Hydraulic properties of drained and cultivated fen soils part I - Horizon-based evaluation of van Genuchten parameters considering the state of moorsh-forming process

Abstract Soil hydraulic properties of drained and cultivated fen soils vary considerably due to structural and physical alteration in the long-term. Although hydrological modelling is a key method to determine soil water conditions for a large number of aims, for instance the utilization of fen soils, general estimators of water retention that reflect the progressive nature of fen soil alteration are rare. Based on 405 horizons from 30 fen soil profiles at 12 different peatland sites in Germany soil hydraulic properties were estimated. Water retention and bulk density measurements were conducted fivefold per each horizon mapped after the German Soil Taxonomy. Considering the varying phases of moorsh-forming process, bulk densities and water retention data were clustered according to horizon category by applying the k-means method. This approach reduced the range of water contents measured at considered pressure heads (pF) and recommended their subdivision into three groups of graduated bulk density (high, moderate, low) per each horizon category. This subdivision resulted in 18 separated data sets for parameterisation of the selected van Genuchten model using the RETC software. Estimated water content at saturation ranged between 0.68 and 0.90 cm 3  cm − 3 and 0.10 and 0.35 cm 3  cm − 3 at a pF-value of 4.2. Shape parameter α varied stronger than n , especially for the strongly altered fen soil horizons. The continuing alteration of the moorshified, earthified, aggregated, and shrunken fen soil horizons classified by increasing bulk density levels was reflected by the estimated van Genuchten parameters. For horizons inventoried as unchanged peat substrates, less variability of bulk densities in combination with deviating van Genuchten parameters indicated the influence of additional factors on soil hydraulic properties.