Representing peatland drainage-ditch hydrology impacts on CO₂ and CH₄ emissions in the ORCHIDEE-PEAT land surface model (r9703)

Abstract. Peatland drainage through ditching is the common practice for cultivating peatlands. Ditches reduce the water table in surrounding peat soils, reducing methane (CH₄) emissions from the peat surface and increasing soil carbon CO₂ emissions. But the ditch itself receives dissolved carbon and litter inputs and emits these two gases, an overlooked process in greenhouse gases (GHG) assessments. We present here a parameterization of drainage ditch hydrology and GHG emission processes within the ORCHIDEE-PEAT global land surface model to simulate CO₂ and CH₄ emissions from peatland drainage ditches. The model prescribes subgrid ditch area and depth in the fraction of each model grid cell covered by peat, and simulates lateral carbon transfers from soils to dissolved organic carbon (DOC), soil organic carbon (SOC) pools in the bottom of ditches, diffusion of dissolved gases, ditch water discharge, and kinetic gas exchange of GHG at the ditch air-water surface. Simulated gas exchange velocities and CH₄ emissions are within the range of field measurements and meta-analysis data. Our simulation results show that ditch emissions offset 7–13 % of drainage-induced peat soils CH₄ emission reductions. Simulated ditch CO₂ fluxes were small or negative, partly reflecting current model limitations in which lateral dissolved CO₂ transport from peat soil to ditch is not represented. This identifies dissolved CO₂ transport as an important process for further model development. This model development is transferable to other models and can contribute to more accurate assessments of peatland ditch drainage hydrology and GHG fluxes by enabling the simultaneous consideration of upland and aquatic ditch fluxes.