1. General Model Information
Name: Gas Diffusion Model
Main medium: terrestrial
Main subject: biogeochemistry
Organization level: ecosystem
Type of model: partial differential equations (finite differences)
Keywords: gas diffusion, atmospheric-soil interactions
USDA-ARS-NPA Soil Plant and Nutrient Research
P.O. Box E
Fort Collins, CO 80522
Richard W. Healy
The Gas Diffusion Model simulates the diffusion of various gases through the
soil. The model is based on a numerical solution of discrete-finite difference
approximations of a one or two dimensional diffusion equation. The model is
designed around a block center grid approach which results in a matrix equation
solved using the preconditioned conjugate gradients method. The model uses a 6
second time step executed over a 30 minute time period.
Validation Procedures: Model is verified by comparison of numerical and analytical solutions for
cases of molecular diffusion, two-dimensional heat transfer, and axisymmetric radial saturated
Author of the abstract:
II. Technical Information
Operating System(s): UNIX workstation or on a IBM PC (PC runtime is about two hours for a 30 minute time interval simulation)
Programming Language(s): FORTRAN 77
III. Mathematical Information
Parameters for mass concentration, thermal gradients,
Parameters for mass concentration, thermal gradients,constants of proportionality, constant-flux nodes, time step, and iteration Mass balance for each time step, including mass in and out of fixed
Mass balance for each time step, including mass in and out of fixedconcentration nodes, mass in and out of fixed flux nodes, changes in storage, mass balance, andrelative error.
Temporal Scale: from one-tenth minute time step to 30 minute intervals.
Spatial Scale: Point
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
This model is useful for the study of atmospheric-soil interactions. Itis a fairly complex model with respect to the mathematics employed. There areseveral unique features of the model that can be applied to otherdispersion/diffusion problems. The model uses a discrete finite-difference approximation method rather than the traditional linear diffusion equations. By employing a grid arrangement, calculations are made using linear algebra anda preconditioned conjugate gradient method which avoids the need to estimate iterative parameters.
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