Direct solution package based on alternating diagonal ordering for the U.S. Geological Survey modular finite-difference ground-water flow model

Abstract

This report documents a direct solver for MODFLOW, the U.S. Geological Survey modular threedimensional finite-difference ground-water flow model. Included in the report are input instructions, a description of concepts and implementation, and detailed program documentation. Although direct solvers require more memory and typically require more computational effort than iterative solvers, a direct solver may execute faster than an iterative solver in some situations. A direct solver is the most efficient when solving small, linear problems. The direct solver uses Gaussian elimination with an alternating diagonal equation numbering scheme that is more efficient than the standard method of equation numbering.