Modeling methane emissions from rice fields : variability, uncertainty, and sensitivity analysis of processes involved

Estimates of global methane (CH4) emissions, to which rice cropping systems contribute significantly, are uncertain. T The variability and uncertainty of variables governing emission rates and the sensitivity of emissions to these variables determine the accuracy of CH4 emission estimates. A good tool for quantification of sensitivities is a process-based model. This paper describes a model that has been validated previously by experimental data. Variability and uncertainty in processes and variables underlying CH4 emissions are reviewed and the sensitivities of modeled CH4 emission estimates for process variables are tested. The sensitivity analysis is carried out for two sites in the Philippines at which CH4 emissions have been measured for several years. The sensitivities of the model are compared with measured sensitivities, both as a function of input parameters. The model sensitivity analysis shows that the system is not sensitive to mechanisms of CH4 production or the pathway of gas transport through the plant. Methane emissions are very sensitive, however, to the description of substrate supply (both from the soil and from organic fertilizers). Unfortunately, this description also represents a main uncertainty. Uncertainty in CH4 emission estimates will thus remain large as long as this process is not well quantified.