Methane Flux - The (continuous) flow of methane from one place in the atmosphere to another. It is usually measured as a rate of flow across a given area where methane is produced; for instance, the average amount of methane being released from a rice field of a specific area every hour would be considered the methane flux rate for that field.
[Dictionary of Ecology and Environmental Science; Henry W. Art; 1993; Henry Holt and Company; New York.]
[Testing of Hydrocarbon Emissions From Vegetation and Methodology for Compiling Biogenic Emission Inventories; Patrick R. Zimmerman; National Center for Atmospheric Research; P.O. Box 3000 Boulder, Colorado 80303.] 
Atmospheric Chemistry Glossary