Modeling peatland hydrology and related elastic deformation

Reversible surface displacements occur at time scales of hours in peatlands that are subject to drying and rewetting cycles. This phenomenon is related to the long term subsidence caused by biological oxidation of the organic matter under aerobic conditions that peat drainage for agricultural purposes induces in temperate regions. Modeling land subsidence of peatland surface requires, at a preliminary stage, the separation between the elastic deformation related to water table and soil moisture changes and the irreversible sinking of land due to peat oxidation. In this paper peat hydrology is modeled together with soil deformation in a one-dimensional column subject to soil moisture fluctuations due to precipitation, evapotranspiration, and drainage. The test case is representative of a field test bog located south of Venice (Italy), where hydrologic and displacement data have been continuously recorded since November 2001. A linear, one-dimensional pattern of elastic peat swelling/shrinkage relating the soil deformation to the gravimetric water content is implemented by coupling a variably saturated finite element flow model to a soil volume change equation. In this study the oxidation of peat is ignored, the time scale of water table fluctuations being much less than that of the long term subsidence process. The comparison of the numerical results with the large dataset available from the Venice monitoring site allows for the calibration of the hydraulic and mechanical parameters of the soil and assesses the validity of the proposed deformation model.