Compatibility of MODPATH with MODFLOW-NWT July 14, 2014 MODPATH works with MODFLOW-NWT provided that the MODFLOW-NWT option flag IPHDRY is set to a positive number. IPHDRY is specified in the UPW input file. When IPHDRY is positive, MODFLOW-NWT assigns the dry-cell head value (HDRY) to all dry cells when it writes the head output file. MODPATH reads the MODFLOW head output file and uses the HDRY value to indicate that a cell is dry and inactive. If the IPHDRY flag is not set to save HDRY values for dry cells, MODPATH assumes dry cells are active and tries to track particles through them. The result is that MODPATH computes negative saturated thickness values in dry cells that cause the program to terminate with an error. When using MODPATH with MODFLOW-NWT particles can enter dry cells laterally and then appear to stop prematurely. MODFLOW-NWT flags dry cells as inactive but continues to compute lateral flow into the dry cells that subsequently is routed downward into the next active cell. MODPATH recognizes when there is a component of flow into a dry cell but stops particles when they reach a boundary face of the dry cell because the cell is flagged as inactive. Unlike MODFLOW-NWT, MODPATH has no way to route the water downward from an inactive cell to the next active cell in an underlying layer. We expect future versions of MODPATH will be able to recognize this situation and move particles downward to the next active cell, but for now the only remedy is to be aware of the issue and recognize it when it occurs.