Water and energy fluxes from a permafrost peat plateau: examining the controls on runoff generation

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Water and energy fluxes from a treed peat plateau in a wetland-dominated discontinuous permafrost basin near Fort Simpson, NWT, Canada, were examined to determine the factors controlling runoff generation from peat-covered permafrost slopes. A water balance approach and the Dupuit-Forchheimer equation were used to quantify subsurface runoff from the peat plateau during spring. These two computations yielded similar results in both years of study (2004-2005), and showed that runoff accounted for approximately half of the moisture loss from the peat plateau, most of which occurred in response to snowmelt inputs. The melt of ground ice was also a significant source of water, which was largely detained in soil storage. The timing and magnitude of runoff was found to be dependant on the amount of water input, antecedent moisture conditions, the saturated hydraulic conductivity of the soil, and frost-table depth. The distribution of frost-table depths on the peat plateau was examined over four consecutive years (2003-2006) at a variety of spatial scales, to elucidate the role of active-layer development on runoff generation. Frost-table depths were highly variable over relatively short distances (0.25 - 1 m), and the spatial variability was strongly correlated to soil moisture distribution, which was partly influenced by lateral flow converging to frost-table depressions. On an inter-annual basis, thaw rates were temporally correlated to air temperature and the amount of precipitation input. Simple simulations show that lateral subsurface flow is governed by the frost-table topography having spatially variable storage that has to be filled before water can spill over to generate flow downslope. The annual surface energy balance and thermal regimes of the peat plateau and an adjacent permafrost-free wetland were compared to identify the site characteristics that control ground surface energy input rates. The plateau tree canopy reduced the amount of energy available for ground thaw by 14% in summer, when compared to the tree-less wetland. The ground heat flux (Qg) was 54% greater than at the bog, largely because the plateau had a much steeper soil temperature gradient than the bog, and the bog released a large fraction of Q* as latent heat of evaporation (Qe).
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