Appropriate frequency and time of day to measure methane emissions from an irrigated rice paddy in Japan using the manual closed chamber method

Rice agriculture is a large anthropogenic source of atmospheric methane (CH₄). The reliable estimation of CH₄ emissions requires frequent measurements to trace diurnal and seasonal variations. To evaluate the appropriate intervals and optimal time of day to measure CH₄ emissions using the manual closed chamber method, this study analysed four datasets of CH₄ fluxes in a Japanese irrigated rice paddy measured at 2-hour intervals using the automated closed chamber method. The typical diurnal variation in the CH₄ flux was observed after the rice's heading stage, during which the daily time-weighted mean CH₄ flux was observed twice, in the 08:00–11:59 and 18:00–21:59 time windows. During the flooded rice-growing period, the CH₄ emissions, which were estimated by weekly measurements once per day during the 10:00–11:59 time window, corresponded to 93–106% of the emissions calculated using the automated measurement method. In contrast, no regular measurement strategies produced a satisfactory estimate of the CH₄ emissions during the non-flooded rice-growing period because of a sharp increase in the CH₄ flux just after the drainage. Consequently, the combination of weekly measurements once per day at approximately 10:00 as local mean time for the flooded rice-growing period and daily measurements once per day during the daytime for 1 week after each drainage event is recommended as a strategy to obtain the estimation with a ±10% error.