A modular three-dimensional finite-difference ground-water flow model is presented, designed to simulate groundwater flow in three dimensions. The model is based on a modular programming structure, allowing users to independently examine specific hydrologic features and add new capabilities without modifying existing modules. The program is written in FORTRAN '66 and can run on most computers with a FORTRAN '66 compiler. It includes a variety of packages for simulating different hydrologic features, such as wells, recharge, rivers, drains, evapotranspiration, and general-head boundaries. The model uses a block-centered finite-difference approach to simulate groundwater flow, allowing for the representation of confined, unconfined, or mixed aquifers. The finite-difference equations can be solved using either the Strongly Implicit Procedure (SIP) or Slice-Successive Overrelaxation (SSOR) methods. The program includes detailed documentation for each module, including narrative descriptions, flow charts, and program listings. The model is designed to be flexible, allowing users to select various output options and to modify the program as needed. The program is organized into a main program and a series of highly independent subroutines called modules, grouped into "packages" that handle specific aspects of the simulation. The model includes a variety of utility modules for reading and printing data, as well as detailed documentation for each package. The program is designed to be efficient in terms of computer memory and execution time, and it can be executed on a variety of computers with minimal changes. The model is used to simulate groundwater flow in a variety of scenarios, including recharge, evapotranspiration, and flow through riverbeds and drains. The program includes detailed input instructions and sample input for each package, allowing users to easily understand and use the model. The model is designed to be user-friendly, with a modular structure that allows for easy modification and expansion. The program is accompanied by a detailed abstract and a list of packages, as well as a variety of figures and tables that illustrate the model's functionality and applications. The model is a comprehensive tool for simulating groundwater flow and is widely used in hydrological studies.A modular three-dimensional finite-difference ground-water flow model is presented, designed to simulate groundwater flow in three dimensions. The model is based on a modular programming structure, allowing users to independently examine specific hydrologic features and add new capabilities without modifying existing modules. The program is written in FORTRAN '66 and can run on most computers with a FORTRAN '66 compiler. It includes a variety of packages for simulating different hydrologic features, such as wells, recharge, rivers, drains, evapotranspiration, and general-head boundaries. The model uses a block-centered finite-difference approach to simulate groundwater flow, allowing for the representation of confined, unconfined, or mixed aquifers. The finite-difference equations can be solved using either the Strongly Implicit Procedure (SIP) or Slice-Successive Overrelaxation (SSOR) methods. The program includes detailed documentation for each module, including narrative descriptions, flow charts, and program listings. The model is designed to be flexible, allowing users to select various output options and to modify the program as needed. The program is organized into a main program and a series of highly independent subroutines called modules, grouped into "packages" that handle specific aspects of the simulation. The model includes a variety of utility modules for reading and printing data, as well as detailed documentation for each package. The program is designed to be efficient in terms of computer memory and execution time, and it can be executed on a variety of computers with minimal changes. The model is used to simulate groundwater flow in a variety of scenarios, including recharge, evapotranspiration, and flow through riverbeds and drains. The program includes detailed input instructions and sample input for each package, allowing users to easily understand and use the model. The model is designed to be user-friendly, with a modular structure that allows for easy modification and expansion. The program is accompanied by a detailed abstract and a list of packages, as well as a variety of figures and tables that illustrate the model's functionality and applications. The model is a comprehensive tool for simulating groundwater flow and is widely used in hydrological studies.