1. General Model Information

Name: Gas Diffusion Model

Acronym: GAS


Main medium: air+terrestrial
Main subject: hydrology, biogeochemical
Organization level: ecosystem
Type of model: partial differential equations (finite differences)
Main application:
Keywords: gas diffusion, hydrogeology, diffusion equation, finite difference numerics, pcg-solver

Contact:

Gordon Hutchinson (Soil Scientist)
USDA-ARS-NPA
Soil Plant and Nutrient Research
P.O. Box
EFederal Building
Fort Collins, CO 80522
Phone: 303-490- 8240
Fax : 303-490-8213
email: glhutch@lamar.colostate.edu

Author(s):

The model developer is Richard W. Healy

Abstract:

GAS is a simulation model, based on a numerical model used to solve the diffusion equation in hydrogeologic systems. The purpose of the model is to simulate the diffusion of various gases (CO2, N2, CH4, etc...) in the soil beneath chambers used to measure gas flux. The model is verified by comparison of numerical and analytical solutions for cases of molecular diffusion, two-dimensional heat transfer, and axisymmetric radial saturated fluid flow. The model is formulated by replacing the continuous linear diffusion equation by discrete finite- difference approximations at each node in a block centered grid. The resulting matrix equation is solved by the method of preconditioned conjugate gradients. The conjugate gradient method does not require the estimation of iteration parameters and is guaranteed convergent in the absence of rounding error. The matrices are preconditioned to decrease the steps to convergence. The model allows the specification of various boundary conditions for any number of stress periods and the output of a summary table for selected nodes showing flux and the concentration of the flux quantity for each time step. The model is written in a modular format for ease of modification. As input the model requires parameters for mass concentration, thermal gradients, constants of proportionality, constant-flux nodes, time step, and iteration. Data input is performed by the modeler. The output data given is mass balance for each time step, including mass in and out of fixed concentration nodes, mass in and out of fixed flux nodes, changes in storage, mass balance, and relative error. The temporal scale of the model is from one-tenth minute time step to 30 minute intervals and the spatial scale is one point. A published description of the model is available - see "References". Author of the abstract: summarized from information given by Gordon Hutchinson


II. Technical Information

II.1 Executables:

Operating System(s): Runs on workstation but can also run on 486 PC with the Microsoft Power Station. Run time is two hours for a 30 minute interval on a PC; less on a workstation.

II.2 Source-code:

Programming Language(s): FORTRAN 77

II.3 Manuals:



II.4 Data:



III. Mathematical Information


III.1 Mathematics


III.2 Quantities


III.2.1 Input

III.2.2 Output


IV. References

Initial model described in:
A.L. Ishii, R.W. Healy, K.G. Striegl. 1989 ''A Numerical Solution for the Diffusion Equation in Hydrologic Systems.'' USGS Water-Resources Investigations Report. 89-4027.

V. Further information in the World-Wide-Web


VI. Additional remarks


Last review of this document by: T. Gabele: 9. 7. 1997 -
Status of the document:
last modified by Tobias Gabele Wed Aug 21 21:44:43 CEST 2002

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