Extension of the Basic Model Interface for tight coupling of MODFLOW to other model components.
The Basic Model Interface (BMI) has been extended to allow tighter coupling of model components than is available in the BMI standard. To enable tighter coupling between models, we have developed the eXtended Model Interface (XMI) which extends the BMI functionality and enables coupling within the non-linear Picard iteration loop. The XMI subdivides the BMI update function into multiple functions. This subdivision allows data from other model components to affect matrix coefficients during each MODFLOW non-linear Picard iteration. Additional functions to subdivide the update function include prepare_timestep, do_timestep, finalize_timestep, prepare_solve, solve, and finalize_solve.
We have developed a hypothetical model application that simulates characteristics common to hydrologic conditions in a large part of the Netherlands. The application tightly couples MODFLOW and MetaSWAP using a shared control volume approach and XMI. MetaSWAP is meta-model that simulates the unsaturated zone using a quasi steady-state formulation based on Richards’ equation. The coupling procedure consists of the following steps. After every solution of the groundwater heads within the non-linear Picard iteration loop, MetaSWAP determines the unsaturated zone flux and primary storage coefficients while ensuring mass balance for the shared control volume. Both variables (groundwater recharge and storage coefficients) are then communicated to MODFLOW and this sequence is repeated until the MODFLOW convergence criteria are met for a time step. The hypothetical model application demonstrates that MetaSWAP makes it possible to simulate the unsaturated zone in more detail than possible with the MODFLOW Unsaturated Zone Flow (UZF) Package and simulate soil moisture-based groundwater irrigation.