A Tree-Ring Derived Reconstruction of Regionalized Low-Flow Season Streamflow for Susceptible Watersheds in South Coastal British Columbia, Canada.

Rising winter air temperatures, snowpack declines, and warmer and drier summers have resulted in an increase in the frequency and severity of late-summer low-flow events across south coastal British Columbia. Negative impacts, particularly those associated with deteriorating water quality, decreased water supply for hydroelectric power generation, and the survivorship of migrating Pacific salmon, have been observed and are expected to intensify. It is not known whether these earlier, longer, and/or lower low-flows are anomalous relative to historical streamflow patterns since available gauged flow data is restricted to the last five decades.

We used networks of high-elevation tree-ring data to model historical low-flow (late-summer) season streamflow regionalized across a group of hydrologically similar watersheds that were identified as susceptible to low-flow events. Our approach contrasts with typical dendrohydrological approaches in that the study watersheds are relatively small in size, and we have not employed moisture-limited tree-ring records as model predictors. In these rainfall-dominated streamflow regimes, late-summer discharge is driven by a combination of persisting snowmelt-derived inputs and variations in summer temperature. We used correlation analysis to prescreen a large pool of tree-ring chronologies as candidate model predictors, retaining those whose annual radial growth is limited by variability in those climate variables driving low-flow season discharge. We conducted principal components analysis on the retained chronologies to derive a set of tree-ring based predictors, which were entered into a stepwise multiple linear regression model to estimate historical regionalized low-flow season flows. Our model explains approximately 60% of streamflow variance, elucidates the frequencies and magnitudes of past low-flow episodes not represented within the instrumental record, and provides a historical context for contemporary shifts in the regional hydrology of south coastal British Columbia.