Airborne observations of methane emissions from rice cultivation in the Sacramento Valley of California

Airborne measurements of methane (CH₄) and carbon dioxide (CO₂) were taken over the rice growing region of California's Sacramento Valley in the late spring of 2010 and 2011. From these and ancillary measurements, we show that CH₄ mixing ratios were higher in the planetary boundary layer above the Sacramento Valley during the rice growing season than they were before it, which we attribute to emissions from rice paddies. We derive daytime emission fluxes of CH₄ between 0.6 and 2.0% of the CO₂ taken up by photosynthesis on a per carbon, or mole to mole, basis. We also use a mixing model to determine an average CH₄/CO₂ flux ratio of -0.6% for one day early in the growing season of 2010. We conclude the CH₄/CO₂ flux ratio estimates from a single rice field in a previous study are representative of rice fields in the Sacramento Valley. If generally true, the California Air Resources Board (CARB) greenhouse gas inventory emission rate of 2.7 × 1010 g CH₄/yr is approximately three times lower than the range of probable CH₄ emissions (7.8-9.3 × 1010 g CH₄/yr) from rice cultivation derived in this study. We attribute this difference to decreased burning of the residual rice crop since 1991, which leads to an increase in CH₄ emissions from rice paddies in succeeding years, but which is not accounted for in the CARB inventory.