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The main hydrological system is the Tonel Sap/Mekong system. The Mekong river and Lake Tonle Sap are connected by the Tonle Sap river, which is approximately 120 km long and twice a year reverses its direction of flow. From July to the end of October, when the level of the Mekong is high, water flows into the Tonle Sap river, which fills Lake Tonle Sap, thereby increasing the size of the lake from 2 600 km2 to about 10 500 km2 at its maximum. The storage capacity of Lake Tonle Sap is about 72 km3. In early November, when the level of the Mekong decreases, the Tonle Sap river reverses its flow, and water flows from Lake Tonle Sap to the Mekong river and thence to the Mekong Delta.
The Tonle Sap Great Lake has several input rivers, the most important being the Tonle Sap river during the rainy season, which contributes 62 percent of the total water supply. The other rivers in the sub-basin and direct rainfall on the lake contribute the remaining 38 percent. Other major rivers are the Sen river, Sreng river, Pursat (Pouthisat) river, Sisophon river, Mongkul Borey river, and Sangker river.
In Cambodia, the Mekong river flows from north to south, over a distance of around 480 km. About 86 percent of Cambodia’s territory (156 000 km2) is included in the Mekong river basin, the remaining 14 percent draining directly towards the Gulf of Thailand. The Kong river is one of the largest tributaries of the Mekong. It originates in Viet Nam, runs through Lao PDR and joins the San river and Mekong river near Stoeng Treng in Cambodia.
Cambodia was a member of the Mekong river Committee between 1957 and 1975. On 5 April 1995, Cambodia, Lao People’s Democratic Republic, Thailand and Viet Nam signed an agreement for the development of the Mekong river. Under the agreement, the Mekong River Committee became the Mekong River Commission. Cambodia represents 20 percent of the total catchment area of the Mekong river basin.
This river system flowing into the Gulf of Thailand is less important, but retains its potential for future development of water resources, owing to much rain and steep slopes in this area (WEPA, 2010).
The average annual discharge of the Mekong river entering Cambodia is estimated to be close to the discharge at Paksé (324.45 km3/year) in Lao People’s Democratic Republic, some 120 km upstream from the border with Cambodia. Other inflows to the Mekong-Tonle Sap system from outside the country amount to 29.9 km3 from Viet Nam and 1.19 km3 from Thailand. On average, 471.51 km3/year flows out of the country to Viet Nam through the Mekong channels (470 km3/year) and tributaries (1.41 km3/year).
The internal renewable surface water resources (IRSWR) have been computed as the difference between outflow and inflow, i.e. 115.97 km3. This figure does not include the unknown discharge of small rivers to the Gulf of Thailand and is thus probably an underestimate. Annual groundwater resources are about 17.6 km3 most of which, an estimated 13 km3, is drained by the rivers and cannot be considered as additional water resources. The total internal renewable water resources of Cambodia are, therefore, approximately 120.57 km3/year (115.97+17.6-13.0) and total renewable water resources at 476.11 km3/year (Table 2).
The alluvial deposits in the Tonle Sap and Mekong floodplain/delta are believed to be excellent shallow aquifers, with high recharge rates and a water table generally within 5-10 m of the surface water. Shallow wells could be used in an estimated 48 000 km2 of the country (WEPA, 2010).
The capacity of the existing dams is very low and has not been estimated. In 2008, there were only two dams. One small dam (Ochum, in the northeastern province of Rotanakiri) is used as a hydropower station with an installed capacity of 1 MW. The Kirirom power plant, which was installed in 1968 in Kampong Speu province with a capacity of 10 MW has not been in operation since 1970, because of war damage but has been reconstructed with a capacity of 12 MW (Sereyvuth, 2008). The government is preparing to build ten hydroelectric and irrigation dams in the northwest provinces. It is hoped that the US$4 000 million project will supply more than 100 000 rural families with water and electricity (ABC, 2008).
The country’s demand for electrical power is projected to increase from 251 MW in 2000 to 746 MW in 2016 (WEPA). Projects under implementation are the 193 MW Kamchay Hydro-project (2010), the 120 MW Atay Hydropower Plant (2012), the 338 MW Lower Russei Chhrum Hydropower Plant (2014), the 18 MW Kirirum III and the 246 MW Tatay Hydropower plant by (2015) (Sereyvuth, 2008).
There are plans to construct a large hydropower dam near the confluence of the San and Srepok rivers in Sesan District, Stueng Treng Province. However, civil society and local communities say that the plan has not adequately considered the project’s negative environmental and social impacts and the needs of affected communities living upstream and downstream of the proposed dam site. The 75 m high dam is expected to inundate more than 30 000 ha of land and forest and to result in the resettlement of an estimated 5 000 people (International Rivers, 2009).
In 1994, the average treated sewage flows were about 157 000 m3/year. Most of the systems combine sewage and drainage water and are in a poor condition and not functioning properly. Drainage water often mixes with drinking water with obvious health implications. Floods are frequent during the rainy season as the sewers clog rapidly.