Modeling moisture retention in peat soils

Soil moisture governs many biogeochemical processes in peatlands. Modeling of those processes relative to climate and anthropogenic influences requires knowledge of the basic hydraulic properties of different peat soils in a function form. Water retention of undisturbed surface peat samples, collected at four depths at each of 38 undrained and drained pine (Pinus sylvestris L.) mire sites, was measured for suction pressures of -0.98, -3.10, -6.19, -9.81, -98.1, and -1554.25 kPa. The obtained data were used to test several well-known water retention models commonly applied to mineral soils. The most suitable model was found to be van Genuchten's model if the residual water content was omitted. Peat characteristics were used to explain the variation in the model's shape parameters. Accounting for the remains of Sphagnum, Carex, Eriophorum, and lignin and the distinction between shallow and deep peat layers considerably improved the moisture retention predictions compared with using bulk density only. The different behavior of the shallow vs. the deep peat layers was mainly attributed to the Sphagnum and lignin residues, but not to the Carex residues. We developed a semiempirical model with only one shape parameter, which was clearly better explained by the peat characteristics than the two shape parameters of the van Genuchten model. We recommend that for statistical investigations or investigations requiring a robust model, the semiempirical model be used. The van Genuchten model is to be preferred in predicting the moisture conditions near saturation.