The Mekong River basin can be divided into two parts:
- The Upper Basin located in Tibet, Burma, and China (where the river is called the Lancang Jiang), and
- The Lower Mekong Basin located in Lao PDR, Thailand, Vietnam and Cambodia.
Flow and catchment area contributions of the six Mekong River Basin countries are as follows (MRC, 2005).
| |
China
|
Burma
|
Lao PDR
|
Thailand
|
Cambodia
|
Viet Nam
|
Total
|
|
Area (km2)
|
165,000
|
24,000
|
202,000
|
184,000
|
155,000
|
65,000
|
795,000
|
|
Catchment (%)
|
21
|
3
|
25
|
23
|
20
|
8
|
100
|
|
Flow (%)
|
16
|
2
|
35
|
18
|
18
|
11
|
100
|
The climate of the Mekong Basin is dominated by the Monsoon, which generates wet and dry seasons of more or less equal length. The Southwest monsoon generates the wet season which usually lasts from May until late September or early October. Tropical cyclones occur over much of the area during August and September and even October (in the delta). The Northeast Monsoon brings lower temperatures from China and causes dry weather in the Lower Mekong Basin from late October until April.
Mean annual rainfall is significantly different between east and west bank tributaries of the Lower Mekong basin(Fig. 1 below). Rainfalls of more than 2500 mm/year occur in the western mountain regions of Lao PDR, where-as less than 1000 mm/year occur in the central regions of Thailand within the Mun–Chi Basin.
Regional mean annual rates of evaporation vary between 1000 and 2000 mm, with little variability from year to year due to the high relative humidity. Maximum summer temperatures during March and April at altitudes of less than 500 mamsl typically reach 35 degrees, while minimum winter figures can fall to 15 degrees and lower during December and January.
The estimated mean annual flow of the Mekong River is almost 460 km3. In an average year, about 75 percent of the total annual flow occurs within just four months between July and October. A map of annual runoff calculated from annual flow volumes for 85 years between 1924 and 2008 (Fig. 2.) showns major flow contributions from the left bank tributary systems in Lao PDR and the Sre Kong, Se San and Sre Pok Rivers which enter the mainstream between Pakse and Kratie (MRC, 2010).
The Mekong River and its tributaries form a dynamic hydro-ecological system supporting unique biodiversity and providing food security for more than fifty million people in the Indo-Burma region. The system is one of the world’s most diverse river ecosystems, containing over 560 endemic fish species and over 300 globally threatened vertebrate species.
Proposed plans to build numerous dams along the Mekong's mainstream and tributaries have raised significant concern for the potential impacts of the dams will have on biodiversity and fisheries due to changes in the flow regime. Reduced seasonal fluctuations are likely to affect wetland habitats situated on the floodplains of the Mekong and adversely impact the hydrodynamic flooding and drainage of the Tonle Sap (Cambodia's Great Lake).
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| Fig. 1. Annual rainfall |
Fig. 2. Annual runoff |
References:
MRC (2005), Overview of the Hydrology of the Mekong Basin
MRC (2010), State of the Basin Report 2010
