Unsaturated hydraulic properties of Sphagnum moss and peat reveal trimodal pore‐size distributions

• Published in: Water resources research 2017-01, Vol.53 (1), p.415-434
• Main Authors: Weber, Tobias K. D., Iden, Sascha C., Durner, Wolfgang
• Format: Article
• Language: English
• Subjects: Atmospheric models; Capillary flow; Carbon; Carbon sources; Computer simulation; Decomposition; Diffusion; Diffusion rate; Dye dispersion; Evaporation; evaporation experiments; Experimental data; Fluxes; Gardens & gardening; Hydraulic properties; Hydraulics; inverse modeling; Laboratories; Mathematical models; Modelling; Moisture content; Mosses; multimodal pore‐size distribution; Organic matter; Oxidoreductions; Oxygen; Parameterization; Peat; Peatlands; Pressure; Properties; Redox properties; Richards equation; Saturated flow; saturated hydraulic conductivity; Series (mathematics); Sinkholes; Size distribution; Soil; soil hydraulic properties; Soil moisture; Sphagnum; Time measurement; Time series; unsaturated hydraulic conductivity; Vadose water; Water flow; water retention
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1002/2016WR019707

In ombrotrophic peatlands, the moisture content of the vadose zone (acrotelm) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Whether peatlands act as sinks or sources of atmospheric carbon thus relies on variably saturated flow processes. The Richards equation is the standard model for water flow in soils, but it is not clear whether it can be applied to simulate water flow in live Sphagnum moss. Transient laboratory evaporation experiments were conducted to observe evaporative water fluxes in the acrotelm, containing living Sphagnum moss, and a deeper layer containing decomposed moss peat. The experimental data were evaluated by inverse modeling using the Richards equation as process model for variably‐saturated flow. It was tested whether water fluxes and time series of measured pressure heads during evaporation could be simulated. The results showed that the measurements could be matched very well providing the hydraulic properties are represented by a suitable model. For this, a trimodal parametrization of the underlying pore‐size distribution was necessary which reflects three distinct pore systems of the Sphagnum constituted by inter‐, intra‐, and inner‐plant water. While the traditional van Genuchten‐Mualem model led to great discrepancies, the physically more comprehensive Peters‐Durner‐Iden model which accounts for capillary and noncapillary flow, led to a more consistent description of the observations. We conclude that the Richards equation is a valid process description for variably saturated moisture fluxes over a wide pressure range in peatlands supporting the conceptualization of the live moss as part of the vadose zone. Key Points Soil hydraulic properties of Sphagnum moss and peat were identified by inverse modeling of transient laboratory evaporation experiments Sphagnum moss and peat both have distinct trimodal pore size distributions which were represented with soil hydraulic property functions Evidence is provided that the Richards equation can be considered as a valid process model for the acrotelm of peatlands