Hydraulic properties of drained and cultivated fen soils part II - Model-based evaluation of generated van Genuchten parameters using experimental field data

Drained and cultivated fen soils are characterised by continuous alteration of soil structure, which considerably complicates the precise simulation of soil water dynamics. Recently estimated van Genuchten parameters considering the gradient of moorsh-forming process were tested by hydrological modelling using HYDRUS-1D. Simulation was conducted for altogether four fen soil profiles of two peatland test sites varying in profile structure, land-use history and climate. Two parameter sets were applied to characterise the hydraulic properties of the fen soil profiles. The first one consisted of so far available or locally measured parameters, the second one of the new parameters, respectively. The quality of model output was assessed by comparing simulated and observed soil water contents or pore water pressure heads in consideration of the applied parameter sets. Model performance varied considerably for the fen soil profile under grassland use, where the mean normalised root mean square error (RMSEN) was 16.0% for the original parameters and 52.9% for the optimised ones. Contrasting results appeared due to the complex stratigraphy at the corresponding plot. For the rewetted plot formerly used as grassland, both parameter sets achieved comparable simulation results for all observation depths (RMSEN = 17.4%, 18.4% and 23.6%). When the locally measured hydraulic parameters were applied to the unused fen soil profiles solely influenced by drainage RMSEN decreased in average from 38.5% to 27.4% and 30.5% to 25.7%, respectively. However, for specific observation depths model's performance improved by using the optimised parameters. The problem of wettability of moorshified fen soil horizons was identified for one soil profile. Additionally, the new parameters created a more robust reaction in simulation when the residual water content was adjusted.