Development and evaluation of local grid refinement methods for forward and inverse groundwater models.

摘要

This work develops a new method of local grid refinement for block-centered finite-difference groundwater models using shared nodes, implemented in conjunction with MODFLOW-2000. Unlike one-way coupled telescopic mesh refinement methods that are commonly used for groundwater models, the shared node method developed here contains a feedback from the embedded grid that influences the outer grid. In contrast to directly embedded approaches, which require the solution of irregular matrix equations because of the irregular finite-difference stencils at the grid interface, the shared node method uses an iterative coupling between the grids and maintains a regular matrix structure. This allows exploitation of efficient matrix solvers based upon standard finite-difference stencils.; The accuracy and computational efficiency of the new local grid refinement method is put into context by comparisons to several other grid refinement methods including variably spaced grids and one-way coupled methods. Test cases in homogenous and heterogeneous, two and three dimensions, and linear and nonlinear flow regimes are investigated. This constitutes the most thorough comparison of these methods for groundwater flow problems known to the author. Results indicate that the shared node method is a compromise between the accuracy of the computationally intensive but inflexible variably spaced grids and the efficient CPU time of the less accurate but flexible one-way coupled methods.; This research also investigates the performance of several grid refinement methods during inverse groundwater modeling. Results indicate that the one-way coupled methods perform poorly because they lack a feedback to relate parameter sensitivity at both grid scales. In contrast, the feedback present in the shared node method produced good results in all cases tested.; Local grid refinement was evaluated as a tool to address the scale issues of representing the features of surface water-groundwater interactions. Results of this investigation indicate that (1) highly refined grids are necessary to represent the small scale hydraulic features of stream-aquifer interactions, and (2) the stream-aquifer connection is grid-scale dependent. The shared node method was able to represent the stream-aquifer interaction with reasonable accuracy (0.6–3.9% error), while a one-way coupled method resulted in much larger errors (6–20%).*; *This dissertation is multimedia (contains text and other applications not available in printed format). The accompanying CD requires the following system requirements: Adobe Acrobat; Microsoft Office.