Forests affect flood and drought risk, UBC studies show

Two UBC studies report that forest loss and how forests are arranged increase the share of fast-moving 'young water' in watersheds, and a separate paper finds that forests can alter flood frequency and probability.

Two new studies from the University of British Columbia examine how forests affect water storage and flood risk. One study, published in Proceedings of the National Academy of Sciences, analyzed data from 657 watersheds across six continents and found that forest loss and changes in landscape pattern increase the share of "young water" — rain and snowmelt that moves through a watershed within roughly two to three months. The authors say a higher young-water fraction means less water is stored in soils and groundwater for use during drier periods. A second paper, published in Ambio, argues that common methods focusing only on peak flows have led scientists to underestimate how forests can change flood frequency and probability. Key findings: - The PNAS analysis used data from 657 watersheds on six continents. - Forest loss and altered forest patterns increase the fraction of "young water," defined as water moving through a watershed within about two to three months. - A higher young-water fraction indicates reduced storage in soils and groundwater for drier periods. - In watersheds with forest cover below about 40–50 percent, the spatial arrangement of remaining forest patches strongly influences how water moves; above that level, pattern had little effect. - The Ambio synthesis by Samadhee Kaluarachchi and Younes Alila finds that forests can alter the frequency and probability of floods and that prior approaches focusing only on event peak flows may have underestimated this role. Summary: The studies indicate that both the amount of forest cover and its landscape configuration influence watershed storage and the timing of runoff, with consequences for seasonal water availability and flood risk. The Ambio paper suggests that broader flood-risk analyses that account for forest effects on frequency and probability could change how flood risk is understood. The authors note that landscape planning and land-use management are relevant to these outcomes.