Estimation of seasonal methane fluxes over a Mediterranean rice paddy area using the Radon Tracer Method (RTM)

Abstract

Abstract. The Ebro River Delta, in the northwestern Mediterranean basin, has an extension of 320 km2 and is mainly covered by rice fields. In the framework of the ClimaDat project, the greenhouse gases atmospheric station DEC was designed and installed in this area in 2013. The DEC station was equipped, among other tools, with a Picarro G2301 instrument and an ARMON (Atmospheric Radon Monitor) to measure both CH4 and CO2 and 222Rn concentrations, respectively. The variability of methane fluxes over this area and during the distinct phases of the rice production cycle was evaluated in this study using the Radon Tracer Method (RTM). The RTM was carried out using (i) nocturnal hourly atmospheric measurements of CH4 and 222Rn between 2013 and 2019 and (ii) FLEXPART-WRF back trajectories coupled with radon flux maps for Europe with a resolution of 0.05° × 0.05° available thanks to the project traceRadon. Prior to the calculation of methane fluxes by RTM, the FLEXPART-WRF model and the traceRadon flux maps were evaluated by modelling atmospheric radon concentrations at the DEC station and comparing them with observed data. RTM-based methane fluxes show a strong seasonality with maximums in October (13.9 mg CH4 m−2 h−1), corresponding with the period of harvest and straw incorporation in rice crop fields, and minimums between March and June (0.2 to 0.6 mg CH4 m−2 h−1). The total estimated methane annual emission was about 262.8 kg CH4 ha−1. These fluxes were compared with fluxes directly measured with static accumulation chambers by other researchers in the same area. Results show strong agreement between both methodologies, having both a similar annual cycle and similar monthly mean absolute values.