A strong El Niño significantly affects CH4, CO2, H2O and energy fluxes as well as grain yield in a Thai rainfed rice field

Rainfed rice fields are among the primary rice ecosystems in many Asian countries, including Thailand. High variability in rainfall amount and its distribution under climate change and climate variability, such as El Niño, has made this rice ecosystem highly vulnerable. Here, we report the impacts of El Niño on methane (CH₄), carbon dioxide (CO₂), water (H₂O), energy exchanges, and grain yield in a Thai rainfed lowland rice using the eddy covariance method during 2015–2017. During the rice-growing season, cumulative rainfall and effective rainfall were reduced in the 2015 strong El Niño year by approximately 18.3% and 19.8% compared to the neutral years (2016 and 2017), respectively. This resulted in a lower water level (WL) above the paddy surface and required more irrigation water (IWR), especially at the pre-booting stage. A correlation analysis revealed that WL had a strong relationship with CH₄ (r = 0.72) and gross primary production (GPP; r = 0.72) at the pre-booting stage. CH₄ emissions during the cropping season in the 2015 El Niño year were reduced by 56.77%. In 2015, gross primary production (GPP) during the cropping season was also reduced by 31.5% and 26.5% compared to 2016 and 2017, respectively, leading to a decrease in the annual net carbon (C) budget (58.2 g C m^-2), up to 66.8% in comparison with 2016 (143.86 g C m^-2). Moreover, a higher fraction of sensible heat (H) exchange as a higher Bowen ratio along with high net radiation (Rn) in 2015 led to higher evapotranspiration, especially in the post-booting stage. The strong El Niño in 2015 reduced grain yield by 2.8–5.7%, with less carbon use efficiency (CUE) and water use efficiency (WUE) relative to the non-strong El Niño year. Strong ENSO decreased net C budget and increased net GHG budget during the cropping season. An evaluation and prediction of the amount of rainfall for water supply during strong ENSO phenomena could be an effective measure before rice cultivation in rainfed paddies to prevent crop yield loss from climate oscillations.