ASIATODAY.ID, YOGYAKARTA – The catastrophic flash floods and landslides that struck West Sumatra, North Sumatra, and Aceh in late November 2025 left a trail of extensive destruction.
Days of relentless heavy rainfall caused rivers to overflow and hillside slopes to collapse. Hundreds of villages were inundated, critical infrastructure was cut off, and the flash floods claimed more than 400 lives across the three affected provinces.
Indonesia’s National Disaster Management Agency (BNPB) reported 2,726 hydrometeorological disasters from January to November 2025 alone. In response to the late-November catastrophe, the governors of West Sumatra, North Sumatra, and Aceh simultaneously declared a 14-day emergency status.
Dr. Hatma Suryatmojo, a hydrology and watershed conservation expert from Gadjah Mada University (UGM), emphasized that the late-November flash floods were not an isolated incident, but part of a worsening pattern of hydrometeorological disasters seen over the past two decades.
According to him, the disaster was triggered by extreme weather but amplified by extensive environmental degradation in upstream watershed areas.
“Rainfall was extremely high—BMKG recorded over 300 mm per day in parts of North Sumatra at the peak of the event. The extreme intensity was influenced by exceptional atmospheric dynamics, including Tropical Cyclone Senyar that formed in the Malacca Strait,” Hatma said at UGM on 1 December 2025.
“But extreme weather was merely the trigger. The real damage occurred because the natural buffers in the upstream regions have already collapsed.”
Forest Loss Undermines Watersheds’ Ability to Absorb Extreme Rainfall
Hatma explained that widespread forest degradation in upstream watersheds has severely reduced their natural capacity to absorb, store, and regulate rainfall. Loss of forest cover disrupts key hydrological processes—interception, infiltration, evapotranspiration—and sharply increases surface runoff, erosion, and landslide risk.
Studies from natural tropical forests in Kalimantan and Sumatra show:
Forest can intercept 15–35% of rainfall in the canopy
Undisturbed soil can infiltrate up to 55% of rainfall
Forest evapotranspiration returns 25–40% of rainfall to the atmosphere
Only 10–20% becomes direct surface runoff
“When upstream forests are degraded or cleared, all these hydrological functions collapse. Heavy rainfall can no longer be absorbed because the soil loses its porosity along with the roots that once stabilized it. As a result, most rainfall turns into rapid surface runoff rushing downstream,” Hatma explained.
Intact forests do have a threshold in holding extreme rainfall, he noted. Under exceptional conditions, saturated soils can trigger landslides that block rivers and create natural dams. When these dams burst, deadly flash floods occur—carrying mud, rocks, and logs downstream.
Decades of Deforestation Heightened the Severity of the Disaster
Hatma highlighted significant deforestation trends across Sumatra’s upstream regions:
Aceh:
Forest cover was still 59% (≈3.37 million ha) in 2020
But the province lost over 700,000 ha of forest between 1990–2020
Losses continue to increase flood vulnerability
North Sumatra:
Forest cover declined to just 29% (≈2.1 million ha) in 2020
Remaining forests are fragmented along the Bukit Barisan range
The Batang Toru ecosystem, a critical watershed and biodiversity hotspot, continues to degrade due to logging, plantation expansion, and gold mining
West Sumatra:
Forest cover stood at 54% (≈2.3 million ha) in 2020
One of the fastest deforestation rates in Indonesia
WALHI recorded 320,000 ha of primary forest loss and 740,000 ha total tree cover loss (2001–2024)
Steep slopes of the Bukit Barisan make the region highly prone to landslides when forests disappear
“The November 2025 disaster was the accumulation of upstream ecological destruction. Extreme weather was only the trigger; the scale of the damage reflected decades of environmental degradation from the headwaters down to the lowlands,” Hatma said.
Weak Land Governance and Expanding Human Activity Intensify Risks
Poor spatial planning, illegal logging, and conversion of forest lands into plantations—especially oil palm—continue to undermine watershed integrity.
In Batang Toru, forests designated as water-catchment zones have been cleared or encroached upon, reducing their ability to hold rainfall during storms.
“Without the forest buffer, the excess rainfall flowed rapidly downstream, overwhelming communities and infrastructure,” Hatma added.
Climate Change Raises the Frequency of Extreme Weather
Sumatra’s naturally wet tropical climate already makes it vulnerable to heavy rainfall. Combined with deforestation and river narrowing, the island now faces the constant threat of large-scale hydrometeorological disasters.
“Nature has limits. When environmental degradation exceeds those limits, disaster becomes inevitable,” he stressed. “Both structural and ecological approaches must be integrated into future mitigation efforts.”
Mitigation: Structural Measures Must Be Paired With Ecological Restoration
Structural measures alone—levees, riverbank rehabilitation, and river normalization—will not be sufficient without upstream conservation.
Hatma urged the government to:
Prioritize forest protection and watershed conservation
Enforce disaster-mitigation-based spatial planning
Halt deforestation in critical upstream regions, including the Leuser Ecosystem in Aceh and Batang Toru in North Sumatra
Accelerate reforestation and rehabilitation of degraded catchments
Strengthen community education and participation in forest protection
Early Warning Systems and Weather Modification as Complementary Tools
Hatma noted that early warning systems must be continuously improved as climate change increases the frequency of extreme events. BMKG has issued high-risk advisories during peak rainy seasons, but local governments must respond quickly with evacuation drills, resettlement programs, and improved emergency readiness.
Weather modification technology (TMC) may be considered in high-risk situations, but Hatma cautioned that technological solutions cannot substitute environmental restoration.
A Turning Point for Indonesia’s Disaster Resilience
“Flash floods like those in November 2025 are a stark reminder that economic development cannot ignore environmental limits,” Hatma said.
“Only through coordinated action—government, private sector, communities, and environmental organizations—can Sumatra and Indonesia strengthen resilience against worsening hydrometeorological disasters.”
He concluded that the tragedy should serve as a turning point: a call to restore balance between human development and ecological integrity, to protect lives and safeguard the future. (AT Network)
Follow Us at Google News and WA Channel