneous colluvial layer over a far less permeable substratum. The parametric analysis targets silty-range K_S and rainfall intensity RI; within literature-based root-uptake ranges (T_R), vegetation effects are minor over 2448 h. Across cases, K_S and RI govern wetting-front advance and the rate/timing of suction change. The framework delivers rapid forecasts of pressure head and infiltration depth, with diagnostics of boundary fluxes, storage, and HM contributions.

3rd International Workshop on Soil-Vegetation-Atmosphere Interaction (RootS2025)

2c. Numerical modelling of the behaviour of rooted soils and boundary value problems under static and dynamic loading conditions