The effects of water table draw-down on the hydrology of a patterned fen peatland near Quebec City, Quebec, Canada

Abstract

Hydrological response to climate change may alter the biogeochemical role that peatlands play in the global climate system, so an understanding of the nature and magnitude of this response is important. Simple hydrological models have predicted the effects of climate change on the hydrology of these systems, and estimated a ~20 cm water table draw down. This draw down amount was modeled to estimate the changing role that wetlands may play in global biogeochemical cycling, but failed to account for modifications of the peatland structure, which has profound implications for the hydrology of these systems. Volume change in compressible soils occurs as the result of different processes, mainly compression and oxidation. Compression occurs instantaneously as a change in water pressure (e. g. , from water table draw down) occurs and the peat matrix is unable to withstand the increased pressures and subsides. Oxidation is the long term chemical breakdown of the peat under aerobic conditions. <br /><br /> Consequently, in 2002 the water table in a fen peatland near Quebec City was lowered by ~20 cm (Experimental site), and the hydrological response was measured compared to a Control (no manipulation) and Drained site (previously drained c. 1994). <br /><br /> As a result of the draw-down, the surface in the Experimental pool decreased 5, 15 and 20 cm in the ridge, lawn and mat topographic locations, respectively resulting in an increased bulk density of ~60% in the Experimental lawn. Hydraulic conductivity (K) generally decreased with depth and from Control (25 to 125 cm) 10^-1 to 10^-5 cm s^-1 to Experimental (25 to 125 cm) 10^-2 to 10^-7 cm s^-1 and to Drained (25 to 75 cm) 10^-2 to 10^-6 cm s^-1. In similar topographic locations (ridge, lawn, mat), K trended Control>Experimental>Drained, usually by an order of magnitude. <br /><br /> Water table fluctuations in the Drained site were, on average, twice that of the Control site, whereas water table fluctuations within sites trended ridge>lawn>mat. The water table in the Control lawn was able to remain at a stable depth relative to the surface (~ -1 cm) because the lawn peat floats with changes in water table position. However, because of the denser, degraded peat, the Drained lawn peat was more rigid, forcing the water to fluctuate relative to the surface, further enhancing peat decay and densification. <br /><br /> While climatic change will not occur instantaneously the limitations of the experiment required an abrupt change in water table position (drainage). However, regardless of how volume change occurs in the peat (compression or oxidation) the direction of change to the hydraulic properties is the same (increased bulk density, decreased hydraulic conductivity) which affects the hydrology of these systems (increase water table fluctuations and decreases surface movement). Thus, valuable information can be obtained regarding the changing role of peatlands in global biogeochemical cycling processes.