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The modified version of the MODFLOW groundwater modelling code described in this user guide is based on the 1996 version of the USGS code: MODFLOW-96 (Harbaugh & McDonald, 1996), which in turn was based on an earlier version of the code: MODFLOW-88 (McDonald & Harbaugh, 1988). This is a revised version of the User Guide originally issued in March 2002. The revised version includes some minor changes to the MODFLOW VKD code (currently MF-VKD7), the results of some further code testing, and an independent peer review of the code and documentation.
The main focus of the modifications to MODFLOW has been to include a representation of the variation of hydraulic conductivity and storage with depth commonly observed in chalk and limestone aquifers.