Long term peatland subsidence: Experimental study and modeling
scenarios in the Venice coastland
F. Zanello, P. Teatini, M. Putti, G. Gambolati
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
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 timeolsiscale 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.
