Assessing modelled methane emissions over northern wetlands by the JULES-HIMMELI model

Northern wetlands are considered to be one of the most significant natural sources of methane (CH₄) emissions. The default wetland CH₄ emission scheme in JULES, a current state-of-art land surface model, only takes into account the CH₄ emissions from inundated wetland areas in a simple manner based on soil temperature and substrate availability. In this work, a process-based peatland CH₄ emission model HIMMELI was integrated with JULES, and the HIMMELI parameters were optimized with measured CH₄ flux at six northern wetland sites for each site separately or multi-sites simultaneously. The simulated CH₄ emission was significantly improved when using the optimized parameter values, with the bias of 54.88 mg m^-2 d^-1 averaged across all the studied sites in the simulation using the default parameter (DPR) values being reduced to -0.70 mg m^-2 d^-1 in the simulations using parameters values derived from the single site optimization (SSO) for each site. In the simulations using parameters values from the averages of single site optimization (SSO_AVG) and the multi-site optimization (MSO), the biases averaged across all the studied sites were -7.39 mg m^-2 d^-1 and -8.36 mg m^-2 d^-1, respectively. The MSO simulations demonstrated more stable root mean square error (RMSE) between the simulated and observed methane emissions than the SSO_AVG simulations over the studied sites, when the RMSEs of SSO simulations were used as reference points. To further reduce the uncertainties in the simulated CH₄ emissions by the JULES-HIMMELI model, model processes related to the environment conditions (e.g. water table, soil carbon and vegetation) of wetland and northern wetland CH₄ emission processes (e.g. snow and ice covering effect) are suggested to be improved in JULES and HIMMELI, respectively. This study presents a comprehensive analysis of the impact of different parameters on the CH₄ emission in the JULES-HIMMELI model and obtains optimal parameter values for modelling CH₄ emissions at the studied northern wetlands. These findings pave the way for accurate regional estimates of northern wetland CH₄ emission.