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There are four main river basins in Brunei Darussalam: Temburong, Belait, Tutong and Brunei:
- The Temburong, which is the smallest of the rivers, drains a catchment area of about 430 km2.
- The Belait is the largest basin, with an area of 2 700 km2. The lower catchment is composed of an extensive area of peat swamp forest. The river narrows at the town of Kuala Belait and a sandbar restricts the discharge of water into the South China Sea. Some areas in the upper catchment have been cleared for agriculture.
- The Tutong basin, which is about 1 300 km2, has a complex estuary system that has formed between two sand spits. Subject to fairly high tidal influence, its lower catchment is mainly floodplain. The upper catchment is jungle with patches of agriculture.
- The Brunei river flows into Brunei Bay. The upper reaches of the river are a major freshwater source particularly for the western part of the country.
In relation to the whole island, the runoff coefficient is estimated as 1.5 m/year corresponding to a surface flow of 8.5 km3. Limited reserves of groundwater have been identified in the Liang and Seria areas of the Belait district and in the Berakas area of the Brunei-Muara district. The estimated safe yield is 17.3 million m3/year. Also, in relation to the whole island, the total groundwater resources are estimated as 0.1 km3/year, all being drained by the rivers. Internal renewable water resources are around 8.5 km3 (Table 2).
Brunei Darussalam has two dams that have a total storage capacity of just over 45 million m3. The Tasek reservoir is used for water supply and has a total capacity of 13 000 m3; its catchment area is 2.8 km2. The Benutan dam, which is an impounded reservoir, used to regulate the Tutong river, has a total storage capacity of 45 million m│ and a catchment area of 28.6 km2.
There is no hydropower dam, though one suitable site has been located within the National Forest Reserve of Temburong.