Model investigation overthrows assumptions of watershed research

Abstract

A 2009 study revealed serious flaws in a standard technique used by hydrological researchers to understand how changes in watershed land use affect stream flow behaviors, such as peak flows. The study caused academics and government agencies alike to rethink decades of watershed research and prompted Kuraś et al. to reinvestigate a number of long‐standing assumptions in watershed research using a complex and well‐validated computer model that accounts for a range of internal watershed dynamics and hydrologic processes. For the test site at 241 Creek in British Columbia, Canada, the authors found not only that deforestation increased the severity of floods but also that it had a scaling influence on both the magnitudes and frequencies of the floods. The model showed that the larger the flood, the more its magnitude was amplified by deforestation, with 10‐to 100‐year‐return‐period floods increasing in size by 9%–25%. Following a simulated removal of half of the watershed's trees, the authors found that 10‐year‐return‐period floods occurred twice as often, while 100‐year‐return‐period events became 5–6.7 times more frequent. This proportional relationship between the increase in flood magnitudes and frequencies following deforestation and the size of the flood runs counter to the prevailing wisdom in hydrological science.

A paradigm shift in understanding and quantifying the effects of forest harvesting on floods in snow environments

“…The lack of an influence of forest harvesting on large floods at all spatial scales is a preconception [DeWalle, 2003] reinforced by a century of paired watershed studies undertaken using inappropriate experimental design and leading to scientifically indefensible conclusions regarding the relationship between land cover changes and flood response [Alila et al, 2009[Alila et al, , 2010Schultz, 2012]. Over the past five decades such a preconception has also been taught to students as an established precept in forest hydrology textbooks [e.g., Jeffrey, 1970;Lee, 1980;Brooks et al, 2003;Calder, 2005;Chang, 2006] creating a bias in the understanding of the influence of forests on floods at the very core of the science.…”

“…Understanding the influence of land cover changes on flood response in forested catchments is critical to ensure forest management is undertaken in a way that minimizes the risk of negative effects to the environment and to humans dependent on surface water ecosystems. A response by Calder et al [2007] published in Nature summarizing the current state of knowledge in the study of forest hydrology reports that “Now forest hydrologists generally agree that, although forests mitigate floods at the local scale and for small to medium‐sized flood events, there is no evidence of significant benefit at larger scales and for larger events.” The lack of an influence of forest harvesting on large floods at all spatial scales is a preconception [ DeWalle , 2003] reinforced by a century of paired watershed studies undertaken using inappropriate experimental design and leading to scientifically indefensible conclusions regarding the relationship between land cover changes and flood response [ Alila et al , 2009, 2010; Schultz , 2012]. Over the past five decades such a preconception has also been taught to students as an established precept in forest hydrology textbooks [e.g., Jeffrey , 1970; Lee , 1980; Brooks et al , 2003; Calder , 2005; Chang , 2006] creating a bias in the understanding of the influence of forests on floods at the very core of the science.…”

Peak flow regime changes following forest harvesting in a snow‐dominated basin: Effects of harvest area, elevation, and channel connectivity

“…[] because they were based on the premise that the effects of harvesting is a difference in the magnitude of peak flows when control and harvested basins are subject to the same storm input (pluvial regime) or freshet season (nival regime). Such chronological type of peak flow event pairing, a hallmark of traditional paired‐basin data analyses in decades of forest hydrology literature, leads to erroneous outcomes because it does not explicitly consider the all‐important highly nonlinear relationship between peak flow magnitude and frequency [ Alila et al ., ; Schultz , ; Green and Alila , ].…”