Water profile of Cambodia
Geography and population
Cambodia is situated in southeast Asia on the coast of the Gulf of Thailand and has a total area of 181,040 km2. It is bordered by Thailand in the west, Lao PDR in the north and Vietnam in the east. These countries share the lower Mekong basin with Cambodia. Water surfaces, including Lake Tonle Sap, occupy approximately 2.2% of the total area of the country. The country is divided into 21 provinces for administrative purposes.
Physiographically, the country comprises an undulating plateau in its eastern part, a continuous flat plain (the Lake Tonle Sap lowland) interrupted only by isolated hills (Phnoms) and the Mekong River in the central part of the country, and by the Cardamone mountains in the southwest of the country.
The cultivable area is estimated at 4.626 million hectares (ha), or 25% of the total area. The total cultivated area has been estimated by a recent remote sensing survey at 3,914,400 ha, or 21.6% of the total area, though this may be an overestimate. Indeed, there is a mosaic of about 1 million ha of crops and secondary vegetal formation or trees, where the area actually cropped and harvested does not exceed 150,000 ha. Moreover, many paddy fields with palm trees are considered as cultivated although they are not actually farmed every year. More realistic estimates give a total cultivated area of about 2.1 million ha for 1993, of which 1,844,000 ha of cultivated rice (1,685,000 ha harvested), 122,000 ha of other annual crops, and 146,000 ha of permanent crops (mainly palm trees, coconut and rubber).
The total population was estimated at 10.3 million in 1996 (79% rural). The population density is 57 inhabitants/km2, varying from 4 inhabitants/km2 in Mondul Kiri in the northeast to 236 inhabitants/km2 in Kandal in the southeast. The population growth rate was 2.5% in 1994. About 73% of the active population is currently engaged in agriculture, and agriculture accounted for 45% of gross domestic product (GDP) in 1994.
Climate and water resources
Cambodia has a wet monsoon climate. The wet season starts in May and ends in October. The rainfall pattern is bi-modal with peaks in June and September/October.
In August, a short period of drought may damage wet-season rice which is not irrigated. In Phnom Penh, the monthly rainfall ranges from 5 millimeters (mm) in January to 255 mm in October. The average annual rainfall is estimated at 1,463 mm but varies from about 1,000 mm in Svay Check in the western province of Banteay Meanchey to nearly 4,700 mm in Bokor in the southern province of Kampot. The mean annual evaporation varies from 1,000 to 2,300 mm/year. April is the warmest month of the year with a maximum temperature of 36°C, while January is the coldest with 21°C.
River basin: the Tonle Sap/Mekong system
Cambodia has a unique hydrological system. The Mekong River and Lake Tonle Sap are connected by the Tonle Sap River which 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 estimated at 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.
About 86% of Cambodia's territory (156,000 km2) is included in the Mekong River basin, the remaining 14% draining directly towards the Gulf of Thailand. Cambodia was a member of the Mekong River Committee between 1957 and 1975. On 5 April 1995, Cambodia, Lao PDR, 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.
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 PDR, 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 Vietnam and 1.2 km3 from Thailand. On average, 471.4 km3/year flow out of the country in the Mekong channels and tributaries to Vietnam.
The internal renewable surface water resources (IRSWR) have been computed as the difference between outflow and inflow, i.e., 115.9 km3. This figure does not include the unknown discharge of small rivers to the Gulf of Thailand and is thus probably an underestimate. Groundwater resources are estimated at 17.6 km3, most of which (an estimated 13 km3/year) is drained by the rivers and cannot be considered as additional water resources. The total renewable water resources of Cambodia are therefore estimated at 476.110 km3/year.
Lakes and dams
The capacity of the existing dams is very low and has not been estimated. Only one small dam (Ochum, in the northeastern province of Ratanakiri) is used as a hydropower station with an installed capacity of 1 megawatts (MW). The Kirirom power plant, which was installed in 1968 in Kompong Speu province with a capacity of 10 MW, has not been in operation since 1970 due to war damage. A number of dams with high storage capacity are planned for the near future.
Water withdrawal was estimated at 520 million m3 in 1987 (Figure 1), of which 94% for agricultural purposes.
The total population with access to water supply was estimated at 19% in 1992. At that time, it was estimated that only 7,000 wells had been constructed (by international organizations) out of the 30,000 needed.
A 1995 survey assessed the quality of water supply, wastewater and sanitation in the main towns of Cambodia. Most of the systems combined sewage and drainage water, and have not been maintained over the past two decades. As a result, they are now 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. In Battambang, in the west of the country, about 13,000 people are served by a water sewage system. The average treated sewage flows are estimated at 157,000 m3/year.
Irrigation and drainage development
Cambodia's history of hydraulic control goes back to before the Angkor period (tenth century). The famous Angkor Wat irrigation system was based on four reservoirs, built between the tenth and thirteenth centuries, and stored some 100-150 million m3 of water to irrigate approximately 14,000 ha.
Modern irrigation systems were first developed in the period 1950-53. Many of the structures built during that period functioned until 1975. Most of these structures, such as the 'colmatage' canals, have become non-functional as a result of the network of irrigation/drainage systems built during the period 1975-79. Since then, most attempts to rehabilitate these newer schemes have failed (Figure 2).
Irrigation potential has never been estimated in terms of physical area which could be irrigated considering water and land resources. However, an assessment has been made of the total potential cropped area if existing and past irrigation systems were rehabilitated and improved. The total area would be 419,344 ha in the wet season and 187,020 ha in the dry season.
Water managed areas were estimated to be 390,461 ha in 1993, of which 69% were equipped with full/partial control irrigation, and 31% were flood recession cropping areas (Figure 3).
Some 841 full/partial control irrigation schemes have been recorded in a recent inventory, covering a total area of 269,461 ha. Only 176 of these schemes were reported to be fully operational, while 115 schemes covering 27,638 ha were equipped but not operating.
The operating full/partial control irrigation schemes can be divided into four main categories (Figure 4):
- River, lake or stream diversion by gravity. These systems are used for wet season supplementary irrigation as there are no storage facilities. Offtakes are generally uncontrolled, although in some cases, water level control is provided by diversion weirs.
- Water pumping from rivers. These systems can provide water for both the wet and dry seasons. Pump stations have been provided by the Government.
- Reservoirs storing water from runoff, streams or rivers for wet season supplementary irrigation. Water is abstracted from the reservoir by gravity or mobile pumps provided by farmers.
- Reservoirs storing flood waters from the Tonle Sap/Bassac/Mekong system and released by gravity or mobile pumps for a dry season recession crop only. These areas also benefit from natural flooding for land preparation. The crop is transplanted as the floodwater recedes and irrigated during the growing season with water stored in nearby reservoirs. This system takes advantage of the large range of water levels in the Tonle Sap/Bassac/Mekong system to fill the reservoirs during the flood to a level sufficient to give gravity command of the paddy fields. Although they are equipped for full/partial control irrigation, these areas are often termed "flood recession areas" as they use natural flooding at the beginning of the season for land preparation and the filling of the reservoirs.
The cropped area in these full/partial control irrigation schemes is estimated at 172,727 ha during the wet season and 103,656 ha in the dry season. Double cropping is practiced only on small areas (estimated at 6,922 ha).
Another classification, used by the Department of Hydrology, defines three irrigation systems (Figure 5):
- Large-scale projects, where water is supplied from a multipurpose dam (generally irrigation and hydropower). The annual irrigated area for these schemes is estimated at 118,225 ha in the wet season and 63,241 ha in the dry season.
- Medium-scale projects, with an irrigated area of 100 ha or more, where water is supplied by single-purpose dams or 'colmatage' canals. The 'colmatage' system uses dikes and sluices to provide controlled annual inundation. Intake and drainage are controlled, allowing a fertile layer of silt to settle on the fields. The annual irrigated area for these schemes is estimated at 46,599 ha in the wet season and 31,225 ha in the dry season.
- Small-scale projects, with an area of less than 100 ha. The annual irrigated area for these schemes is estimated at 7,903 ha in the wet season and 9,190 ha in the dry season.
There are about 121,000 ha of floating rice, mainly in the provinces bordering Lake Tonle Sap: Battambang, Banteay Meanchey, Pursat, Siem Reap, Kompong Thom and Kampong Cham. This general category consists of two subcategories:
- floating rice, with a straw length up to 4 meters (m);
- deep water rice, with a straw length of 1-2 m.
Both subcategories are adapted to continuous, unregulated flooding. The rice varieties have a rapid elongation with increase in water depth, and submergence tolerance to flash floods. In this country profile, these 121,000 ha are considered as deep-water/flood recession cropping areas.
A recent survey has estimated that the development of one hectare irrigated by pumping would require an investment coast of US$2,800, and US$85/year for operation and maintenance (O&M), while the respective figures for a hectare irrigated from a reservoir would be US$3,600-4,300 and US$40-65/year.
The major crop in Cambodia is rice, with a total harvested area of 1.84 million ha in 1993. This figure comprises irrigated rice, floating rice, but also upland rice (about 24,000 ha in 1993) and rainfed rice which is cultivated in the lowland during the wet season and which covers most of the rice-cropped area in Cambodia. The average rice yield is estimated at 1.39 t/ha under rainfed conditions and 2.07 t/ha under irrigated conditions.
A recent Food and Agriculture Organization (FAO) survey indicates that a number of areas appear suitable for groundwater exploitation, though there are still uncertainties about water quantity and quality. The lack of data, particularly on water quality, is a cause for concern as there are reports on iron toxicity from Svay Rieng province, close to the border with Vietnam, as well as increased tidal saline incursion from the Mekong River in May-June.
Two of the most common water-related diseases linked to the development of irrigation are malaria and schistosomiasis. Malaria is already a serious problem throughout the country as a consequence of the natural ecosystem. Estimates of about 500,000 cases of malaria per year are common. Each year, 5,000-10,000 persons die from malaria. Schistosomiasis was reported in the Kratie area in 1993. Dengue haemorrhagic fever has recently become an important cause of child morbidity in Cambodia. In 1990, about 7,000 cases resulting in 340 deaths were recorded.
The public institutions involved in the water sector are:
- The General Directorate of Irrigation, Meteorology and Hydrology of the Ministry of Agriculture, Forestry and Fisheries, with:
- the Department of Water Management, which is responsible for the O&M of all irrigation infrastructure in Cambodia, including the operation and repair of pumps. The office also undertakes rural water supply, including well drilling;
- the Department of Engineering, which is responsible for the design and construction of hydraulic structures;
- the Department of Hydrology, which carries out the installation and maintenance of a network of hydrological stations, and collects and processes data;
- the Department of Meteorology, which is in charge of meteorological data collection and forecasting;
- the Department of Research, Training and Extension.
- The Mekong Secretariat: the River Mekong, whose lower basin covers large areas of Cambodia, Lao PDR, Thailand and Vietnam, is a major regional resource. In 1957, the Mekong River Committee was established under the auspices of the United Nations to coordinate the efforts of these countries in developing the resource. In principle, all proposals to utilize the waters of the Lower Mekong Basin require the unanimous approval of the country representatives on the now renamed Mekong River Commission.
An informal 'water resources law task force' has been established through the Irrigation Sector Meeting of the interested parties. As part of this process, an adviser to the Ministry of Agriculture, Forestry and Fisheries (MAFF) has compiled a draft law on the water resources of Cambodia, which was due to be submitted in 1996.
Domestic water supply is the responsibility of several institutions: the Department of Hydrology; the Ministry of Public Works; and the Ministry of Rural Development.
Trends in water resources management
Under the National Socio-Economic Development Plan, 1996-2000, water supply and wastewater treatment have been set as priorities by the Government.
While precise comprehensive data on access to water supply are not available, it is estimated that some 1.75 million people (19% of the population) have access to clean drinking water. This is about 40% of the urban population and 15% of the rural population.
Similarly, precise comprehensive data on the provision of environmental sanitation are not available. Access to sanitation is limited to an estimated 1.24 million people (13% of the population); about 53% of the urban population (mostly in Phnom Penh) and 6% of the rural population.
It is estimated that providing a safe water supply to 65% of the rural population by 2000 would require a capital investment of nearly US$31 million, or an average of about US$6 million/year.
The target is to provide an additional 1.5 million rural people with access to environmental sanitation facilities in the period 1996-2000. This is based on a realistic assessment of the Government's absorptive capacity, its ability to implement programs and the prospects for external funding to the sector. It would increase the coverage of rural sanitation from 6% to about 22% by 2000.
In the recent past, sedimentation of Lake Tonle Sap has given cause for concern. This concern is mainly due to the Mekong silt load, and to deforestation in the upper reaches of the Tonle Sap watershed. In the absence of reliable data on hydrology and sediments in this area, many scenarios have been developed. The most pessimistic ones forecast a drying up of the lake in a ten-year period, while other studies estimate that the lake would take 600 years to dry up. All these estimates reveal a need for reliable hydrological data. What is agreed by all concerned is the negative effect of sedimentation on the environment, particularly on fish.
As new irrigation scheme development has a low economic internal rate of return (1-6%), the rehabilitation of existing schemes has been set as a priority by the Government. Priority is given to small-scale schemes, as large-scale schemes have serious O&M problems. The estimated potential of irrigated agriculture production is high for small-scale irrigation schemes with active community participation and in combination with other agricultural technology packages, especially balanced fertilizer use. Indeed, soil fertility is a major problem in Cambodia and production increase with irrigation alone would remain relatively limited.
In the Mekong Delta, the development of groundwater irrigation might be a valid alternative to the present water managed systems (in certain areas with sufficient and easily accessible groundwater reserves) whose efficiency depends heavily on the level fluctuations of the Mekong River. Recently, sprinkler and micro-irrigation have been introduced on very small areas in Cambodia.
Another priority is the development of well-designed flood control devices in conjunction with irrigation facilities to enable drainage in times of flooding, and irrigation in the dry season. Another priority is the construction of several dams, mainly for hydropower purposes. Investigations have been carried out by the Mekong River Commission. Two of these dams (Sambor and Stung Treng), with a total estimated cost of US$11,907 million would have a power capacity of 4,208 megawatts (MW). The environmental costs would include 31,700 ha of agricultural land and 75,300 ha of forests flooded, and more than 14,000 people having to be resettled.
Another project has been prepared for regulating the level of Lake Tonle Sap and for hydroelectricity generation (140 MW of capacity) at an estimated cost of US$435 million.
The Asian Development Bank is investigating the feasibility of building dams on the Stung Chinit (a tributary of the Tonle Sap River), and on the Se Kong and Se San rivers, both in the province of Ratanakiri in northeast Cambodia.
- Water profile of Cambodia, Food and Agriculture Organization.
- World Factbook: Cambodia, Central Intelligence Agency.
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