AIR is a numerical model for three-dimensional simulation of compressible or incompressible gas flow in the unsaturated zone. The pressure based governing flow equation is solved using a node-centered finite difference method using rectangular prismatic cells. The resulting set of algebraic equations are solved with a preconditioned conjugate-gradient method (PCG). The model assumes that:

• the soil moisture distribution is constant in time;
• the flow is single-phase gas flow only;
• the free ground-water surface (water table) is an impervious boundary for gas and its location does not change with the air pressure field; and
• flow is laminar and can be described by Darcy's law.

The model can handle heterogeneous and anisotropic soil permeability conditions, injection/pumping wells with constant flow rates, constant flux boundaries, and fixed pressure boundaries. The model has the option to take compressibility into consideration for pressures greater than 40,000 Pa. The model supports a grid of up to 25 x 25 x 10 cells and includes a detailed air flow balance.

AIR has a menu-driven, interactive user-interface with context-sensitive help screens. The program includes various on-screen graphical options including pressure distribution contours and three-dimensional pathlines. Output includes spatial distribution of pressures in HPGL format, and ASCII (text) results files for postprocessing with a commercial graphics program.

AIR is distributed on DOS-formatted disks containing source codes, runtime version, and example files. The user's manual includes installation and operation instructions, and example problems. AIR is a proprietary program.

SYSTEM REQUIREMENTS

Intel 80i86 based computer or compatible with at least 400 Kb free RAM, about 1.5 Mb free disk space to install program and example files, DOS 3.1 or higher, EGA or VGA graphics.

Developers of ASM: W. Kinzelbach (University of Heidelberg), and R. Rausch (University of Stuttgart).