Tree-ring data suggest B.C. is facing harshest droughts in 350 years

Mark Hume, Globe and Mail, May 04, 2016

VANCOUVER — British Columbia is facing droughts more severe than any in the past 350 years, according to new research that used tree-ring data to reconstruct the coastal climate back to the 17th century.

The study suggests recent droughts in the West – which have dried up streams, led to provincewide restrictions on water use, and triggered intense fire seasons – are likely to be surpassed by more severe events in the next few decades.

Dr. Coulthard adds, for the Cowichan Watershed Board:

Droughts on Vancouver Island, British Columbia, are happening more often and becoming more severe. Both the provincial government and communities in BC are developing new strategies for coping with the impacts of drought. Usually, hydrologists and water managers base drought management plans on hydrometric data records. These records are only a few decades long, and this snapshot rarely records 'worst-case scenario' droughts that have occurred over history. As a result, we may be underestimating how severe Vancouver Island droughts can be.

Our research shows that natural (climate-driven) droughts on Vancouver Island are underestimated byhydrologists and decision-makers. We used tree-ring data to model historical streamflow over the past 300-500 years, and found that at least 16 droughts occurred that were worse than any in the hydrometric data period. Our results apply to streams where a large part of summer runoff comes from snow, such as the Cowichan River. Several conclusions can be drawn from our work:

  • We can expect droughts worse than any in the instrumental record to occur on Vancouver Island
  • Those droughts are also more likely to occur than hydrologists would determine based on the streamflow data
  • Largely, our models do not take into account climate change or human impacts to watersheds (deforestation, human water use). These added impacts would 'surf on top of' a worst case scenario natural drought, making it potentially much worse than any drought that occurred in the past 300-500 years.
  • This type of unprecedented drought could have long-term and disastrous consequences for stream ecology and local salmon populations.

Bethany Coulthard, a postdoctoral research associate at the University of Arizona, collected tree-ring data from old-growth forests in three watersheds on Vancouver Island and one on the Lower Mainland.

By studying the growth rates in two different sets of trees, one that reflected snowpack levels in winter and another that indicated hot, dry summers, she and her colleagues reconstructed what conditions were like dating back to 1666.

“In the one set, a [narrow] ring means a deep snowpack, and higher run off from snowmelt water, and then in the other set a [narrow] ring means drier conditions, the tree is thirsty, it’s not getting enough moisture, so it tells us about the summer aridity conditions,” Dr. Coulthard said in an interview on Wednesday.

“You can combine those two data sets and use them to tell you about summer drought conditions in every year, going back over the length of the tree-ring record,” she said.

The data showed that British Columbia experienced 16 droughts more severe than any since the 1970s, when climatologists began using instruments to record weather and climate information.

“Our reconstruction suggests that preinstrumental droughts were more severe than those in 1992, 1996, 2003 and 2009, when water scarcity throughout south coastal B.C. severely impacted municipal, hydroelectric, and agricultural water supplies,” her research paper says.

Data from the 2014 and 2015 droughts in British Columbia were not available in time to be included.

But Dr. Coulthard said the record, which includes a period before climate change and before widespread logging, clearly shows that British Columbia can expect worse droughts in the years ahead.

“What the model does not account for is [human-caused] climate change or land-use change like deforestation, urbanization, these other pressures that also lend themselves to drought,” Dr. Coulthard said.

“So when we take the projected climate change and we take the projected land-use change and we put them on top of the 16 worst-case scenario droughts, it is reasonable to assume that all those factors combined would translate into a drought that would be worse than anything we’ve seen over the period of the reconstruction, which was 350 years.”

The tree-ring data show droughts occurred about every 21 years on average, and the three most severe were in 1674, 1682 and 1958.

The droughts did not occur in two consecutive years except for 1868 and 1869.

The past two summers in British Columbia have been extremely dry, with water restrictions throughout Metro Vancouver and many other jurisdictions. Current U.S. long-range forecasts by AccuWeather, Inc. predict drought conditions “may return in the middle and latter part of summer and may result in another year of rampant wildfires for northern California and the Northwest.”

This spring has been hot and dry in British Columbia, and water restrictions are to start on May 15 in Metro Vancouver, two weeks earlier than last year. But the snowpack throughout most the province has been average, or slightly above average, except on the Central Coast, and in the north and northeast regions of the province, where it was below average.

Dr. Coulthard said while the tree-ring data clearly showed when droughts occurred, they did not indicate to researchers just how low the streams got.

“It is really hard to say exactly how bad the droughts were in terms of actual streamflow measurements, like how low did the water get? We can’t really answer that question. But what we can say is that they were worse, they were lower than in anything in the severe drought years we’ve seen,” she said.

Dr. Coulthard, who got her Ph.D. from the University of Victoria, worked on the research project with Dan Smith of UVic’s tree ring laboratory, and David Meko of the University of Arizona. Their paper is in the Journal of Hydrology.

Is worst-case scenario streamflow drought underestimated in British Columbia? A multi-century perspective for the south coast, derived from tree-rings

Coulthard, B., Smith, D. J., & Meko, D. M. (2016). Journal of Hydrology. DOI:10.1016/j.jhydrol.2015.12.030

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