While numerical groundwater models are widely used, analytic element models (AEMs) offer advantages for specific applications. A key advantage of AEMs is their continuous nature, which can represent model values at any spatial point without requiring a computational grid. However, AEMs have several limitations that restrict their universal applicability compared to numerical models. Coupling numerical models with AEMs can combine the strengths of both modeling approaches.

This presentation demonstrates how coupling MODFLOW 6 (MF6) with the AEM TTim can enhance various aspects of groundwater modeling. The coupling was implemented using pymf6, which enables runtime control of MF6 through dynamic reading and modification of model variables. The coupling process creates a new TTim model for each MF6 cell at every time step. The TTim model inherits properties and boundary conditions from the parent MF6 model. Consequently, each MF6 cell becomes a fully functional TTim model capable of representing spatial and temporal details within that cell for one time step. Multiple TTim models can be coupled, one per MF6 cell.

The first use case demonstrates runtime reduction by coupling a TTim model containing a well to a cell in a coarsely discretized MF6 model. The objective is to obtain more accurate water levels in the well. Rather than implementing grid refinement or converting from rectilinear discretization (DIS) to vertex-based (DISV) or unstructured discretization (DISU), we employed TTim coupling. Results show that the coupled model reproduces well water levels comparable to those from MF6 models with significantly finer discretization, substantially reducing computational time by orders of magnitude.

The second use case involves placing multiple TTim wells within a single MF6 cell. This approach enables representation of closely spaced wells in an MF6 grid that is much coarser than the inter-well distances. Model outputs include water levels for all wells and the drawdown distribution within the MF6 cell. pymf6 can dynamically adjust the pumping rate of wells in the MF6 cell based on TTim results during runtime. For instance, the MF6 pumping rate can be regulated to maintain water levels in one or more wells within specified constraints.

Additional TTim features enable further coupling applications. TTim line sinks could represent small drainage ditches interacting with groundwater. TTim area sinks could model multiple small-scale infiltration structures with different hydraulic properties within a single MF6 cell. TTim models could also provide outer boundary conditions for MF6 models, such as infinite boundaries. This approach could reduce the numerical model domain size by allowing boundary conditions to be positioned closer to the area of interest, with the coupled TTim model accounting for distance effects.