The effects of cumulative forest disturbances on hydrology in the interior of British Columbia, Canada

Research on forest disturbances and their impacts on hydrology in large watersheds (>500 km2) is limited. Forest disturbances such as harvesting, wildfire and insect infestation are cumulative over space and time in large watersheds. The major objectives of this study were: 1) to assess the impacts of cumulative forest disturbances on annual and seasonal mean flows; 2) to evaluate the impacts of cumulative forest disturbances on flow regimes (magnitude, timing, duration, frequency, and variability); and 3) to conduct an integrated analysis, and discuss their ecological implications. The six selected large watersheds experiencing various levels of forest disturbances along environmental gradients in the BC interior include the Willow River, Cottonwood River, Baker Creek, Moffat Creek, Tulameen River, and Ashnola River watersheds. Cumulative equivalent clear-cut area (CECA) was used to indicate cumulative forest disturbances. A novel methodology combining advanced statistical methods (e.g., time series analysis) with graphical methods (modified double mass curve and flow duration curve) was employed to conduct statistical and quantitative assessments. The analysis demonstrates that forest disturbances produced significant impacts on annual mean flows and some of the seasonal mean flows in the intensively disturbed watersheds including Willow River, Baker River, Moffat Creek, and Tulameen River watersheds. Forest disturbances and climate variability have produced counteracting effects or joint positive effects on streamflow in these watersheds. The comparisons suggest that the wetter years or the wetter watersheds were more sensitive to forest disturbances as compared with the drier years or the drier watersheds. Using the long-term data on the hydrological responses in the Willow and Tulameen River watersheds, a pattern of reduced impact strengths over time probably due to watershed resilience was discovered. Forest disturbances significantly altered the magnitude, timing, variability, duration, and frequency of high flows in the Baker Creek, Moffat Creek, and Willow River watersheds. Less pronounced impacts on low flow regimes and inconsistent relationships between forest disturbances and low flow regimes were found. These findings are of great implications for designing forest and watershed management strategies to protect watershed ecological functions and public safety in the context of future forest and climate changes.Arts and Sciences, Irving K. Barber School of (Okanagan)Earth, Environmental and Geographic Sciences, Department of (Okanagan)Graduat