Supplementary material to "Modelling northern peatlands area and carbon dynamics since the Holocene with the ORCHIDEE-PEAT land surface model (SVN r5488)"

Abstract

In the model, energy and water exchanges between land and the atmosphere, and the soil water budgets are simulated at a 30 min time step. The parameterization of soil water dynamics is from De Rosnay et al. (2000, 2002). Vertical diffusion of water in the 11 soil layers (with a total depth of 2.0 m) of the soil column is solved by the Fokker-Planck equation, with hydraulic conductivity and diffusivity defined by the Mualem-Van Genuchten model (Van Genuchten, 1980; Mualem, 1976). Processes related to the carbon cycle such as plant phenology, photosynthates allocation, litter and soil C decomposition are calculated at a daily time step (Krinner et al., 2005). A new plant functional type (PFT) with shallow roots was introduced for peatland vegetation (Largeron et al., 2018). In grid-based simulations, peatland is a sub-grid hydrological soil unit (HSU) that receives surface runoff from surrounding nonpeatland HSU in the same grid cell, and has its bottom drainage flux reduced to zero, thus a high water content can be maintained in the peatland HSU where the standing water above the soil surface can reach up to 10 cm (Largeron et al., 2018). Qiu et al. (2018) improved the representation of peatlands in a revision referred to as ORCHIDEE-PEAT by implementing peat-specific hydraulics in the peatland HSU with high water content at saturation and high saturated hydraulic conductivity, while hydraulic parameters of non-peatland HSUs are determined by the dominant soil texture in the grid cell. Text S2. Following the CENTURY model (Parton et al., 1988), prescribed fractions of plant residues are added to the metabolic and structural litter pools. Litter from leaf and fruit of peat PFT is added to the top layer of litter pools, while belowground litter from root is added in depth discretized according to the exponential root profile of the peat PFT. To account for the fact that peatland vegetation develops shallow and extensive root systems to survive in the high stress conditions (a scarcity of oxygen and nutrients; spongy, acid soil) (Boutin and Keddy, 1993), the exponential root profile of the peat PFT is set an e-folding length of 30cm (Largeron et al., 2018). The fraction of litter that