A NUMERICAL APPROACH TO EFFICIENTLY SIMULATE WETTING AND REWETTING IN A TRANSIENT UNCONFINED SIMULATION

Many practical groundwater projects require simulation of unconfined aquifers in transient conditions. Unfortunately, the moving water table problem is notoriously computationally demanding. Accurate solutions can be obtained using two-phase (air/water) simulations, but these are not practical for many field-scale problems where large (hundreds of thousand to millions of gridblocks) models are often used. In addition, these models require calibration (hence hundreds to thousands of forward runs) and so efficient run-times are necessary.

Simplified single-phase approaches have been developed, such as that implemented in MODFLOW2000. There has been concern, however, when using the wetting/re-wetting capability on large scale problems with uniqueness of the solution (Naff et.al. 2003) and the ability to re-wet efficiently.

We have developed an alternative single-phase, simplified numerical approach, which incorporates selected aspects of multi-phase flow theory. We incorporate this approach into the code FEHM and apply it to analysis of unconfined aquifers. This method has some advantages (and a few disadvantages) when compared to three established methods: that used in MODFLOW, the full two-phase numerical formulation, and the Richards' equation approach. The new formulation is compared with the other methods on several problems ranging a simple drawdown analysis model to a complex basin scale model. The formulation is evaluated in terms of accuracy, numerical stability, uniqueness of the solution, and computational efficiency.

Naff, R., Banta E., and J. McCord, Obtaining a Steady State Solution with Elliptic and Parabolic Groundwater Flow Equations under Dewatering Conditions: Experiences with a Basin Model, MODFLOW and More 2003, September 16-19 2003, Golden Colorado.