Evaluation of CO 2 fluxes from an agricultural field using a process-based numerical model

Abstract

During 2004, soil CO 2 fluxes, and meteorological and soil variables were measured at multiple locations in a 30-ha agricultural field in the Sacramento Valley, California, to evaluate the effects of different tillage practices on CO 2 emissions at the field scale. Field scale CO 2 fluxes were then evaluated using the one-dimensional process-based SOILCO2 module of the HYDRUS-1D software package. This model simulates dynamic interactions between soil water contents, temperature, and soil respiration by numerically solving partial–differential water flow (Richards) and heat and CO 2 transport (convection–dispersion) equations using the finite element method. The model assumes that the overall CO 2 production in the soil profile is the sum of soil and plant respiration, whose optimal values are affected by time, depth, water content, temperature, and CO 2 concentration in the soil profile. The effect of each variable is introduced using various reduction functions that multiply the optimal soil CO 2 production. Our results show that the numerical model could predict CO 2 fluxes across the soil surface reasonably well using soil hydraulic parameters determined from textural characteristics and the HYDRUS-1D software default values for heat transport, CO 2 transport and production parameters without any additional calibration. An uncertainty analysis was performed to quantify the effects of input parameters and soil heterogeneity on predicted soil water contents and CO 2 fluxes. Both simulated volumetric water contents and surface CO 2 fluxes show a significant dependency on soil hydraulic properties.