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Irrigation and drainage
Evolution of irrigation 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 twelfth century, which stored some 100-150 million m3 of water to irrigate approximately 14 000 ha.
Modern irrigation systems were first developed in the period 1950-1953. 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 and drainage systems built during the period of the Democratic Kampuchea (1975-1979), when the regime put practically the entire population to work planting rice and digging irrigation dykes and canals during which over 20 percent of the population died of exhaustion, starvation, disease and execution (Himel, 2007).
The nation’s irrigation infrastructure has grown gradually over the past two decades. This has been a major focus of the government to enable farmers to achieve higher crop yields, reduce vulnerability to drought, stabilise rice production potential, and increase national food security or self-sufficiency. Despite this growth, Cambodia’s irrigation remains significantly underdeveloped. In comparison to similar rice cultivation environments in the lowlands of Thailand or the Mekong Delta in Viet Nam, Cambodia has little rice land under irrigation. The Ministry of Water Resources and Meteorology (MOWRAM) estimates that approximately 24 percent of the country’s rice land is irrigated (USDA, 2010).
Recently, some 946 operating irrigation systems have been inventoried. However, many are not operational. The area equipped for full control irrigation in 2001 was around 284 177 ha (MRC, 2003). Rice was cultivated on 275 177 ha; sugarcane on 8 000 ha and citrus on 1 000 ha. In the dry season, rice cultivated in flood recession areas covered around 63 000 ha and in the wet season deep floating rice covered around 137 753 ha (MAFF, 2006). This brings the total water managed area for 2001 to 484 870 ha. The area equipped for full control irrigation in 2006 was about353 566 ha. Rice cultivated in flood recession areas covered 367 688 ha, of which 63 000 ha in the dry season. This brings the total water managed area in 2006 to 721 254 ha (Table 4).
Irrigation potential has never been estimated for the physical area that could be irrigated considering water and land resources. However, it could be at least 1 million ha.
The operating irrigation schemes can be divided into four main categories:
- 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 is 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 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.
Another classification, used by the Department of Hydrology, defines three irrigation systems with the following areas in 1993 (Figure 2):
- Large-scale projects, where water is supplied from a multipurpose dam (generally irrigation and hydropower). The annual irrigated area in 1993 for these schemes is an estimated 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 dykes 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 in 1993 for these schemes is an estimated 46 599 ha in the wet season and 31 225 ha in the dry season.
- Small-scale projects, are less than 100 ha. The annual irrigated area in 1993 for these schemes is an estimated 7 903 ha in the wet season and 9 190 ha in the dry season.
All irrigation in Cambodia is surface irrigation. During the 1990s, sprinkler and localized irrigation was introduced on very small areas.
In 2010, the government implemented the ‘Hegemonization of Irrigation System Strategy’. The project cost US$61 million, including a credit loan of US$47 million from China and US$14 million from the government of Cambodia. Once completed, the project will be capable of irrigating over 49 000 ha of agricultural land.
The Asian Development Bank (ADB) in 2003 approved a loan of US$18 million to develop irrigated agriculture to boost production in poor and neglected rural areas of northwest Cambodia. The loan for the Northwest Irrigation Sector Project assists in improving water resource management, providing rehabilitation and improvement of irrigation schemes and other water control infrastructure, and strengthening management of the irrigation infrastructure. The project focuses on four northwest provinces, Pursat, Battambang, Banteay Mean Chey and Siem Reap, which are among the poorest and most isolated areas in Cambodia. The total cost of the project is an estimated US$30.9 million. Agence Francaise de Developpement provided a grant of about US$3.7 million, while the Government of Cambodia contributed US$6.9 million equivalent and beneficiaries US$2.3 million. MOWRAM is the executing agency for this project, which had a completion date of 2010 (ADB, 2003).
Male-headed households irrigate on average 0.15 ha more than female headed households (FAO/SIDA, 2010).
Role of irrigation in agricultural production, the economy and society
Total harvested irrigated cropped area in 2006 on full control irrigation schemes is an estimated 384 531 ha, of which the most important crop is rice accounting for 373 331 ha (97.1 percent), followed by sugarcane on 9 956 ha (2.6 percent) and citrus on 1 244 ha (0.3 percent) (Table 4 and Figure 3). Double cropping is practiced on a small area. Only a few irrigation schemes are capable of irrigating year round (WEPA, DATE).
Rice ecosystems in Cambodia are influenced by rainfall and flooding patterns, soil suitability and the country’s topography. As a result, rice growing ecosystems can be grouped into the following broad categories, considering a total cultivated rice area of 2.44 million ha (WFP, 2010):
- Rainfed lowland rice: Represents 82 percent of the total annual rice cropping area. It is characterized by flat bounded rice fields, which mostly depend on rainfall or surface runoff for their water supply. It includes areas with supplementary irrigation. In the higher fields, where the water depth is 15-20 cm, short duration (fast growing) varieties are normally grown, while in the lower fields, where the water depth is 20-60 cm, medium and long duration varieties are normally grown.
- Rainfed upland rice: Represents 2 percent of the total annual rice cropping area. The areas are unbounded fields in the mountainous and rolling hill areas (Mondulkiri, Rotanakiri, Kratie, Koh Kong, Kampong Cham and Kampong Thom). In the shifting cultivation areas of the northeast of Cambodia upland rice is an integral part of the ‘chamkar farm’, practiced mostly by ethnic minority groups. It is also known as swidden agriculture, which is the common practice of clearing and using a plot of land for 1-5 years and then clearing another plot of land. As it is associated with burning, it is also called ‘slash and burn’. Permanent upland rice production is commonly practiced by Khmers, where a field of rice is grown annually either on its own or as an intercrop or in rotation with other upland crops.
- Deepwater or floating rice: Represents 4 percent of the total annual rice cropping area. This is practiced in low lying areas and depressions that accumulate flooded water to a straw length of 1-2 m (deepwater rice) or with a straw length up to 4 m (floating rice) at least one month during the growing period. Both subcategories are adapted to continuous, unregulated flooding. These areas are located mainly around the Tonle Sap Lake (Battambang, Banteay Mean Chey, Pursat, Siem Reap, Kampong Thom and Kampong Cham) and along the Mekong and Bassac rivers. The rice varieties have a rapid elongation with increase in water depth, and submergence tolerance to flash floods.
- Irrigated rice: Represents 12 percent of the total annual rice cropping area. The distribution of dry season production is primarily in those areas close to the major rivers and their floodplains. Dry season rice production is associated with higher yields than wet season production because of higher solar radiation, better water control and the cultivation of more fertilizer-responsive varieties of rice.
In the Tonle Sap area, irrigation schemes are largely designed to manage floodwater to supplement rainfall for wet season rice production at the start and/or the end of the wet season from May to November. Only a few schemes are designed to divert water from the Mekong or Tonle Sap catchment for dry-season crops during the main part of the dry season or for flood recession irrigation early in the dry season (CDRI, 2008).
Dry season irrigated rice usually includes improved varieties of rice grown for cash income, while wet season rice includes traditional varieties cultivated for subsistence and food security (WFP, 2010).
In 2006, the average rice yield was an estimated 2.26 tonnes/ha and 3.90 tonnes/ha in the wet and dry season respectively (MAFF, 2006). In 1993, the average rice yield was an estimated 1.39 tonnes/ha under rainfed conditions and 2.07 tonnes/ha under irrigated conditions.
A survey in 1999 estimated that the development of one hectare irrigated by pumping would require an investment of US$2 800, and US$85/year for O&M, while for a hectare irrigated from a reservoir this would be US$3 600-4 300 and US$40-65/year.