Thursday, June 9, 2011

Forest harvesting impacts on peak flows

As a Hydrotechnical Engineering (one of the areas of specialization in Civil Engineering) student, my area of focus is reservoir operation during high flow events. Although I just use the inflow data other specialists provide me with, hydrology and inflow forecast is the starting point of reservoir operation planning process. Forest harvesting as an influential element which can change the inflow amount until forest regrowth is complete, has been a controversial area of study in forest hydrology.

In the eyes of the public and usually policy-makers, logging trees and removing forests increases the magnitude of floods and consequently exacerbates its destructive impacts. On the other hand, historically, forest hydrologists have tended to rely on the chronological pairing analysis of peak flow events to study the impact of forest harvesting on peak flows which has led them to have opposite views ("Forest impact on floods due to extreme rainfall and snowmelt in four Latin American environments 1: Field data analysis" as one of the most recent examples). Throughout the years, many scientists have demanded that the public and policymakers improve their understanding of natural phenomena such as floods. But is it the public who need to improve their understanding of the behavior of nature?

In order to study the impacts of forest harvesting on peak flows, there are usually two small neighboring watersheds (which are quite similar) that are chosen, one as control watershed which remains unchanged and the other one as treatment watershed which is clear-cut, or treated in another way depending on the objectives of the study. Using statistical methods, the relationship between paired peak flows, which stem from the same meteorological events in the neighboring watersheds, is captured and used to figure out how this relationship changes as the result of clear-cutting in the treatment watershed. This type of pairing of the events is known as chronological pairing. To avoid getting too technical or going through statistical complexities involved in making inferences from the graph results of this type of pairing, let us suffice to say that the related studies tend to conclude that the impact of forest harvesting on the magnitude of peak flows diminishes as the size of the meteorological event increases so that the highest peak flows are almost unchanged. Moreover, in some cases there are suggestions that peak flows with the return period of higher than a particular number of years, like 10, are not affected by logging.

Recently, Younes Alila, a professor at the Department of Forestry in University of British Columbia, and his colleagues have published a paper on the topic utilizing frequency-based pairing of events. In the paper, the authors reveal how chronological pairing of events leads to an irrelevant hypothesis and how the blind use of some statistical methods to support the hypothesis without giving sufficient thought to the process has misled forest hydrologists for several decades. The authors, through appropriate and insightful use of statistical methods accompanied with physical reasoning of natural phenomena, finally prove that the public view on the impact of forest harvesting on floods turns out to be closer to reality than traditional view in the forest hydrology studies.

The publication of the paper as a new paradigm to dismiss years of controversy over the impact of forest harvesting on flood magnitude, at least for small watersheds, has been objected by some scientists who have spent several years of their career on similar studies with opposite conclusions. For further reading, below you can find the links to the original paper, a critique to the paper, and the reply of the authors to the critique.

Forests and floods: A new paradigm sheds light on age-old controversies.

Comment on “Forest and floods: A new paradigm sheds light on age-old controversies” by Younes Alila et al.

Reply to comment by Jack Lewis et al. on “Forests and floods: A new paradigm sheds light on age-old controversies”

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