Kingdom of Cambodia:comprises the land area of 181,035 square
kilometers in the south west part of the Indochina peninsula,
about 20% of which is used for agriculture. It lies completely
within the tropics with its southernmost points slightly more
than 10º 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 Vietnam 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,
3 municipalities, 172 districts, and 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 Cambodia landscape
are the large, almost centrally located, Tonle Sap (Great
Lake) and the Bassac River systems and the Mekong River, which
crosses 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 Planteau of Thailand;
and the Ratanakiri Plateau and Chhlong highlands on the east
merging with the Central Highlands of Vietnam.
The Tonle Sap Basin-Mekong Lowlands region
consists mainly 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 reaches 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 Bassak River form 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 Vietnam 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% of the annual rainfall occurring during the
southwest monsoon in May-October. The Mekong River’s
average annual flow at Kratie of 44km3 is estimated as 93%
of the total Mekong run-off discharge into the sea. The discharge
at Kratie ranges from a minimum of 1,250m3/s to a maximum
66,700m3/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 g flow
of the Mekong and the Bassak 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 km2 to 10,000 km2, at times exceptionally to 13,000
km2, 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, 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%, 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 20-30km 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 border
is about 7,000km2.
