Forest Conditions, Not Rain, Are More Influential in the Scale of Disasters in Sumatra

Cyclone Senyar has triggered the highest intensity rainfall ever recorded in Indonesia. Aceh, North Sumatra, and West Sumatra have been devastated, resulting in hundreds of fatalities and many others missing. However, the pattern of destruction that emerged does not entirely correlate with the level of rainfall.

In many locations, it is precisely the areas that do not receive the highest rainfall that experience the greatest devastation. This indicates that the environmental carrying capacity, particularly the condition of forests, land cover, and the stability of river basins, determines the level of disaster vulnerability more significantly than rainfall volume alone.

Data from the Meteorology, Climatology, and Geophysics Agency (BMKG) indicates that the highest rainfall during Cyclone Senyar occurred in Bireuen Regency, Aceh, which recorded 411 mm within 24 hours on Tuesday (25/11/2025). This figure is the highest record in all of Indonesia. Other regions in Aceh also reported extreme rainfall, such as Aceh Tamiang with 397.4 mm, Langsa with 382 mm, and South Aceh with 376.6 mm per day. "The rainfall in one day exceeds the total accumulation of a full month," said BMKG Head Teuku Faisal Fathani.

Climatological data in Bireuen indicates that in November, the average rainfall is 116 mm per month or approximately 19.33 mm per day, with 6 rainy days. The peak of the rainy season in Bireuen typically occurs in December, with an intensity of 362 mm per month or 24.13 mm per day, accompanied by 15 rainy days.

The Director of Meteorology at BMKG, Andri Ramdhani, stated that the period of extreme rainfall with an intensity of more than 150 mm per day also occurred in various other regions in Aceh during the period of November 25–26, 2025. "East Aceh, North Aceh, and South Aceh experienced extreme rainfall for more than two consecutive days," said Andri.

The highest recorded rainfall in North Sumatra reached 390 mm per day, noted in Langkat Regency on Wednesday (26/11/2025). The regions of Langkat, Humbang Hasundutan, and Mandailing Natal experienced extreme rainfall for two consecutive days. "The peak of extreme weather events in North Sumatra occurred on 26–27 November," said Andri.

In West Sumatra, the highest daily rainfall of 261 mm was recorded in the city of Padang on Wednesday (26/11/2025). Similar to North Sumatra, the period of extreme rainfall in West Sumatra also occurred on 26–27 November. "The city of Padang and Padang Pariaman experienced extreme rainfall for more than two consecutive days," he said.

In North Sumatra, the highest daily record reached 390 mm in Langkat Regency (26/11/2025). Meanwhile, in West Sumatra, the city of Padang recorded 261 mm in one day. The Meteorology, Climatology, and Geophysics Agency (BMKG) stated that the period of extreme weather was concentrated on November 26–27 in the two provinces.

Although the highest rainfall occurred in Aceh, the worst impact was actually experienced by North Sumatra. As of December 3, 2025, BNPB recorded 753 fatalities and 650 missing persons, with 301 deaths in North Sumatra, 234 in West Sumatra, and 218 in Aceh. However, the number of refugees indicates a much larger scale of crisis, totaling 1.7 million in North Sumatra, 1.5 million in Aceh, and 14,800 in West Sumatra.

If traced down to the district level, this pattern of inconsistency becomes increasingly evident. Central Tapanuli and South Tapanuli recorded the highest number of victims, even though these areas did not experience extreme rainfall as high as that in Aceh.

From these facts, we can clearly see that the causes of the ecological disaster currently occurring are the state administrators and corporations.

The North Sumatra Disaster Management Agency (BPBD) recorded over 296,000 people affected in Central Tapanuli, with 86 deceased and 85 missing. South Tapanuli reported 279,000 affected and 79 fatalities. Meanwhile, the rainfall in this region is significantly below the figures recorded in Bireuen or Langsa.

The areas with the worst impacts are located within and around the Harangan Tapanuli Ecosystem or Batang Toru, the last essential tropical rainforest in North Sumatra that serves as a natural buffer against floods, erosion, and provides clean water for the downstream watersheds. Approximately 66.7 percent of this area is in North Tapanuli, 22.6 percent in South Tapanuli, and 10.7 percent in Central Tapanuli.

However, this ecosystem has experienced massive degradation for more than a decade. The Executive Director of Walhi North Sumatra, Rianda Purba, stated that the acceleration of damage began with the issuance of the Minister of Forestry Decree No. 579/Menhut-II/2014, which opened up new investment opportunities. "Since then, many permits have been issued, ranging from gold mining to palm oil plantations," he said. Walhi recorded that seven large companies operate in the area, including mining, industrial timber estates, geothermal, and palm oil.

The Coordinator of Jatam, Melky Nahar, stated that based on the analysis of the time series of Google Satellite/Google Imagery as of November 28, 2025, the Batang Toru hydropower project has cleared at least 56.86 hectares of forest area along the river flow for the main structures, ponds, roads, and supporting areas, which is clearly visible as an expansion of open areas within the ecosystem.

"The presence of large-scale hydropower plants modifies river flows, alters sediment patterns, and increases the risk of flooding and landslides downstream when extreme rainfall and poor dam management occur simultaneously," said Melky.

Greenpeace Indonesia assesses that the Batang Toru watershed is currently in a critical condition. From 1990 to 2022, deforestation reached 70 thousand hectares (21 percent of the watershed area). The remaining natural forest is only 167 thousand hectares or just 49 percent. Land-based licensing areas cover 94 thousand hectares or 28 percent. The annual erosion potential now reaches 31.6 million tons, with approximately 56 percent originating from erosion-prone areas, or more than 180 tons per hectare per year.

This vulnerability is not limited to Batang Toru. Almost all watersheds in Sumatra are now in critical condition. According to data from Greenpeace Indonesia, natural forest cover in Sumatra is less than 25 percent in many areas. Overall, natural forests across Sumatra are estimated to remain at 10–14 million hectares, less than 30 percent of the total area of the island, which reaches 47 million hectares.

Walhi's analysis shows that out of 954 river basins in Aceh, 60 percent are located within forest areas, and 20 of these river basins are in critical condition. For instance, the Krueng Trumon river basin, which covers an area of 53,824 hectares, has lost 43 percent of its forest cover from 2016 to 2022. Currently, only 30,568 hectares, or approximately 57 percent of the forest, remain.

The Singkil watershed, as established by the government based on Decree 580, covers an area of 1,241,775 hectares. However, the remaining forest cover in 2022 was only 421,531 hectares. This means that over the past 10 years, there has been a degradation of forest cover in the Singkil watershed amounting to 820,243 hectares, or 66 percent.

Other watershed areas are also experiencing severe damage. The Jambo Aye watershed, which originally covered 479,451 hectares, has suffered 44.71 percent damage. The Peusangan watershed, with an area of 245,323 hectares, has experienced 75.04 percent damage, the Krueng Tripa watershed, with a total area of 313,799 hectares, has sustained 42.42 percent damage, and the Tamiang watershed, covering 494,988 hectares, has incurred 36.45 percent damage.

In West Sumatra, the Aia Dingin watershed, which is one of the important administrative watersheds in Padang City, covers an area of 12,802 hectares. Topographically, the upstream area of the watershed has slopes ranging from flat to steep, with the upper part located in the conservation forest area of the Bukit Barisan, which should function as a primary ecological barrier.

However, this area has been severely degraded. From 2001 to 2024, the Aia Dingin watershed lost 780 hectares of tree cover, with the majority of deforestation occurring in the upstream region, which plays a vital role in mitigating surface runoff and preventing flash floods.

"From these facts, we can clearly see that the causes of the ecological disaster currently occurring are the state administrators and corporations. Therefore, the responsibility of the state administrators is to evaluate all existing company permits in Indonesia, particularly in critical and essential ecosystems," said the Forest and Plantation Campaign Manager of Walhi National Executive, Uli Arta Siagian.

Uli urged that companies responsible for forest destruction, particularly in watershed areas, should have their licenses revoked. "Moreover, the Minister of Forestry has already stated that an evaluation will be conducted, so now we demand accountability. We have the names of the companies; please evaluate and enforce the law. Do not just make promises while hundreds of thousands of people are mourning in Sumatra," she said.

Observing the degradation of forests and the environment in Sumatra, extreme rainfall, regardless of its intensity, will result in disasters that are far greater than during the time when the forests were still intact. With climate change increasing the likelihood of extreme weather, the risk of hydrometeorological disasters will continue to rise in the coming years.

In reality, the floods and landslides in Sumatra are not merely the result of unusual rainfall. They are the most tangible consequence of the gradual collapse of environmental carrying capacity over decades.