Deforestation's impact on flood risk often overlooked, study warns

Forests play a crucial role in managing the growing threat of floods worldwide, and it’s high time we adopt more sustainable forestry practices. This is the key finding of a study led by Dr. Yunus Alila, a hydrologist and professor at the Faculty of Forestry, and researcher Henry Pham.

They analyzed years of hydrology studies and found that the role of forests in controlling flood risks is often underestimated. This has led to ill-informed forestry policies and practices.

The study, published in the Science of the Total Environment journal, emphasizes the need for a better understanding of how deforestation increases flood risk. Dr. Alila explains that it’s not just about the direct impact of tree cutting. Factors like snow cover, melting rates, and rainfall also play a part, and they interact in complex ways over time.

Dr. Younes Alila, a hydrologist and professor in the faculty of forestry, and his graduate student Henry Pham synthesized decades of hydrology studies and found that many "severely and consistently underestimated" the impact of forest cover on flood risk. Because of this underestimation, they led to the creation of forest management policies and practices that were either unsound or poorly informed.

Cause and effect

Dr. Alila explained that for over a century, scientists clung to a "deterministic" analysis. The reasoning compares to a strategic board game analogy where you look at each move in isolation, thinking, "If I move here, then I should win." However, this approach does not consider factors that can change the game – such as the roll of the dice, the cards you draw, and the potential actions of your opponents.

When it comes to understanding how logging might increase flood risk, a deterministic approach would look at the logging alone and try to figure out its direct effect.

Many things such as the amount of snow on the ground, whether it's melting or not, the amount of rain falling, and the characteristics of the landscape itself, however, influence the risk of flooding.

These factors interact over time in complex ways. "We call taking them all into account a "probabilistic" approach and it provides a better overall picture of flood risk." It's like a savvy board game player considering all the game's variables instead of just one.

"The probabilistic approach is already well established in other disciplines such as climate change science. It is the most accurate method for evaluating the effects of deforestation on floods," said Henry Pham, a student in UBC's master of Science in forestry program.

Forests can lower flood risk

Dr. Alila highlights the use of a probabilistic framework to predict the impact of factors like climate change, land use changes, and logging on events like the 2021 Fraser Valley floods.

This method can also be used to study flood risks in different regions.

He warns that in British Columbia, the risk of flooding is rising due to the loss of forests from extensive logging and wildfires. To lessen the impact of disasters like the recent floods, we need to rethink our forest management strategies. He suggests regenerative practices like selective logging and small patch cutting as alternatives to clear-cutting.

Pham noted that clear-cut logging results in more severe and much frequent floods, and these floods can create harsh consequences.

"They can impact river ecosystems negatively, degrade the water quality in community watersheds, and cause sedimentation issues downstream. Clear-cut logging could potentially affect thousands of lives and many ecosystems further downstream."

Dr. Alila concludes, "Forests serve as the most effective natural defense against a global escalating flood risk attributed to factors such as climate change. Now is the time for the most up-to-date and defensible science to start guiding water and forest management policies."

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