## 3 Results and Discussion
### Seasonal Variations in Biophysical Factors and CH\({}_{4}\) Fluxes  
The seasonal variations in biophysical factors and CH\({}_{4}\) fluxes are shown in Figures 4 and 5. Owing to instrument failure, the data of net solar radiation (R\({}_{\rm n}\)) for early season 2020 was lost. The net solar radiation showed decreasing/increasing trends during the growing seasons of late/early rice that were similar to those of T\({}_{\rm a}\). The soil temperature at 5 cm depth (T\({}_{\rm s5}\)) differed slightly from T\({}_{\rm a}\) and fluctuated less. Early rice in 2020 remained flooded until the middle of the reproductive stage. Late rice in 2018 was flooded except for some drying downs in the reproductive stage. Owing to severe drought, late rice in 2019 experienced several periods of dry and wet alternates. Late rice in 2020 was flooded except for mid-season drainage, as well as ripening stages. R\({}_{\rm eco}\) varied similarly to T\({}_{\rm a}\) in both early and late rice. GPP and NEE increased in the vegetative stage, declined slightly  
Figure 2: Diurnal variations in half-hourly CH\({}_{4}\) fluxes in the vegetative, reproductive and ripening stages of the rice growing seasons of 2018–2020 late rice and 2020 early rice. The grey shading represents the standard deviations of the half-hourly CH\({}_{4}\) fluxes. in the reproductive stage and decreased slowly in the ripening stage. The GPP and NEE of early rice were slightly lower than those of late rice. Large seasonal variations in daily CH4 were observed in the two double-cropping paddy fields (Figure 5a). In late rice in 2018, the paddy field was flooded six days before transplanting, and the daily CH4 fluxes decreased gradually after transplanting. The mean CH4 fluxes in the vegetative stage, reproductive stage and ripening stage were 0.42, 0.09, and 0.01 umol m\({}^{-2}\) s\({}^{-1}\), respectively, and the largest daily CH4 flux was 0.71 umol m\({}^{-2}\) s\({}^{-1}\) on the first day after transplantation. In late rice in 2019, the CH4 fluxes increased rapidly after basal fertilizer application, reached maximum daily emissions of 1.304 umol m\({}^{-2}\) s\({}^{-1}\), and declined rapidly thereafter. Then, the CH4 fluxes increased from 2 August and reached a second peak before mid-season drainage. During this period, the temperature and SWC fluctuated slightly, while GPP increased significantly. Therefore, it is assumed that the increase in daily CH4 flux was related to the growth of rice plants (Figure 5d). The CH4 fluxes declined sharply during mid-season drainage and remained low after mid-season drainage. The average CH4 fluxes during the vegetative stage, reproductive stage, ripening stage and fallow season were 0.345, 0.046, 0.011 and 0.001 umol m\({}^{-2}\) s\({}^{-1}\), respectively. In early rice in 2020, the CH4 fluxes increased gradually 30 days after seeding, declined slowly during the reproductive stage, and remained low at approximately 0.02 umol m\({}^{-2}\) s\({}^{-1}\). Figure 3: The wavelet coherence of CH4 flux with (**a**) gross primary production (GPP), (**b**) air temperature (T\({}_{\rm a}\)) in 2018 late rice, (**c**) GPP, (**d**) T\({}_{\rm a}\) in 2019 late rice, (**e**) GPP, (**f**) T\({}_{\rm a}\) in 2020 early rice, (**g**) GPP, (**h**) T\({}_{\rm a}\) in 2020 late rice. The direction of the arrows indicates the phase angle between the two time series; an arrow with an inclination of zero points to the right indicates zero lag (i.e., the series are perfectly correlated). The average CH\({}_{4}\) fluxes during the vegetative stage, reproductive stage and ripening stage were 0.084, 0.232, and 0.022 umol m\({}^{-2}\) s\({}^{-1}\), respectively. The first peak occurred in the vegetative stage, at 0.380 umol m\({}^{-2}\) s\({}^{-1}\), while the second occurred in the reproductive stage, at 0.377 umol m\({}^{-2}\) s\({}^{-1}\). In late rice in 2020, CH\({}_{4}\) fluxes increased rapidly after seeding, reaching the first peak of 0.952 umol m\({}^{-2}\) s\({}^{-1}\) at 11 days after seeding.