Linking an analytic element code to MODFLOW - implementation and benefits
Sensitivity to boundary conditions is seldom investigated during model calibration because it is more convenient to adjust values of hydraulic conductivity and recharge. We used an analytic element (AE) model to develop boundary conditions for an existing complex three-dimensional finite-difference (FD) model.. Using an AE screening model to develop conditions on the grid perimeter for a smaller FD model is conceptually similar to a telescopic mesh refinement process. A preprocessor was created which can extract a detailed FD model from a regional AE model. The AE representation of hydrological features are converted into an equivalent FD representation. Either head-specified or flux-specified conditions at the FD mesh boundary are computed from the AE model, ensuring that at the mesh perimeter, flow conditions in the AE and FD representations are the same. Detail may then be added to the FD model, so long as effects at the mesh perimeter created by the additional detail are negligible. Combining analytic element modeling with finite-difference modeling takes advantage of the strengths of both approaches - one can include near-field complexities not easily handled by analytic element techniques while still incorporating the effect of remote hydrological features not easily incorporated in a finite difference model.
Golden, Colorado, Colorado School of Mines