Why deforestation is causing our water bodies to rain

It is a proven fact that forests and water are closely related. For decades, integrated water tests – a scientific method of evaluating the effects of land use on the quantity or quality of water – have shown that when we lose forests, the total amount of water flowing into our rivers tends to rise.

But an important question has remained unanswered: does this additional water come from past sources, or is it just “new” rain that the country is unable to handle?

In other words, does the loss of forests cause our water bodies to lose their internal integrity and leak like a filter?

Our latest study at the University of British Columbia examined 657 water bodies around the world. Using a tool called Young Water Fraction, we found that forest loss significantly accelerates how quickly rainfall moves across the country.

We estimate that for every piece of forest lost, the “fresh water” in our streams increases by 0.17 percent.

Importantly, our research reveals that it’s not just about how many trees are cut down – it’s also about how much space is left. The way we manage forest land can exacerbate or reduce this leakage.

Why do water bodies leak?

Young Water Fraction tells us how much of the stream has been formed by recent rain – usually within the last two to three months. Ideally, we want a low percentage of small water.

The small amount means that the area acts like a sponge, filtering rain from the soil and groundwater, which keeps the river flowing in times of drought. However, the large amount of small water suggests a “leaky” watershed that provides immediate new rain.

The reasons for this leakage are related to the way we treat the country. When the canopy is removed, the raindrops hit the ground hard instead of being absorbed by the leaves. In addition, heavy machinery and logging roads compact the soil, making it difficult for water to sink.

Finally, when there are no trees to breathe water from the atmosphere through respiration, the soil remains saturated. When the next rain comes, there is no room left to store it, forcing the water to flow faster into the stream.

This loss of water storage capacity is particularly severe in areas with shallow groundwater. In these areas, soil layers are thin, limiting the amount of rain that can be stored. Any disturbance in the soil cover is not protective, which immediately translates into an altered, smaller flow.

The edge of the forest

The most remarkable thing found in our work is that the landscape – the organization of the area of ​​trees – is just as important as the entire lost area.

Consider two scenarios: one large, solid log of harvested wood versus many scattered pieces. Our work has confirmed that even if the total cut area is the same, the hydrological effect is different.

We looked specifically at forest edges, the border where trees meet open spaces. It is interesting that as the weight of these edges increases, the small water tends to decrease.

This happens because these sides are exposed to sunlight and wind. These “effects” can enhance evapotranspiration, driving more soil water to leave the watershed as evaporation rather than runoff.

This management impact is particularly strong in sparsely forested areas with less than 40 percent forest cover. This is because the strengthening of the wind requires large enough openings to allow sunlight to penetrate from the side and drive small changes in the weather.

In densely forested areas, the increasing density of open space becomes more abundant, creating smaller gaps that mitigate these effects.

Rethinking forest management

Our research confirms the important role of forests in water management. Beyond the obvious connection between forests and water, we now know more about how the loss of forests damages the integrity of water bodies. The relationship between our forests and our water is closer than we first expected.

This study also challenges the concept of a binary forest system that focuses on saving or losing trees. This is especially important in areas where the timber industry is part of the economy.

The direct lesson for the industry is that we should avoid clearcuts that are uneven, with a regular shape, that result in a low rate. Instead, we advocate mimicking the complex, unusual patterns found in nature. This can be achieved through silvicultural practices such as variable harvesting, selective logging and continuous forestry.

Taking into account forest structure provides a way to balance forest conservation and development: by improving the way forests are managed, it is possible to reduce the negative hydrological effects caused by logging.

By designing with landscape in mind, we can help ensure that our watersheds continue to act as sponges rather than filters.