Geography

Cambodia has a land area of 181,035 square kilometers in the southwestern part of the Indochina peninsula, about 20 percent of which is used for agriculture. It lies completely within the tropics with its southernmost points slightly more than 100 above the Equator. The country's capital city is Phnom Penh. International borders are shared with Thailand and the Lao People's Democratic Republic on the west and on the north, and the Socialist Republic of Viet Nam on the east and the southeast. The country is bounded on the southwest by the Gulf of Thailand. In comparison with its neighbors, Cambodia is a geographically compact country administratively composed of . 20 provinces, three of which have relatively short maritime boundaries, 2 municipalities, 172 districts, 1,547 communes. The country has a coastline of 435 km and extensive mangrove stands, some of which are relatively undisturbed.

The dominant features of the Cambodian landscape are the large, almost centrally located, Tonle Sap (Great Lake) and the Bassac River systems and the Mekong River, which traverses the country from north to south. Surrounding the Central Plains which covered three quarters of the country's area are the more densely forested and sparsely populated highlands, comprising: the Elephant Mountains and Cardamom Mountains of the southwest and western regions; the Dangrek Mountains of the north adjoining the Korat Plateau of Thailand; and the Ratanakiri Plateau and Chhlong highlands on the east merging with the Central Highlands of Viet Nam.

The Tonle Sap Basin-Mekong Lowlands region consists chiefly of plains with elevations generally of less than 100 meters.

As the elevation increases,. the terrain becomes more rolling and dissected. The Cardamom Mountains in the southwest rise to more than 1,500 meters and is oriented generally in a northwest-southeast direction. The highest mountain in Cambodia-Phnom Aural, at 1,771 meters-is in the eastern part of this range.

The Elephant Range, an extension of the Cardamom Mountains, runs toward the south and the southeast and rises to elevations of between 500 and 1,000 meters. These two ranges are bordered on the west by a narrow coastal plain facing the Gulf of Thailand that contains Kampong Som Bay. The Dangrek Mountains at the northern rim of the Tonle Sap Basin, consisting of a steep escarpment on the southern edge of the Korat Plateau in Thailand, marks the boundary between Thailand and Cambodia. The average elevation of about 500 meters with the highest points reaching more than 700 meters. Between the northern part of the Cardamom ranges and the western part of the Dangrek, lies an extension of the Tonle Sap Basin that merges into the plains in Thailand, allowing easy access from the border to Bangkok.

The Mekong River, Cambodia's largest river, dominates the hydrology of the country. The river originates in mainland China, flows through Myanmar, Laos, Thailand before entering Cambodia. At Phnom Penh, with its alternative arms,. the Basak River from the south, and the Tonle Sap River linking with the 'Great Lake" itself -- Tonle Sap -- from the northwest, it continues further southeastward to its lower delta in Viet Nam and to the South China Sea.

The section of Mekong River passing through Cambodia lies within the tropical wet and dry zone. It has a pronounced dry season during the northern hemisphere winter, with about 80 percent of the annual rainfall occurring during the southwest monsoon in May-October. The Mekong River's average annual flow at Kratie of 441 km' is estimated as 93 percent, of the total Mekong run-off discharge into the sea. The discharge at Kratie ranges from a minimum of 1,250 m3/s to a maximum 66,700 m3 / s.

The role of the Tonle Sap as a buffer of the Mekong River system floods and the source of beneficial dry season flows warrants explanation. The Mekong River swells with waters during the monsoon season reaching a flood discharge of 40,000 M3/s at Phnom Penh. By about mid-June, the flow of the Mekong and the Basak Rivers fed by monsoon rains, increases to a point where its outlets through the delta cannot handle the enormous volume of water, flooding extensive adjacent floodplains for 4-7 months. At this point, instead of overflowing its banks, its floodwaters reverse the flow of the Tonle Sap River (about 120 km in length), which then has a maximum inflow rate of 1.8 m/s and enters the Great Lake, the largest natural lake in Southeast Asia, increasing the size of the lake from about 2,600 km' to 10,000 km2, and exceptionally to 13,000 km' and raising the water level by an average 7m at the height of the flooding. This specifity of the Tonle Sap River makes it the only "river with return" in the world.

After the Mekong's waters crest (when its downstream channels can handle the volume of water), the flow reverses and water flows out of the engorged lake. The Great Lake then acts as a natural flood retention basin. When the floods subside, water starts flowing out of the Great Lake, reaching a maximum outflow rate of 2.0 m/s and, over the dry season, increase mainstream flows by about 16 percent, thus helping to reduce salinity intrusion in the lower Mekong Delta in Viet Nam. By the time the lake water level drops to its minimum surface size, a band 20-30 km wide of inundated forest is left dry with deposits of a new layer of sediment. This forest, which is of great significance for fish, is now greatly reduced in size through siltation and deforestation. The area flooded around Phnom Penh and down to the Vietnamese border is about 7 000 km'.

The Tonle Sap Basin and Mekong Lowland areas of the Central Plain consist of four main agro-ecological systems:

• The river banks or levees (mouat tonle), which are primarily devoted to vegetables., fruit trees, and, in the lower areas, to corn, sesame, and beans-- grow instead of rice. The soil is relatively rich and water supply is rarely a problem owing to the high water table. The fields (chamcar) are usually narrow and elongated and lie perpendicular to the river bank. These areas (totaling about 500,000 ha) are usually the most densely populated and often the most valuable, especially if they are dose to markets for their produce. Provinces with large areas of this type of agro-ecological system include Kandal, Kompong Cham, and Battambang.
  
  
• The lowland floodplains (beng), which lie between the levees and the terraces. They flood easily, depending on the height of the rivers during the rains. The lower parts of the lowlands are planted with floating rice, while the higher elevations are planted, with rice varieties that flourish in receding waters. The soil can be fertile. These areas (estimated at 1.5 million ha) are less densely populated, but normally produce much of the rice that is consumed by the settlements on the levees. They can bee found mainly in east Kandal, Kompong Chhnang Prey Veng west Svay Rieng, and south Takeo provinces.
  
  
• The upper terraces, where mainly rain-fed agriculture is practiced ( up to 200,000 ha ). The soils are often poor, apart from the rich alkaline clays of Battambang, where rice is the main crop. In addition to Battambang, most of Kompong Speu province, north Takeo, west Kandal, west Kompong Chhnang, and Pursat have this type of ecosystem.
  
  
• The Great Lake plain surrounding Tonle Sap, which consists of flooded forest and plain. The area has rich soils in which a considerable quantity of floating and deep water rice was grown in the period prior to the 1970s. It formerly covered up to 600,000 ha but now accounts for less than 100,000 ha.