Effective hydraulic properties of unsaturated sphagnum moss and peat

English Abstract:

The moisture state of the vadose zone (acrotelm) in ombrotrophic peatlands decisively determines whether carbon is contained in soil organic matter or released to the atmosphere. As the pore space is variably saturated with water throughout the year, oxygen diffusion, heat, and solute transport and thus the redox state are a function of water content over time. With computer models predictions are possible by establishing a link between the terrestrial water cycle and the carbon cycle. This requires proper representation of effective soil hydraulic properties which are a mandatory input to the Richards equation, the standard model for variably-saturated flow processes in porous media. By applying the Richards equation in peatland hydrology the assumption is madethat the acrotelm may be conceptualised as a rigid porous material. To test this approximation and to select the most adequate set of soil hydraulic property functions, we conducted a series of specifically designed laboratory evaporation experiments on sphagnum moss and decomposed sphagnum peat. Sampling was carried out on a vertical profile in five in a bog in the Harz mountains. We selected sphagnum moss as it is a very common plant species colonising bogs of the Boreal. To test the adequacy of different parameterizations of soil hydraulic property functions inverse modelling was used. We used pressure head data measured at two depths in the objective function to identify soil hydraulic properties. The Richards equation served as the process model decribing the unsaturated water fluxes. We critically assess the applicability of the van Genuchten/Mualem model, which finds frequent application in peatland hydrology, and discuss alternatives which account for (1) multimodal pore size distributions, (2) physical plausibility towards the dry end, (3) capillary and non-capillary storage and flow, and (4) isothermal flow of water vapour. Finally, our results indicate that applying the Richards equation to water flow under evaporation conditions in sphagnum moss and peat is a feasible approximation.