Improved water table dynamics in MODFLOW

Area of Science:

• Hydrogeology
• Numerical Modeling
• Computational Fluid Dynamics

Background:

• Standard block-centered finite-difference models like MODFLOW place cell nodes at cell centers.
• Simulating dynamic water tables requires accurate node placement for improved model performance.
• Existing models may lack precision in representing complex water table behaviors.

Purpose of the Study:

• To enhance the accuracy of groundwater flow simulations, particularly for dynamic water tables.
• To modify the MODFLOW model to position cell nodes at the water table in convertible cells.
• To evaluate the impact of this node placement modification on simulation accuracy and convergence.

Main Methods:

• Modified the Layer Property Flow (LPF) package of MODFLOW-2000.
• Implemented a change to position cell nodes at the water table for cells containing a dynamic water table.
• Compared simulation results with the standard node-at-center approach, focusing on partially penetrating wells in unconfined aquifers.

Main Results:

• Improved simulation accuracy in the upper regions of unconfined aquifers when pumping from partially penetrating wells.
• Introduced nonlinearity into the flow equations, leading to a slight increase in convergence time (approximately 7% in the demonstration).
• Accuracy gains are most significant where vertical flow is dominated by water table movement, less so with steep water table gradients.

Conclusions:

• Positioning cell nodes at the water table enhances the accuracy of MODFLOW simulations for dynamic water table conditions.
• The modification is particularly beneficial for unconfined aquifers and scenarios involving partially penetrating wells.
• While computational cost increases slightly, the improved accuracy justifies the modification for specific hydrogeological applications.