Case Studies of Modelling CH4 Fluxes from Chinese Coastal Wetlands

English Abstract:

Coastal wetlands are important CH₄ source to the Atmosphere. The coastal wetlands accounts for ~10% of the total area of natural wetlands in China, but the size of this source remains highly uncertain. We modified a biogeophysical model CH4MOD_wetland by adding submodels to describe the influence of salinity on CH₄ production as well as CH₄ diffusion, so that it can be used in the coastal wetlands. The modified model was then validated against independent field measurements from different coastal wetland sites, including flooding tidal marshes from Liao river delta in Liaoning Province and Chongming island in Shanghai Province, as well as mangroves from Guangdong Province and Hainan Province. Model validation showed that CH4MOD_wetland can generally simulate the seasonal and annual variations of CH₄ emissions in different years from different coastal wetlands. But it overestimated CH₄ fluxes from mid-June to mid-July 1997 at LH site and underestimated CH₄ fluxes from May to June 2012 at CM site. The regression of the computed versus observed total seasonal CH₄ emissions (Fig. 4b) resulted in an R² of 0.99, with a slope of 1.09 and an intercept of -0.67 g m⁻² (n=8, p < 0.001). The RMSE, RMD and EF between the simulated and observed values were 15.7%, 3.5% and 0.98, respectively. Model sensitivity analysis showed that at the no flooding site, water table depth is the most sensitive environmental factors to CH₄ emissions. Increase of salinity will reduce CH₄ fluxes. In general, the CH4MOD_wetland model can simulate seasonal CH₄ emissions from different types of coastal wetlands under various conditions (climate, water, soil and plant species). However, key processes such as the plant growth should be enhanced in future so that it can be more accurate in simulating seasonal variations of CH₄ emissions.