## 3 Results and Discussion
### Effect of Drainage on CH\({}_{4}\) Fluxes  
As shown in Figure 5a, daily CH\({}_{4}\) fluxes declined sharply after mid-season drainage in 2019 late rice. This phenomenon seemed to occur because CH\({}_{4}\) oxidizes when exposed to air. Meanwhile, electron acceptors in the aquifer become reduced, so the soil environment is not favourable for methanogenic bacteria even when the paddy field is refolooded [40]. This finding is consistent with that of [PERSON] [70], who observed that if paddy fields were drained for more than two days, CH\({}_{4}\) fluxes would not be restored to their emission level before mid-season drainage. Similarly, [PERSON] et al. [71] reported that the pattern and magnitude of seasonal CH\({}_{4}\) fluxes in the whole growing season varied with mid-season drainage and that up to 80% of CH\({}_{4}\) fluxes would be reduced when soils experienced aerobic conditions for 6-10 days in paddy fields. [PERSON] et al. [31] also reported that mid-season drainage decreased CH\({}_{4}\) emissions from an irrigated rice paddy. However, [PERSON] et al. [1] showed that after 21, 12 and 15 days of mid-season drainage in paddy fields in Korea from 2016 to 2018, the peak daily CH\({}_{4}\) fluxes were greater than those before mid-season drainage. The finding was different from those presented in this study and may be attributable to differences in the cropping system and regional climate. The paddy fields in Korea applied a single-season system in which the T\({}_{\rm a}\) increased continuously for a while after mid-season drainage, while the paddy fields in China applied a double-cropping rice system in which the T\({}_{\rm a}\) tended to decrease throughout the growing season. Combining our observations with those of others, we concluded that the changes in daily CH\({}_{4}\) fluxes after mid-season drainage are controlled by multiple factors and are thus site dependent.  
Mid-season drainage was performed during the four seasons observed in this study. In addition to drainage, natural field drying also caused the disappearance of the water layer in the field. The daily CH\({}_{4}\) fluxes showed a brief rise followed by a sharp fall on 26 August 2019, 19 September 2019 and 23 July 2020, while the daily CH\({}_{4}\) fluxes declined directly after the disappearance of the water layer on 9 September 2018 and 15 June 2020. Taking the mid-season drainage on 26 August 2019 as an example, the half-hourly CH\({}_{4}\) fluxes, T\({}_{85}\) and SWC are shown in Figure 6. On 26 August, the CH\({}_{4}\) fluxes increased, SWC decreased and T\({}_{85}\) varied little, implying that the production of CH\({}_{4}\) did not change; thus, the increase in CH\({}_{4}\) fluxes on 26 August was mostly likely attributable to the increase in transport efficiency. This finding could be explained by the fact that when the standing water had just disappeared, the soil in the paddy field experienced anaerobic conditions, and the resistance that CH\({}_{4}\) gas needed to overcome to move from the soil to the air was reduced; therefore, the transport efficiency for CH\({}_{4}\) increased [41]. Then, with the continuous decline in SWC, the soil environment changed from anaerobic to aerobic, resulting in a lower rate of methane production and a faster rate of oxidation; consequently, CH\({}_{4}\) fluxes decreased. This finding was also reported by [PERSON] et al. [66], [PERSON] et al. [49] and [PERSON] et al. [41]. Researchers have also found that the magnitude and duration of the increase in CH\({}_{4}\) flux depended on how much CH\({}_{4}\) was stored in the soil before mid-season drainage.