Multi-century records of snow water equivalent and streamflow drought from energy-limited tree rings in south coastal British Columbia




Coulthard, Bethany L.

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Anthropogenic climate change has triggered widespread shifts in the global hydrological cycle. In south coastal British Columbia, these changes have led to more winter precipitation falling as rain rather than snow, more rain on snow events, and generally reduced snowpacks. Since snowmelt is a primary source of summer surface runoff and groundwater, snowpack declines have caused severe seasonal streamflow droughts in recent decades. For accurate water supply forecasting under future climate change, it is crucial to know if snowpack and runoff declines are unprecedented in the last several hundred years. This research focused on developing multi-century, annually-resolved records of snow water equivalent and streamflow drought to determine if recent conditions deviate from long-term norms. The research targeted small temperate watersheds that are not usually conducive to application of dendrohydrological methodologies. Traditional dendrohydrology relies on moisture-limited tree-ring records from arid settings. This dissertation presents a new method for developing tree-ring based reconstructions from energy-limited trees. Tree-ring records from high-elevation mountain hemlock (Tsuga mertensiana (Bong.) Carrière) and amabilis fir (Amabilis (Dougl.) Forbes) stands were collected at sites in south coastal British Columbia. Ring-width measurements were used to develop multi-century dendrohydrological models of snow water equivalent and streamflow drought. A 322-year reconstruction of May 1 snow water equivalent for Vancouver Island explains 56% of the instrumental SWE data variance and suggests snowpacks in 2015 were lower than in any year since 1675. A 477-year reconstruction of summer streamflow for Tsable River explains 63% of gauged streamflow variance and indicates that since 1520 twenty-one droughts occurred that were more extreme than recent “severe” droughts. Finally, a reconstruction of regionally synchronous streamflow among four south coastal rivers explains 64% of the regionalized streamflow variance. In addition to snow-sensitive tree-ring data, the latter model incorporated a paleorecord of the Palmer Drought Severity Index as a summer temperature and aridity proxy. The reconstruction suggests that since the mid-1600s sixteen regional-scale droughts occurred that were more extreme than any within the instrumental period. All three models were particularly accurate at estimating lowest snow and runoff years, and reflected the long-term influence of cool phases of the Pacific Decadal Oscillation on regional snowmelt and summer discharge trends and patterns. The reconstructions suggest: 1) snowpack declines in 2015 were unmatched in the past ~340 years; and, 2) existing water management strategies based on hydrometric data records underestimate potential magnitudes of natural droughts. Worst-case scenario droughts compounded by land use change and climate change could result in droughts more severe than any in the past several hundred years. Energy-limited tree-ring records have strong potential as paleohydrological proxies and for expanding applications of dendrohydrology to arid settings. For some of the tree-ring chronologies examined in this study, the correlation with snow water equivalent became non-significant after the mid-1990s, possibly due to warming spring temperatures. Future studies using this type of tree-ring data must carefully evaluate the recent stability of climate-growth relationships.



Dendrochronology, Dendrohydrology, Dendroclimatology, Paleoclimatology, Drought, Streamflow, Hydrology, Water management, British Columbia, Tree rings