Importance of shrinkage and compression in determining water storage changes in peat: the case of a mined peatland

Abstract

This study examines changes in peat volume in a mined peatland near Lac St Jean, Quebec, during the spring and summer of 1995 and 1996, and the implication for water storage changes. Lowering of the water table caused drainage above the water table, but the specific yield (S y ) of the peat was relatively small (0.48), and did not adequately describe the water storage change. Lowering of the water table also caused surface subsidence, which was shown to be partly due to shrinkage above the water table (<3.6 cm), and partly due to compression of the saturated peat (about 6 cm). Measured total surface subsidence ranged from 6.5 to 10 cm. The change in peat volume occurred over the entire depth of the peat deposit (h), and so the storativity due to peat compression (bS s ), estimated to be 0.13, was more important than specific yield in determining water storage changes. Total storativity (S tot ) was best estimated as the sum S y + bS s . Changes in peat volume in the 0-3 cm layer were evident in the temporally variable bulk density (83-101 kg m -3 ). Its relationship with volumetric moisture content was highly hysteretic, reflecting the complexity of the process in the unsaturated zone. However, there was a linear relationship with water tension, suggesting a more direct causal relationship. Changes in peat volume were also recorded below the water table, as the volumetric water content of the saturated peat decreased by 3.5% over the season. Since this applied to a relatively thick layer of peat, its total effect was greater than shrinkage in the zone above the water table. It was concluded that most peatland water balances should make account of storage changes associated with peat volume changes, and that peat volume changes may increase the water limitations to plants when the water table drops below the surface.