Documentation of computer programs to compute and display pathlines using results from the U.S. Geological Survey modular three-dimensional finite-difference ground-water flow model

! Introduction.... .2 Method...... ......3 Theory ......3 Modifications for special cases.. ....12 Non-rectangular vertical discretization..... .12 Water table layers. ....14 Quasi three-dimensional representation of confining layers 14 Boundary conditions and discharge points.. ........15 Backward tracking .....18 Limitations. .19 Limitations due to underlying assumptions of the method.. ..19 Limitations due to discretization effects 19 Limitations due to uncertainty in parameters and boundary conditions...... ........20 Modpath....... ........22 Organization and structure. .22 Flow system files .....22 Main data file. ......22 Stress package data files.... ..24 Cell-by-cell budget file .28 Opening flow system files 28 Output summary of flow field data.. ....29 Interactive input. .29 Name of file containing flow system files. ......29 Oulput mode. .....30 Starting location data.. ......31 Direction of tracking computation 34 Criteria for terminating path lines.. ......34 Mass balance check 35 Cell-by-cell data summary.. .35 Output. ...35 Execution without interactive input 36