Dynamic Non-Equilibrium Water Flow in Sphagnum Moss and Peat.

It is necessary to characterize the parameters governing unsaturated flow in mosses, since their living upper capitula layer relies on unsaturated vertical water transfer to maintain a critical water storage threshold for photosynthetic activity. Being the standard process model to simulate variably saturated water flow in soils, the Richards equation finds widespread application in vadose zone hydrology. With a long lasting tradition in mineral soils, there is evidence on its applicability in organic soils. An example is the successful inverse modelling of transient evaporation experiments on sphagnum moss and peat over a wide moisture range. However, experiments devised to assess the hydraulic properties of organic soils reveal “dynamic effects” expressed by non-equilibrium conditions between water content (?) and water potential (h). These dynamic effects have not been fully evaluated in organic soils. Based on previously established steady-state methods specifically adapted to organic soils, we conducted transient multistep outflow experiments on Sphagnum moss and peat, with the additional step of measuring h. By changing the pressure at the lower boundary, a prolonged relaxation in the sample’s water content occurs. There is a whole suite of reasons for this, and this cannot be simulated using a classical Richards’ parameterisation. To accommodate for this, such as the dual-fraction non-equilibrium (DNE) model has been show to successfully simulate transient multi-step outflow experiments. It is promising, in that it can be also be used for a variety of boundary condition types. Therefore, the objectives were to i) investigate the applicability of the Richards equation under these boundary conditions, ii) investigate the question of the existence of “dynamic effects”, and iii) test the DNE model for a variety of boundary conditions types on experimental data from sphagnum moss and peat soils.