Our recent study at the University of British Columbia analyzed 657 watersheds across the globe. By using a tool called the Young Water Fraction, we found that forest loss significantly accelerates how fast precipitation travels through a landscape.

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Young Water Fraction tells us what proportion of a stream is made up of rain that fell recently—typically within the last two to three months. Ideally, we want a low percentage of young water.

A low amount means the landscape is acting like a sponge, filtering rain through the soil and into groundwater, which sustains the river during dry seasons. A high amount of young water, however, suggests a "leaky" watershed that sheds new rain almost immediately.

The reasons for this leakage are tied to how we treat the land. When a forest canopy is removed, raindrops hit the ground with full force instead of being intercepted by leaves. Furthermore, heavy machinery and logging roads pack the soil tight, making it harder for water to sink in.

Lastly, without trees to breathe water back into the atmosphere through transpiration, the soil stays saturated. When the next rain comes, there is no room left to store it, forcing the water to run off quickly into the stream.

This loss of retention capacity is especially potent in watersheds with shallow groundwater. In these areas, the soil layers are thin, limiting how much rain can be stored. Any disturbance to the land cover lacks a buffer, immediately translating into altered, younger streamflow.