Long Term Peatland Subsidence: Experimental Study and Modeling Scenarios in the Venice Coastland

Land subsidence in drained cultivated peatlands is responsible for a number of serious environmental concerns and economical problems at both the local and the global scale. In low-lying coastal areas it enhances the risk of flooding, the saltwater contamination of shallow aquifers, and the maintenance costs of the systems that help keep the farmland drained. Since the subsidence is a major consequence of the bio-oxidation of the soil organic fraction in the upper aerated zone, cropped peatlands in temperate and tropic regions are important sources of CO2 into the atmosphere. A 4-year long experimental study has been performed in a drained peatland located south of the Venice Lagoon, Italy, to help calibrate a land subsidence model developed to predict the expected behavior of the ground surface elevation. Continuous monitoring of the hydrological regime and land displacements shows that the vertical movement of the peat surface consists of the superimposition of daily/seasonal time-scale reversible deformations related to soil moisture, depth to the water table, and temperature fluctuations, and long term irreversible subsidence due to peat oxidation. A novel two-step modeling approach to separate the two contributions from the available observations is presented. First, the elastic component is computed by integrating the peat vertical deformations evaluated by a constitutive relationship describing the porosity variation with the moisture content and pore pressure changes implemented into a variably saturated flow equation-based numerical code. The observed trend is then filtered from the computed reversible displacement and is used to calibrate an empirical relationship relating land subsidence rate to drainage depth and soil temperature. The results show that in recent years the subsidence rate ranged from 3 to 15 mm a−1. The large variability is due to the different climate conditions underlying the monitoring period, in particular a wet 2002 and a very dry 2003. The model is then used to investigate the effect of human activities and climate change on the expected reduction of the peat thickness. The results suggest that the long term subsidence is mainly controlled by the manner in which the water table is managed in the peatland. For example, the subsidence rate is almost halved if the current 0.5 m mean water table depth is kept at 0.2 m. Conversely, even the extreme warning scenario provided by the IPCC in 2007 do not suggest significant changes on the expected subsidence trend, at least in low lying reclamation areas where the average temperature rise should not be accompanied by a significant reduction of water availability.