Sept 7 class During class, you will immediately begin modeling using MODFLOW, so it is important to get some understanding of MODFLOW before you start. The manual is very helpful, so read Ch 2, focusing on the areas noted below. Be sure to ask questions on parts that you don’t understand. MODFLOW uses finite-difference approximation for 3D saturated, single phase flow. MODFLOW uses the terminology of “rows”, “columns”, and “layers”. Look closely at figure 1 Note the numbering scheme is a little different than typical (it starts in the upper left hand corner). MODFLOW uses a finite difference approximation to solve for hydraulic head at a node in the center of a grid cell (“block-centered” Figure 2). However, the solution is an average over the cell. Flow only takes place across the faces between adjacent cells (Fig 3 and Fig 4). That means that there is no “diagonal” flow. Flow is horizontal within layers. Between layers, flow is vertical. • Note in the development of the finite-difference equations, grid spacing can vary. A
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