The effect of forest harvesting on streamflow recession curves at Carnation Creek, British Columbia Nelson, Brett
At the Carnation Creek Experimental Watershed on southwestern Vancouver Island, British Columbia, the effect of harvesting, regeneration and road building were analyzed through the use of stream discharge data collected at a weir on the catchment outlet. The study was separated into a Pre-Logging period from 1971-1975, Logging from 1976-1981, and two Post-Logging periods from 1982-1985 and 1985-90 respectively. The current study focussed on the effects of harvesting on streamflow recession curves, which are an indicator of the ability of coastal watersheds to maintain low flows (base-flow) during the water-limited dry season. Approximately 30 years of discharge data along with rainfall and temperature were segmented into corresponding forestry operation periods. Using the linear relationship between log[|dQ/dt|] and log[Qm] according to storage-discharge theory, multiple linear models were created and a regression was used to investigate the significance of each of the forestry operations. It was found that the effect of the roads increased lateral slope interception of sub-surface flow and directed water along the ditch systems to the channel at a greater rate, steepening the recession curves at all discharge levels in the short term, but only persisting at low discharge levels. Harvesting increased the water table height, because of the reduction in transpiration via loss of interception, and low flows and total flows increased over both post-logging periods, which partially offset the effect the roads had on the recession. Regeneration began to occur over the harvested sections of the catchment and it was found their effect between post-logging periods was only significant with the inclusions of extremely low discharge levels. However, a second logging pass in 1987, removing 21% of the forested catchment in the headwaters is believed to have confounded the regeneration effects.
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