Multi-region approach to modeling subsurface flow

A multi-region approach to modeling subsurface flow is presented. In the approach, the media are assumed to contain n pore-regions at any physical point. Each region has different pore size and hydrologic parameters. Inter-region exchange is approximated by a linear transfer process. Based on the mass balance principle, a system of equations governing the flow and mass exchange in structured or aggregated soils is derived. This system of equations are coupled through linear transfer terms representing the interchange among different pore regions. A numerical MUlti-Region Flow (MURF) model, using the Galerkin finite element method to facilitate the treatment of local- and field-scale heterogeneities is developed to solve the system of equations. A sparse matrix solver is used to solve the resulting matrix equation, which makes the application of MURF to large field problems feasible in terms of CPU time and storage limitations. MURF is first verified by applying it to a ponding infiltration problem over a hill slope, which is a single-region problem and has been previously simulated by a single-region model. Very good agreement is obtained between the results from the two different models. The MURF code is thus partially verified. It is then applied to a two-region fractured medium to investigate the effects of multi-region approach on the flow field. The results are comparable to that obtained by other investigators.