Saturday, September 17, 2011

Watershed

http://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Phou_si_-_Mekong_River_-_Luang_Prabang_Laos_%E3%83%97%E3%83%BC%E3%82%B7%E3%83%BC%E3%81%AE%E4%B8%98%E3%80%81%E3%83%A1%E3%82%B3%E3%83%B3%E5%B7%9D_%E3%83%A9%E3%82%AA%E3%82%B9%E3%83%BB%E3%83%AB%E3%82%A2%E3%83%B3%E3%83%97%E3%83%A9%E3%83%90%E3%83%BC%E3%83%B3_DSCF6787.jpg/250px-Phou_si_-_Mekong_River_-_Luang_Prabang_Laos_%E3%83%97%E3%83%BC%E3%82%B7%E3%83%BC%E3%81%AE%E4%B8%98%E3%80%81%E3%83%A1%E3%82%B3%E3%83%B3%E5%B7%9D_%E3%83%A9%E3%82%AA%E3%82%B9%E3%83%BB%E3%83%AB%E3%82%A2%E3%83%B3%E3%83%97%E3%83%A9%E3%83%90%E3%83%BC%E3%83%B3_DSCF6787.jpgThe point at which it rises in its mouth, most of the sharp drop occurs in the Mekong what is called "the Upper Mekong Basin" (see below), a stretch of about 2200 km (1367 miles). Here, you go down to 4500 meters (14,764 feet) meters before entering the lower basin where the borders of Thailand, Laos, Burma, China, and combines the Golden Triangle. Downstream of the Golden Triangle, but the river is 2600 km 2600 km (1616 miles), through Laos, Thailand and Cambodia, in front of the South China Sea through a complex delta in Vietnam.

In the province of Yunnan in China, the river and its tributaries are limited by narrow gorges. Tax systems in this part of the basin are small. Only 14 watersheds that exceed 1 000 km2 (0 sq mi). In southern Yunnan, in the prefectures of Simao and Xishuangbanna, changes in the river, the valley opens, the floodplain widens and the river widens and slower.

Laos is located almost entirely in the Lower Mekong Basin. The 'landscape climate and land use are important factors in the formation of the river hydrology. The mountainous landscape means that only 16% of the land is cultivated in the plain terrace or in mountainous areas, moving the crops. When you move in mountain areas for agriculture (slash and burn), improve the soil 10-20 years, but the vegetation does not. The transition to agriculture is common in the highlands of northern Laos, and 'was reported to constitute up to 27% of all the land under rice cultivation. As elsewhere in the pool of forest cover has been steadily reduced over the past three decades, through permanent agriculture, and agriculture. The cumulative effects of these activities of the river system has not been measured.

Loss of forest cover in Thai territory at the bottom was the highest in all countries of the Mekong decline over the past 60 years. At the Korat Plateau, which includes Mun and Chi-dependent systems have been reduced forest cover from 42% in 1961 to 13% in 1993. Although this part of northern Thailand has an annual rainfall of over 1,000 mm, a high evaporation rate means it is classified as a semi-arid region. Therefore, although the Chi and Mun basins draining 15% of the entire Mekong Basin, they contribute only 6% of mean annual flow. Sandy soil and saline soil types are most common, which makes most land suitable for irrigated rice. Despite low fertility, but agriculture is intensive. The glutinous rice, maize and cassava are the main crops. Drought is by far the greatest water hazards in the region.

In Cambodia, the wet rice the main crop, and grown in the floodplain of the Tonle Sap, Mekong and Bassac rivers. More than half of Cambodia is still covered with forests and mixed evergreen deciduous, but forest cover has decreased by 73% in 1973 to 63% in 1993. Here, the river landscape is flat. Small changes in water level to determine the direction of water movement, including the reversal of the high flow in and out of the Tonle Sap basin of the Mekong River.

Mekong Delta in Vietnam is an intensive agriculture and left little natural vegetation. The forest cover is less than 10%. In the central highlands of Vietnam, forest cover has been reduced by over 95% in 1950 to nearly 50% in the mid-1990s. Agricultural expansion and population pressure are the main causes of land use and landscape changes. Both droughts and floods are common hazards in the delta, which many people believe to be most sensitive to hydrological changes upstream.

The Mekong Basin is divided into two parts: the upper basin in Tibet and China, and the Lower Mekong downstream from Yunnan of China to South China Sea. The upper basin accounts for 24% of the total area and contributes 15-20% of the water that flows into the Mekong River. The basin is narrow and steep here. Soil erosion is a serious problem and 50% of the sediment in the river basin is high.

The major tax systems to develop in the lower basin. These systems can be separated into two groups: the tributaries that contribute to significant flows during the rainy season and its tributaries that drain areas of low relief of decreased rainfall. The first group are left bank tributaries draining the high rainfall areas of Laos. The second group are those of the right bank of the rivers, mainly Mun and Chi, which drain a large part of northern Thailand.


0 comments:

Post a Comment