Read the full profile
Thailand can be divided into seven river basins, but in the literature it is generally divided into 25 subbasins. Figure 1 and Table 2 show the location and the characteristics of the 25 major river subbasins and indicate the total surface water resources, 213.35 km3/year. Aquifer recharge from rainfall is around41.90 km3/year (about 5-6 percent of the total precipitation). Approximately 30.70 km3/year are estimated to return to the river system (overlap). The total internal water resources of Thailand are therefore about 224.55 km3/year.
Thailand shares three major river systems with its neighbours: the Mekong river forms the border with Lao People’s Democratic Republic in the north and east (about 18 percent of the total Mekong catchment area is located in Thailand), the Salween river is on the northwestern border with Myanmar, and the Kolok river is on the southern border with Malaysia. This last river, originating in Thailand and then bordering between Thailand and Malaysia, is very short with a total length of just over 100 km.
The Mekong river constitutes an additional external resource for Thailand, which has been estimated as half the discharge of the river, Thailand’s contribution to this river has to be deducted over a long distance. The flow of the Mekong river at the point where it enters Lao People’s Democratic Republic near Pakxé is about 280 km3/year. The contribution of Thailand to the Mekong river is an estimated 51.9 km3/year This gives an accounted flow of the Mekong border river for Thailand of 114.05 km3/year. The Salween river on the border with Myanmar, with an estimated flow of 200 km3/year, flows only over a relatively short distance on the border. It is therefore considered that there is not much contribution from Thailand over that short distance and the accounted flow is 200/2=100 km3/year.
By adding the internal and external resources together, the total renewable water resources are approximately 438.6 km3/year (Table 3).
Total exploitable water is an estimated 125.98 km3/year, consisting of 75.64 km3/year regular renewable surface water, 27.34 km3/year irregular renewable surface water and 23.00 km3/year regular renewable groundwater.
Total large dam capacity is an estimated 68.28 km3 in 2007, which is about 32 percent of the annual runoff. However, many dams have been over-designed, compared with the annual recharge obtainable. There are four categories of dams in Thailand:
- Large dams with hydropower component are built by the Electricity Generating Authority of Thailand (EGAT), the Royal Irrigation Department (RID) or the Department of Energy Development and Promotion and managed by the EGAT. Their total capacity is an estimated 62.87 km3. All these dams are multipurpose dams, and the irrigation component receives priority over the other components.
- Large dams without hydropower, and therefore mainly destined for irrigation, are operated by the RID. Their total capacity was an estimated 5.41 km3 in 2003.
- Medium dams, similar to large dams with no hydropower, are also under RID. Reasons for not classifying these as large dams are: (i) to avoid environmental assessment, and (ii) to shorten budget processing and construction times to cope with high priority areas.
- Small dams used to be under RID, but are now under local governments. Most of these small dams are for domestic and subsistence irrigation purposes.
There are five dams with a capacity of more than 5 km3: Srinagarind (17.75 km3), Bhumipol (13.46 km3), Sirikit (9.51 km3), Vaijiralongkorn (8.86 km3) and Rat Cha Prapa (5.64 km3).
There are 5 main dams on the Mekong river basin in Thailand, the Sirindhorn (1 966 million m3), Chulabhorn (188 million m3), Ubol Ratana (2 264 million m3), Pak Mun (114 million m3) and Lam Ta Khong (310 million m3).