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Mongolia is situated on three international river basins (Mongolian River Resources, 2010):
- The Arctic Ocean Basin in northern and central Mongolia, also known as the Yenisei river basin, drains in a northerly direction through the Russian Federation into the Arctic Ocean and covers 20 percent of the country. The many lakes and rivers are fed by water from the northern Khangai mountains and the western slopes of the Khentii mountains. The total length of the rivers in the basin is 35 000 km, which is about 50 percent of the total length of all Mongolia’s rivers. The basin’s flow accounts for 51.4 percent of the country’s total annual runoff. Major rivers in the basin are the Selenge and its tributary the Orkhon, the Ider and the Delgermurun.
- The Pacific Ocean Basin in eastern Mongolia, also known as the Amur River Basin, drains in an easterly direction through China, the Russian Federation and the Democratic People’s Republic of Korea into the Pacific Ocean and covers 12 percent of the country. This Basin encompasses rivers in the eastern part of Mongolia, which originate in the Khentii and Khyangan mountains. The basin’s flow accounts for about 15 percent of the country’s total annual runoff. Major rivers in this basin are the Onon, Ulz, Khalkh and Kherlen.
- The Central Asian Internal Drainage Basin in southern and western Mongolia covers 68 percent of the country, does not drain into an ocean, occupies much of the arid Gobi Desert and hence has few rivers (Hovd, Zavkhan, Bulgan, Uyench, Bodonch and Buyant) and limited groundwater resources. It has a series of internal drainage systems: the Khar-Us Nuur, the Uvs Nuur, and the Pu-Lun-To. It is home to 78 percent of Mongolia’s wetlands.
Located within these international basins are eight major regional basins, determined by their economic and environmental significance (Mongolian River Resources, 2010):
- Arctic Ocean Basin:
- the Selenge river basin, located in semi-arid northern Mongolia, is the country’s largest basin. It is composed of two main rivers, Selenge and its tributary Orkhon. Its major sub-basins are the Egiin, Ider, Orkhon and Tuul river basins;
- the Tuul river basin covers almost 3.2 percent of the country and is home to more than half of Mongolia’s population. It has a catchment area of 49 840 km2.
- the Khuvsgul lake basin in northern Mongolia is the location of the second biggest freshwater lake in the world.
- Pacific Ocean Basin:
- the Kherlen river basin covers 116 455 km2 in semi-arid eastern Mongolia;
- the rivers of the Onon, Ulz, and Khalkh basins are among the largest in eastern Mongolia and originate in the upper reaches of the Khentii and Khyangan mountains; They account for about 11 percent of the country’s total surface water runoff.
- Central Asia Internal Drainage Basin:
- the Great Lakes basin in western Mongolia contains Central Asia’s most important wetlands. The basin is divided into four parts: the Uvs, Khyargas, Khar-Us and Sharga depressions. It features a series of large lakes: the Uvs, Khyargas, Khar-Us, Khar, Airag and Shargiin Tsagaan;
- the Northern Gobi river basins; and
- the Southern Gobi river basins.
There are about 4 113 rivers in Mongolia, with a total length of 67 000 km. Large rivers originate in the mountainous areas in the north and west of the country – primarily in the Mongol Altai, Khangai-Khuvsgul and Khentii mountain ranges – where small rivers and mountain streams merge to create well-developed water networks. In contrast, the southern, central and southeastern parts of the country have few rivers or other water resources. In the interior drainage basins, in the western and southern areas of Mongolia, seasonal or intermittent streams end in salt lakes or disappear into the desert. The rivers’ main water sources are rainfall, groundwater, snow and glaciers, with melting snow accounting for 15-20 percent of the annual runoff. From November to May, the rivers in the north are frozen. Waterways in the Gobi Desert are fed almost exclusively by groundwater. Sixty percent of Mongolia’s river runoff drains into the Russian Federation and China, while the remaining 40 percent flows into the lakes of the Gobi Desert. The longest rivers within the Mongolian territory are (Mongolian River Resources, 2010):
- The Orkhon river (1 124 km) originates in the Khangai mountains. It initially flows eastward, before heading north and joining the Selenge river as its major tributary, which then continues northwards into the Russian Federation and Lake Baikal, by volume this is the world’s largest freshwater lake. It has a drainage area of 132 855 km2 and occupies 47 percent of the Selenge river basin. The Tuul and Kharaa rivers drain into the Orkhon river.
- The Selenge river (1 024 km) is Mongolia’s principal waterway, accepting 30.6 percent of the flow of all the rivers in Mongolia. It is formed by the confluence of the Delgermurun and Ider rivers. It flows north into the Russian Federation, eventually draining into Lake Baikal, of which it is the most substantial source of water. Its main tributaries are the Egiin, Orkhon and Uda rivers. It is also the headwater of the Yenisei-Angara river.
- The source of the Kherlen river (1 090 km) is in the Khentii mountains. It flows to China, where it subsequently empties into Lake Hulun. Its main tributaries are the Iluur, Burkh, Baidrag, Terelj and Tenuun rivers.
- The Zavkhan river (808 km) starts at the confluence of the Buyant and Shar Us rivers in the Khangai mountains. It empties into Lake Airag in the Great Lakes basin, which then connects with Lake Khyargas. The river provides most of Lake Khyargas’ tributary flow.
- The Tuul river (704 km) originates at the confluence of the Namiya and Nergui streams. It flows in a southwesterly direction, passing through the southern part of the Mongolian capital, Ulaanbaatar, before joining the Orkhon river.
- The source of the Hovd river (593 km), is on the northern side of the Mongol Altai mountains, rises from the permanent snows of Tavan Bogd mountain. It flows into Lake Khar-Us in Hovd province in western Mongolia. Its main tributaries are the Tsagaan and Sagsai rivers.
- The Eruu river (323 km) starts in the Khentii mountains at the confluence of the Khongiin and Sharluun rivers. It flows into the Orkhon river and has a drainage area of 11 860 km2.
- The Onon river (298 km) originates in the Khentii mountains, from where it flows in a northeasterly direction, eventually converging with the Ingoda river in the Russian Federation to produce the Shilka river. At the border with China, the Shilka joins the Argun river to form the Amur river, which eventually drains into the Pacific Ocean. The Onon river has a drainage area of 94 010 km2. Its main tributaries are the Barkh, Balzh and Khurkh rivers.
- The Kharaa river (291 km) originates in the mountains north of Ulaanbaatar and passes through Selenge and Darkhan-Uul provinces before flowing into the Orkhon river.
Mongolia’s long-term average annual renewable water resources include 32.7 km3 of surface water and 6.1 km3 of groundwater. Part of the groundwater flow, estimated at 4 km3/year, returns to the river system as base flow and is called overlap (Table 2). This gives a total of 34.8 km3/year (32.7+6.1-4.0) for internal renewable water resources (IRWR). It is estimated that no water enters the country from neighbouring countries, but that 25 km3/year flows into the Russian Federation and 1.401 km3/year into China.
There are some 3 060 natural lakes with surface area larger than 100 ha or 0.1 km2. The lake with the largest surface area is Lake Uvs (3 518 km2), which is a saline lake without an outlet (Table 3). Lake Khuvsgul has the greatest volume (384 km3) and depth (139 m). It contains 74 percent of the total freshwater resources of Mongolia, and is fed by 46 rivers and other large lakes. In the higher mountainous regions the potential evaporation is lower than the annual precipitation and, therefore, the lakes never dry up and persist against periods of drought. In areas such as the Valley of Lakes, however, it is the opposite and therefore the lakes there can become quite shallow in very dry areas. Most of the medium lakes such as Orog, Taatsyin Tsagaan, Adgiin Tsagaan and Ulaan in the Valley of Lakes dry up once or twice every 11-12 years, which can lead to an ecological crisis when millions of fish, aquatic plants and animals die in isolated spots of concentrated saline mud left by the drying lake (Davaa et al., 2007).
In 1999, about 27 earth dams were constructed to store water for sprinkler irrigation systems. A small part (55 km2) of the catchment drained by the Boroo river is intercepted by the Shariin Am dam and storage reservoir facility. The Shariin river is a narrow and shallow river with a small dam about 4 m high, capable of impounding a small storage reservoir with a regulating capacity of about 250 000 m3.
The theoretical hydropower potential in 1999 was an estimated 5 500-6 000 MW. There is a 528 kW mini-hydroplant in operation (the Kharakhoum scheme) on an irrigation canal that diverts water from the Orkhon river.