Flood watch – 4 major dams and 32 reservoirs under close monitoring
ONWR says Thailand’s reservoirs are 57% full, with four major water sources above upper rule curve and 32 medium reservoirs over 80%
The Office of the National Water Resources (ONWR) has reported that overall reservoir storage across the country stood at 57% of capacity on May 24, 2026, while warning that four major water sources and 32 medium-sized reservoirs require close monitoring as water levels rise.
The update came as the southwest monsoon continued to influence weather conditions across Thailand. Although rainfall is expected to ease in many areas, heavy rain remains possible in some parts of the North and South.
Highest rainfall by region
ONWR’s daily water and weather summary showed that Phitsanulok and Phatthalung recorded the highest accumulated rainfall in the country.
The highest regional rainfall figures were:
- North: Phitsanulok, 125 millimetres, the highest nationwide
- South: Phatthalung, 115 millimetres
- Northeast: Nong Bua Lamphu, 62 millimetres
- East: Chonburi, 39 millimetres
- Central region: Bangkok, 20 millimetres
- West: Kanchanaburi, 18 millimetres
From May 25-27, a moderate southwest monsoon is forecast to continue covering the Andaman Sea, Thailand and the Gulf of Thailand. Rainfall across the country is likely to decrease, but isolated heavy rain is still expected in the North, the East and the western coast of the South.
Reservoir storage nationwide
ONWR said Thailand’s reservoirs currently hold a combined 45.785 billion cubic metres of water, or 57% of total storage capacity. Usable water stands at 21.677 billion cubic metres, or 38%.
By region, reservoir storage was reported as follows:
- West: 18.496 billion cubic metres, or 65%
- South: 4.637 billion cubic metres, or 61%
- North: 15.312 billion cubic metres, or 56%
- Northeast: 5.611 billion cubic metres, or 46%
- East: 1.117 billion cubic metres, or 35%
- Central region: 612 million cubic metres, or 31%
Four key Chao Phraya Basin dams
The four main dams serving the Chao Phraya River Basin reported the following storage levels:
- Bhumibol Dam: 7.715 billion cubic metres, or 57%
- Sirikit Dam: 5.257 billion cubic metres, or 55%
- Khwae Noi Bamrung Daen Dam: 237 million cubic metres, or 25%
- Pasak Jolasid Dam: 172 million cubic metres, or 18%
Four large water sources above upper rule curve
ONWR said four large water sources had exceeded their upper rule curve and must be monitored closely.
They are:
- North: Kio Lom Dam and Mae Chang Dam
- Northeast: Nam Pung Dam and Lam Pao Dam
The agency also reported that 32 medium-sized reservoirs nationwide are holding more than 80% of capacity. These comprise 13 in the North, 14 in the Northeast, two in the West, two in the South and one in the Central region.
People living in low-lying areas downstream of dams and other flood-risk locations have been advised to keep following official announcements and water situation updates closely.
