State of water : Cambodia
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2.2.4. Tributaries of the Tonle Sap Great Lake

The Tonle Sap Great Lake has many surrounding tributaries. Therefore, during the wet season, the Tonle Sap Great Lake receives not merely a tremendous quantity of water from the Mekong River, but from its tributaries as well. The major tributaries include the following.

  • The Stung Chinith flows into the Tonle Sap Great Lake which is connected via the Tonle Sap River, and has a basin area of 4,504 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 6,711 m³/s.
  • The Stung Sen flows into the Tonle Sap Great Lake which is connected via the Tonle Sap River to the Mekong River, and has a basin area of 13,653 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 11,685 m³/s.
  • The Stung Stoung flows into the Tonle Sap Great Lake which is connected via the Tonle Sap River to the Mekong River, and has a basin area of 2,012 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 4,486 m³/s.
  • The Stung Chickreng flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 1,871 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 4,326 m³/s.
  • The Stung Sreng flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 9,471 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 9,732 m³/s.
  • The Stung Sisophon flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 4,343 km². Only 2,388 km² is located in Cambodia. The area receives about 1,500 mm of rain per year and the average runoff is estimated at 6,590 m³/s.
  • The Stung Boribo flows into the Tonle Sap River which is connected to Mekong River, and has a basin area of 827 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 2,876 m³/s.
  • The Stung Mongkol Borey flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 4,444 km². Only 3,156 km² is located in Cambodia. The area receives about 1,500 mm of rain per year and the average runoff is estimated at 6,666 m³/s.
  • The Stung Krang Ponley flows into the Tonle Sap River which is connected to the Mekong River, and has a basin area of 861 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 2,934 m³/s.
  • The Stung Battambong flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River. The area receives about 1376 mm of rain per year and the average runoff is estimated at 5,683 m³/s.
  • The Stung Daontri/Moung Russey flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 819 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 2,862 m³/s.
  • The Stung Sangker flows into the Tonle Sap Great Lake which is connected to Mekong River via the Tonle Sap River, and has a basin area of 3,225 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 5,679 m³/s.
  • The Stung Pheas flows into the Tonle Sap Great Lake which is connected to the Mekong River through the Tonle Sap River, and has a basin area of 822 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 6,590 m³/s.
  • The Stung Svay Chek flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a total basin area of 2,104 km². Only 1,249 km² is located in Cambodia. The area receives about 1,500 mm of rain per year and the average runoff is estimated at 3,464 m³/s.
  • The Stung Preah Neat Preah flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 1,387 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 3,724 m³/s.
  • The Stung Kampong Krasaing flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 159 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 1,261 m³/s.
  • The Stung Siem Reap flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 842 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 2,902 m³/s.
  • The Stung Roluos flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 551 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 2,347 m³/s.
  • The Preak Thnal Dach flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 339 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 1,838 m³/s.
  • The O Leu flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 385 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 1,962 m³/s.
  • The Stung Neang Sa Sngach flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 1,148 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 3,388 m³/s.
  • The Stung Sraka Moan flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 1,341 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 3,662 m³/s.
  • The Stung Taing Krasaing flows into the Tonle Sap Great Lake which is connected to the Mekong River via the Tonle Sap River, and has a basin area of 1,148 km². The area receives about 1,500 mm of rain per year and the average runoff is estimated at 3,384 m³/s.

2.2.4.1. Water and related resources

Water resources analysis is complex and requires the availability of comprehensive, accurate and up-to-date scientific data. In contrast to the neighbouring countries of the Lower Mekong Basin, Thailand and Vietnam, Cambodia crucially lacks necessary data. For instance, according to N. Bonheur (2003), hydrological data related to the rivers of the Tonle Sap basin and river are available for only one hydrological year as collected by Carbonnel and Guiscafre (1962-63). With the support of the MRC, there have been some gauging stations installed in some rivers of the basin. In the below paragraph, an attempt is made to provide existing data that has been collected from various sources. Only fragmentary information has been gathered, and thus the possibility for serious and comprehensive analysis is limited.

According the BDP Inception Report (MRC, 2002j), water resources analysis should contain three main components:

• Availability of water
• Demand of water
• Use of water

2.2.4.2. Availability of water

This can be defined as the flow of water into a sub-area from upstream, plus the (surface and groundwater) resources generated by net rainfall in the sub-area, minus the ecological demand within the area at its downstream boundary. The availability changes slowly, from one decade to the next, due to medium-term climate variations or to the construction of reservoirs or diversions. The availability can be measured and/or determined by numerical modelling with an accuracy that is conditioned by the coverage and quality of the basic hydrological data.

2.2.4.2.1. Rainfall and surface water

Cambodia has access to substantial surface water resources. On average, the annual inflow from upstream countries is estimated at 410 km³ and the internally generated flow at 90 km³ per year (MOWRAM, 2003). In Cambodia, the Mekong River flows from north to south, over a distance of about 480 km. Its drainage basin covers about 86 percent of the land area of the country. The Mekong River brings yearly floods of about 475 km³, and before flowing downstream, inundates the lowlands and where the floodwaters partially enter the Great Lake and eventually flow down the Mekong and Bassac Rivers.

A summary of rainfall in different locations within the Delta is presented in the figures below. The rainy season occurs from May to October, which is the monsoon season. The mean annual rainfall ranges from around 1,280 mm/year to 1,520 mm/year. Figures have shown that there are two peak mean rainfall periods: one takes place in October which is near the end of rainy season; and the other generally occurs in May. The driest month is February.

The average rainfall in the eastern part of the Delta Sub-area is high compared with other locations, ranging from 1,173.4 to 1,866.6 mm/year. The southwest zone of the Sub-area generally has the smallest annual rainfall, ranging from 936.8 to 1,816.5 mm/year.

Figure 1:Rainfall in Phnom Penh City

Figure 2:Rainfall in Daun Keo, Takeo Province

Figure 3:Rainfall in Prey Veng

Figure 4:Rainfall in Kampong Speu

2.2.4.2.2. River discharge

Discharge during the dry season period (April to May) in 2003 is shown in the table below. A study by JICA has indicated that the dry season flows in the Cambodian floodplains are strongly affected by tidal fluctuations.

Table 3: Effects of tidal fluctuations onto the dry season flow

Station Average Flow
(m³/sec)
Range of Fluctuation
(m³/sec)
Fluctuation Rate
(percent)
Kampong Cham 1,600 100 6
Chrui Changvar 2,000 1,500 75
Neak Luong 3,000 3,000 100
Monivong Bridge 100 150 150
Source: WUP-JICA, 2004.

2.2.4.3. Groundwater

Groundwater is presently used for two main purposes: domestic and drinking water supply and irrigated agriculture. Groundwater resources within the country have been subject to a few investigations but have not been comprehensively studied to date such that serious analysis or conclusions can be drawn. Most provinces include significant areas where groundwater is used as the main source of domestic water supply. As of 2001, withdrawal of groundwater for domestic and drinking water supply was approximately 2,147 cubic meters per day (CNMC, 2003).

Table 4: Groundwater withdrawal for domestic supply

Station Pump Capacity
(m³/day)
Prey Veng 227
Kampong Cham 1,500
Svay Rieng 420
Source: CNMC, 2003.

2.2.4.4. Surface water extraction for domestic water supply

The volume of domestic water supply for Phnom Penh city is much higher than that of provincial towns. A study by WUP-JICA has shown that the estimated amount of urban water usage is approximately 68 million m³ per year. Water extraction at the station in Phnom Penh city is illustrated in the table below.

Table 5: Major river intakes for domestic water supply in Phnom Penh

Station
River
Water Extraction
(m³/day)
Chrui Changvar Mekong 65,000
Chang Kampong Bassac 20,000
Phnom Penh Port Tonle Sap 100,000
Source: Sstudy by WUP-JICA

2.2.4.5. Water demand and use

According to Nielsen (2004), the water demand is the amount required for a given purpose. The demand can be based on the present or future, and it can be actual (i.e. related to an available infrastructure) or potential (assuming full infrastructural development and no raw water shortage). The serviceable (part of the) demand is limited both by infrastructure and raw water availability.

A distinction can be made between consumptive demand (for households, industries and agriculture), and non-consumptive demand (for fisheries, navigation, and environmental preservation).

2.2.4.6. Domestic water use

Domestic water use includes water for normal household purposes, such as drinking, food preparation, bathing, washing clothes and dishes, flushing toilets, and watering lawns and gardens. Domestic water use also includes drinking water and bathing water for livestock–this is an important issue for rural livelihoods.

Increase of domestic demand is mainly based on population growth. Assuming that the annual domestic demand is between 20 m³ and 100 m³, the total demand for domestic water is in the range of 286,000m³ and 1.4 million m³ per day.

It is estimated that the domestic demand increased from 3.1m³/s to 3.3m³/s in 2002. The domestic water demand is shown in the below table.

Table 6: Domestic water demand

Population Water Demand, m³/day
1998 2002 1998 2002
4,930,320 5,198,981 271,168 285,944
Source: BDP, 2003.

2.2.4.7. Industrial demand

Water is necessary for all industrial activities, including cooling, processing or manufacturing operations, power generation, cleanup and other sanitary purposes, and fire protection. The quality and quantity of industrial water demand varies significantly by country, industry and particularly uses, ranging from high water quality for the beverage industry to brackish water or treated municipal effluent for cooling purposes.

According to the MRC-BDP Planning, Regional Sector Overview (2002), urban water usage is estimated at about 100 litres per person per day, for a total average annual supply of 36 million m³ for other urban centres in Cambodia. The total water usage represents about 0.01 percent of the flow in the Mekong River (WUP, 2001).

It is assumed that most Phnom Penh wastewater transported to the Delta will travel via the Bassac River. However, with the distance from Phnom Penh to the Vietnam border (about 110 km), and the large amount of dilution from the major river flows and high water temperature (about 29 degrees Celsius) in the Bassac River, the rate of decomposition of any organic matter discharged from Phnom Penh will be rapidly broken down and organic types of pollution are not likely to reach the border of Vietnam (Hart, 2001).

Most manufacturing and warehouses in Phnom Penh are located along the embankment of the Tonle Sap River north of the town or at the Bassac River south of the town mixed with commercial and residential areas. Such locations allow direct access to river transport and high consumption of water (MRC, 2002i). The water requirement by industrial sector is based upon the size of the factory. An estimate of water use volume for different sizes and types of factories are as follows (CNMC, 2003):

  • Major industry: 1,000 to 20,000 m³/day (for instance: paper production, chemical manufacturing, iron and steel production, oil refining, etc.);
  • Large scale industry: 100 to 500 m³/day (food processing, vegetable washing, drink bottling, ice production, chemical products, etc.); and
  • Medium and small scale industry: 50m³/day.

2.2.4.8. Agriculture use

Cambodia has inventoried 946 operating irrigation systems which can service 256,120 ha of the two million ha wet season cultivated area. In the dry season, rice is grown on 225,000 ha; of this, 143,490 ha are fully irrigated from irrigation schemes. Hence, only 12 percent of the wet season rice is irrigable, the remainder being rainfed. and just over half of the dry season crop is irrigated, the remainder being recession rice receiving supplementary irrigation from manually operated and diesel driven pumps. Very few irrigation schemes are capable of irrigating all year round.

Fully irrigated crops utilize approximately 10,000 m³/ha. Some irrigation engineers in Cambodia estimate that recession rice receives approximately 4,000 m³/ha of irrigated water.

The water consumption for rice is high compared to other crops. The total water consumption is dependent on crop type, stage of crop growth, soil type, irrigation method, and so on. The water consumption for different kinds of crops and for rice is presented in the table below.

Table 7: Water consumption within a critical period

Activity Water Flow
(l/sec/ha)
Water Requirement
(m³/ha/month)
Crop Irrigation Life
(month)
Critical Period Consumption
(m³/ha)
Irrigated rice 0.8 2,074 3.5 7,258
Upland crops 0.6 1,555 4 6,221
Fruit trees 0.4 1,037 4 4,147
Source: BDP, 2003.

Assuming that the water requirement during the critical period (February to May) for dry season irrigated rice and that for non-rice crops is approximately 0.8 l/sec/ha and 0.6 l/sec/ha respectively, the total water use for rice and non-rice crops is estimated at about 1,571 million m³/month and 509 million m³/month respectively.

Table 8: Water consumption within the Delta Sub-area during a critical period

Crop Type Water Flow
(l/sec/ha)
Water Consumption
(m³/month)
Paddy rice 0.8 1,570,876,636
Non-rice crop 0.6 509,125,660
Total 20,800,022,963
Source: BDP, 2003.

Water from the Mekong River is utilised for irrigation, hydropower generation, domestic and industrial purposes. Much of the water emerging from hydropower stations is also consumed downstream for irrigation, domestic and industrial purposes. Irrigated agriculture is responsible for 80 to 90 percent of water abstractions from the basin (FAP from MRC, 2002) and is utilised in the form of receding flood water storage, and diversion of water from streams and groundwater sources.

On a nationwide basis, the LMB countries do not fully utilise their renewable water resources. Renewable water resources are equal to the total precipitation in the country minus evapo-transpiration. Cambodia and Laos use only one percent of their total renewable water resources for agriculture while Vietnam and Thailand use 5 and 20 percent respectively. Based on these figures, an average Mekong River flow of 460 km³ each year can service the irrigation requirements of all LMB countries 11 fold. An annual Mekong River flow can also service approximately 64 million ha of fully irrigated rice based on a consumption of 10,000 m³/ha (one meter of water) per crop. This compares with the 1999/2000 area of two million ha of dry season cropping in the LMB watersheds.

Therefore, there is no shortage of water in the Mekong River to service agriculture in its watersheds if all water is captured and redistributed when required. This is, of course, not the case, with a majority of water flowing through to the ocean during the wet season when crops receive most of their water requirements directly from rainfall. Water shortages may occur (especially in the Mekong Delta) during the months of February to May when water flows in the Mekong River are at their lowest. Crop irrigation is the major consumer.

2.2.4.9. Ecological demand

Ecological demand (of water) is a minimum stream flow or water level required for prevention of irreversible ecological degradation. The ecological demand varies from year to year and from place to place. The flow must be high in the wet season in order to maintain a healthy environment; for instance, fish species mainly rely on annual floods for reproduction.

According to the MRC (2003e), a value of 1 l/sec per km² has been applied as an indicator for ecological demand in the dry season for the Kok River basin. At the regional level, it is an important aim to preserve the Mekong Delta as a freshwater regime. This requires maintenance of a certain minimum flow, estimated at 2 l/sec per km² (Nielsen, 2002).

2.2.4.10. Other consumption

According to the MRC (2003e), the consumption of irrigation water in the Mekong Delta fish ponds for aquaculture is approximately 6,000m³/ha/month.

A study has shown that the total water use for tourists is much higher than local inhabitants, which affects the water use within the city and provincial towns. If the water use per tourist is approximately 0.5m³/day, the total water use for the tourism sector within the Delta Sub-area is estimated at 1.4 million cubic meters per year (including local and foreign tourists).

Some figures on water demand and use:

Total water demand/capita = 150 m³ per capita per year
Share agriculture = 94%
Municipal and industrial share = 6%
Municipal and industrial withdrawal in 1990 = 78 million m³
Municipal and industrial withdrawal in 2020 = 187 million m³
(Source: Ringler, 2001 in MRC-BDP Planning Regional Sector Overview 2002)

Total water demand = 0.5 Bm³ per year
Internal water supply = 1,004 m³ per person
Water for domestic use = 5%
Water for industrial use = 1%
Water for agricultural use = 94%

These figures will change due to future development in the LMB that will bring about changes in the river hydrology (Source: CNMC, 2003)

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Table of contents > 2. River Basins > 2.2. Tonle Sap River System
2.2.1. Introduction
2.2.2. The World network of biosphere reserves
2.2.3. The Royal Decree
2.2.4. Tributaries of the Tonle Sap Great Lake
2.2.5. Detailed survey of surface water in the Tonle Sap Great Lake catchment
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