Determining forest disturbance thresholds for managing cumulative hydrological impacts

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The impacts of forest disturbance on hydrology have been receiving growing attention due to increasing forest disturbance and climate change, and their effects on water supply and other aquatic functions. However, the majority of this research has been conducted on small watersheds (500 km2), as forest disturbance often occurs cumulatively over space and time, its hydrological impacts are cumulative. With support from my NSERC Discovery Grants, I established a research program entitled "Forest Disturbance and Hydrology in Large Watersheds" where we have successfully developed a method referred to as modified double mass curve (MDMC) for quantifying the cumulative hydrological effects caused by forest disturbance in any individual watershed. Using this method and other statistical techniques, we have studied critical research questions such as cumulative hydrological effects, interactions between forest disturbance and climate, and spatial scaling of hydrological impacts across nested watersheds. Our results have been published in high-impact journals (e.g., Water Resources Research, Journal of Hydrology, Global Change Biology, Nature Communications). Our progress has led to an important research question: what are forest disturbance thresholds over which significantly cumulative hydrological impacts can be detected? To our surprise, there is very limited research on this topic given its significance in sustainability, mainly due to several methodological challenges. Answering this question can greatly advance watershed science and promote watershed sustainability. In this original and innovative study, 50 watersheds from the interior of British Columbia (BC) will be selected, whereby several statistical techniques including MDMC will then be applied to determine forest disturbance thresholds for each watershed individually. The short term objectives (2021-2026) are: (1) to quantitatively determine forest disturbance thresholds on annual mean flows; (2) to determine forest disturbance thresholds high and low flows; and (3) to above results from all 50 watersheds to assess whether climate and watershed property are also controls on these thresholds. My long-term objectives are to assess ecological thresholds or tipping points and resilience in a broader context considering more ecological processes, and to expand spatial scopes to examine forest disturbance thresholds at larger spatial scales. The BC interior is an ideal region for this proposed study because there are numerous gauged watersheds where long-term historic data on hydrology, climate and forest disturbance are available. The proposed research will provide important and timely information for BC to manage forest disturbance to protect aquatic functions. It will allow development of forest disturbance thresholds for effectively managing cumulative hydrological risks. Finally, this project will train 2 HQPs and 5 undergraduate students.