Estimation of Unsaturated Hydraulic Properties of Peat Soils From Evaporation Method and Multi-Step Outflow Method.

Knowledge of water and solute transport in unsaturated peat soils are needed for effective management of wetlands and surrounding-agriculture fields. Inverse modeling has increasingly been used for estimating unsaturated soil hydraulic functions. However, reports dealing with parameter estimation of peat soils using inverse methods are scarce. The objective of this study was to determine inversely unsaturated hydraulic properties of peat soils from data collected using evaporation(EVAP) and multi-step outflow(MSTO) methods. The Bibai bog(141‹48`E, 43‹19`N) is located in the northeast of Hokkaido, Japan. Soils at the site are categorized high moor-peat soils. 100 cm3 soil core samples are taken at 10, 30, and 50 cm depths in June 2008. Groundwater level was about 40-50 cm below the surface. After the MSTO method, the same soil samples were saturated again for the EVAP method. The measured soil water retention(WRC) was determined by hanging-water method and pressure-plate method. HYDRUS-1D was used to analyze data to estimate soil unsaturated hydraulic parameters. In the MSTO method, outflow volumes increased substantially from 0 to -10 cm pressure head without a clear air entry value. Therefore, at the high pressure head, the measured WRC from the MSTO method would be in poor agreement with the estimated one from the EVAP and MSTO method. In contrast, at the low pressure head, the measured WRC from both methods were in good agreement with the estimated ones. Relatively-high residual moisture content values estimated from both methods were found for soils collected from 10 and 30 cm depths. When values of saturated water content (ƒÆs) were optimized in the MSTO procedure, the estimated WRC was much lower than the measured WRC. In the EVAP method, however, the estimated WRC was higher than the measured WRC. ƒÆs needs to be a fitting parameter for peat soils when the MSTO method is used.