INNOVATION May-June 2012

developed and extensively tested at a site containing a rich set of internal catchment process observations to examine the effects of forest harvesting on peak flows with return periods of up to 100 years. For their test site at 241 Creek, one of three 5 km 2 drainages in the Upper Penticton Creek Watershed Experiment (Okanagan Highlands, British Columbia), Kuraś and co-researchers found that not only did forest harvesting increase the severity of all peak flows in the 100 years of simulated record, but that it had a scaling influence on both the magnitudes and frequencies of floods. The model showed that the larger the flood, the more its magnitude was amplified by forest harvesting, with 10- to 100-year return period floods increasing in magnitude by 9 to 25%. Following a simulated removal of half of the watershed’s trees, the authors found that while 10-year return period floods occurred twice as often, 100- year return period floods became 5 to 6.7 times more frequent. This proportional relation between an increase in flood magnitudes and frequencies following forest harvesting and the size of floods is in direct contrast to the prevailing perception in forest hydrological science. These findings can have major implications for the lifespan and safety of structures like bridges and dams, human settlements, drinking water quality, and the sustainability of river ecosystems. The outcomes of decades of small paired watershed studies have been and continue to be used to advocate policies related to land-use management over larger basins in hydroclimate regimes worldwide.

It is important, therefore, to expose the caveats of small paired watershed studies, if only to put an end to using their potentially erroneous outcomes to influence land-use policy. Understanding the influence of forests on floods in larger basins is critical to the development of land use policy, especially in light of a changing climate and wide spread mountain pine beetle infestations that have affected the North American Cordillera from Northern BC to southern Mexico. In addition, evaluating the impact of harvesting on floods over time coupled with forest regrowth (ie, hydrological recovery) is critical for establishing sustainable annual harvest volumes and long-term timber supply rates. This also serves as a reminder that critical thinking is essential even when relying on deep-seated science, and that shifts in scientific paradigms are often long drawn out. v Piotr Kuraś PEng PE RPF MASc is a hydrologist with Northwest Hydraulic Consultants Ltd. Dr Younes Alila PEng is an Associate Professor in the Faculty of Forestry at the University of British Columbia. More information on the 2009 study by Dr Alila et al is available online: www.agu.org/pubs/crossref/2009/2008WR007207.shtml. The full journal publication and AGU research spotlight on the 2012 study by Kuraś et al are available at: www.agu.org/cgi-bin/highlights/ highlights.cgi?action=show&doi=10.1029/2011WR010705&jc=wr.

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