Potential of prolonged midseason drainage for reducing methane emission from rice paddies in Japan: a long-term simulation using the DNDC-Rice model

Water management practices, such as midseason drainage (MD) and intermittent irrigation, are effective in reducing methane (CH4) emission from irrigated rice paddies. In a previous study in which two-year field experiments were conducted at nine sites across Japan, prolonged MD was found to reduce the seasonal total CH4 emission by 30.5 ± 6.7 % (mean ± 95 % confidence interval) compared with conventional MD without compromising rice grain yield. However, the degree of CH4 reduction by water management is dependent on prevailing weather conditions. To estimate the mean effect of prolonged MD on CH4 emission at the nine sites with rice straw application, we conducted a long-term (20 years) simulation using a process-based biogeochemistry model, the DNDC-Rice. The model adjusted using site-specific parameters was able to reproduce the measured magnitude of the total CH4 emission and the suppressive effect of prolonged MD. The number of nonrainy days during MD explained the degree of CH4 reduction for each site and all sites combined. In the simulation, mean reduction percentage was 20.1 ± 5.6 % when acceptable prolonged MD (i.e., having less than 15 % yield loss) was applied compared with conventional MD. The discrepancy of the percentage between measurement and simulation was primarily attributable to longer nonrainy days during prolonged MD at several sites in the measurement than the mean of 20-year simulation. We therefore conclude that the long-term simulation better represents the mean reduction percentage of CH4 emission by prolonged MD relative to conventional MD at the nine sites across Japan.