Cyclone Senyar (Tropical Depression 34W) brought extreme rainfall to the Malay Peninsula and Sumatra and extending its impacts to central and southern Thailand. Across the region, the cyclone claimed at least 1,200 lives and caused an estimated US$19.8 billion damage. Sumatra bore the heaviest toll, with more than thousands recorded deaths. In just eight days, parts of Sumatra received over 700 mm of rainfall, nearly one-third of the region’s average annual precipitation. Such an extraordinary volume overwhelmed soil systems, saturating pore networks and forcing excess water to accumulate rapidly in downstream areas. Under intact conditions, forests act as natural sponges: studies show they can intercept and retain approximately 15–35% of rainfall through canopy interception and soil infiltration. When upstream forests are removed, this hydrological buffer collapses. Groundwater recharge declines, surface runoff accelerates, and flood risks intensify.
It is therefore reasonable to infer that extensive deforestation in Sumatra’s upper watersheds significantly amplified the impacts of Cyclone Senyar.
According to WALHI records, approximately 1.4 million hectares of forest were lost across Aceh, North Sumatra, and West Sumatra between 2016 and 2025. This loss is a stark reminder of the so-called “intangible” values of forests ecosystem functions that are rarely accounted for economically yet prove indispensable for human safety and environmental stability.
