Cumulative hydrological impacts, disturbance thresholds and hydrological sensitivities in forested landscapes in British Columbia, Canada
Creators: name: Hou, Yiping
Publisher: name: University of British Columbia
Year: 2023

description: Understanding hydrological responses to cumulative forest disturbance is important to ensure hydrological and ecological functions. While extensive studies have examined how forest disturbance may affect hydrology, forest disturbance thresholds for cumulative hydrological impacts and hydrological sensitivities are rarely assessed. The major objectives of this study are to determine forest disturbance thresholds for cumulative hydrological impacts and the hydrological sensitivities of annual mean flows and summer low flows, as well as their contributing factors. This study is conducted in forested landscapes in the interior of British Columbia with forest disturbance from timber harvesting, mountain pine beetle infestation, and wildfire. Cumulative equivalent clear-cut area (CECA) is used to indicate the level of forest disturbance. A well-tested methodology (the modified double mass curve) and a novel hydrological response curve (the double mass curve for summer low flows) are applied to determine forest disturbance thresholds and hydrological sensitivities of annual mean flows in forty-two watersheds and summer low flows in twenty watersheds, respectively. This study shows that forest disturbance thresholds on annual mean flows range from 7 to 52% of CECA among forty-two watersheds. The corresponding immediate hydrological sensitivities vary from 0.5 to 3.6, indicating that a 1% change in CECA results in a 0.5 to 3.6% change in annual mean flows. There is a significant and negative relationship between forest disturbance thresholds and hydrological sensitivities. Climate (inter-annual and intra-annual) and watershed properties (vegetation and topography) play significant roles in forest disturbance thresholds and hydrological sensitivities. Watersheds with more desynchronizations of energy demand and water supply, higher snow fractions, steeper slopes, less diverse ecosystems, and higher vegetation capacities have lower disturbance thresholds and higher hydrological sensitivities. Forest disturbance thresholds on summer low flows range from 8 to 52% of CECA among the twenty study watersheds, which are also significantly and negatively associated with hydrological sensitivities. There is a non-significant difference in forest disturbance thresholds between summer low flows and annual mean flows, but a significant difference in hydrological sensitivities between them. The impacts of cumulative forest disturbance on summer low flows are variable, with both increased and decreased summer low flows being detected. This study clearly indicates large variations in forest disturbance thresholds and hydrological sensitivities, highlighting that the one-threshold-fits-all application and management are inappropriate and watershed characteristics (climate and watershed properties) should be carefully considered to manage water supply and other hydrological functions under increasing forest disturbance and climate change impacts in the region.