A sweeping international study led by researchers at the University of British Columbia suggests that deforestation does more than shrink tree cover it fundamentally alters how watersheds store and release water, potentially undermining long-term water security.
The research, published in the Proceedings of the National Academy of Sciences (PNAS), analyzed hydrological data from 657 watersheds across six continents. The findings show that forest loss along with changes in how forests are spatially arranged increases the proportion of “young water” flowing out of watersheds.
Forest Loss Speeds Up Water Movement
“Young water” refers to rain and snowmelt that moves through a watershed within roughly two to three months of falling. A higher proportion of young water indicates that water is traveling quickly through the system instead of being stored in soils and groundwater reserves.
“Young water is a signal that water is moving quickly through a system,” said lead author Ming Qiu, a doctoral student at UBC Okanagan. “When the young-water fraction is high, it means less water is being stored for use during drier periods.”
The study found that when forests are removed, watersheds release more young water, effectively becoming “leakier.” This rapid runoff reduces the natural buffering capacity that sustains rivers, agriculture, ecosystems and communities during dry seasons.
Co-author Dr. Adam Wei noted that the findings challenge the traditional framing of forest management as a simple trade-off between conservation and economic development. “Forest loss clearly reduces a watershed’s ability to retain water,” he said. “But how forests are arranged on the landscape can either worsen or help mitigate that impact.”
Why Forest Pattern Matters
While previous studies have focused primarily on the amount of forest removed, this research adds a crucial new dimension: forest landscape pattern.
In watersheds where forest cover falls below roughly 40% to 50%, the arrangement of remaining forest patches significantly influences water behavior. In these sparsely forested landscapes, increased forest edges where forest meets cleared land were linked to lower young-water fractions.
Forest edges experience altered microclimates, including greater solar radiation and lower humidity, which can boost evapotranspiration and reduce rapid runoff.
By contrast, in watersheds with dense, contiguous forest cover, spatial pattern had little measurable effect. When forests remain largely intact, edges are closer together and microclimatic changes are muted.
“This was one of the most surprising findings,” Qiu said. “Forest pattern matters most when forest cover is already low. Above a certain threshold, its influence largely disappears.”
Implications for a Warming World
As climate change intensifies extreme weather events, water availability is becoming increasingly unpredictable. Researchers say understanding how land-use decisions affect watershed storage capacity is now more urgent than ever.
Watersheds function as natural water-storage systems, regulating flows across seasons. If deforestation continues without careful landscape planning, communities may face heightened flood risks during wet periods and reduced water supplies during droughts.
The study’s authors argue that smarter landscape design not just limiting forest loss could help preserve watersheds’ capacity to retain water. By considering both forest quantity and spatial configuration, policymakers may be able to reduce hydrological risks while balancing economic needs.
The findings underscore a growing scientific consensus: forests are not only carbon sinks and biodiversity reservoirs, but also critical infrastructure for sustaining the global water cycle.