For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/. Preface This report describes the "XT3D" option in the Node Property Flow (NPF) Package of the Groundwater Flow Model for the U.S. Geological Survey (USGS) modular hydrologic

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... mulation program, called MODFLOW 6. The MODFLOW 6 pro gram can be be downloaded from the USGS for free. The performance of the XT3D option has been tested in a variety of applications. Future applications, however, might reveal errors that were not detected in the test simulations. Users are requested to send notification of any errors found in this model documentation report or in the model program to the MODFLOW contact listed on the Web page. Updates might be made to both the report and to the model program. Users can check for updates on the MODFLOW Web page (https://doi.org/10.5066/F76Q1VQV).