In this report we present a predictive simulation model, SOILCO2, based on process oriented relationships. The model includes one-dimensional water flow and multiphase transport of CO2 utilizing the Richards' and the convection-dispersion equations, respectively, as well as heat flow and a CO2 production model. The flow equation incorporates a sink term to account for water uptake by plant roots. In the model we consider that the transport of CO2 in the unsaturated zone can occur both in the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which are dependent on water content, temperature, growth, salinity, and plant and soil characteristics. Heat flow is included in the model, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.
We also select parameter values for optimal plant and microbial CO2 production, production dependence on temperature, water content, and gas composition for plant and microbial respiration, all based on literature review. The predictive capabilities of the SOILCO2 model are evaluated by comparing model simulations to published field data from Missouri for two growing seasons under transient conditions, as well as a data set collected in Riverside, CA under relatively constant water content at depth.
The program is written in ANSI standard FORTRAN 77. Required computer memory is a function of the problem definition. This report serves as both a user manual and reference document. Detailed instructions are given for data input preparation. Example input and selected output files are also provided, as is a listing of the source code.
Simunek, J., and D. L. Suarez. 1994. The SOILCO2 Code for Simulating One-Dimensional Carbon Dioxide Production and Transport in Variably Saturated Porous Media, Version 1.2. Research Report No. 127, U. S. Salinity Laboratory, USDA-ARS, Riverside, California.
The program and manual are available upon request from: Donald Suarez USDA-ARS U.S. Salinity Laboratory 450 W Big Springs Road Riverside, CA 92507-4617