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UBC Theses and Dissertations

Revealing forest harvesting effects on large peakflows in rain-on-snow environment with new stochastic physics Rong, WeiTao


Using nine pairs of control-treatment watersheds with varying climate, physiography, and harvesting practices in the Rain-On-Snow (ROS) environment of the Pacific Northwest region, this thesis demonstrates the linkage between environmental control and the sensitivity of peakflow response to harvesting effects. Compared to previous paired watershed studies in ROS environment, this study, for the first time, employed an experimental design of Frequency Pairing to isolate the effects of disturbances on systems’ response. The use of frequency distributions for evaluating the relation between forest harvesting and peakflows is a well-established framework outside forest hydrology literature. The results show how harvesting can dramatically increase the magnitude of all peakflows on record and how such effects can increase with increasing return periods, as a consequence of substantial increases to the mean and variance of the peakflow frequency distribution. Most critically, peakflows with return period larger than 10 years can increase in frequency, where the larger the peakflow event the more frequent it may become. The sensitivity of the upper tail of the frequency distribution of peakflows was found to be linked to the physiographic and climatic characteristics via a unifying synchronization / desynchronization spatial scaling mechanism that controls the generation of rain-on-snow runoff. This new physically-based stochastic hydrology understanding on the response of watersheds in ROS environments runs counter the deterministic prevailing wisdom of forest hydrology, which presumes a limited and diminishing role of forest cover as the magnitude of the peakflow event increases. By demonstrating the need for invoking the dimension of frequency in the understanding and prediction of the effects of harvesting on peakflows, this study added another brick to the pile of evidence in calling for the abandonment of the outdated pure deterministic hypotheses and experimental designs that have misguided forest hydrology research for over a century on this topic.

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