The geographical environment of Lhasa

The climate belongs to the semi-arid monsoon climate zone in the temperate zone of the plateau. The annual sunshine hours are 3,000 hours, which is more than 1.800 hours in Chengdu, Sichuan, and more than 1.654 38+0 hours in Shanghai, the largest city in eastern China. It ranks among the best in China, so it has the reputation of "Sunshine City".

Lhasa is located on the north side of the Himalayas. Affected by the downward flow, the weather is sunny all year round, with little rainfall, no severe cold in winter and no heat in summer, and it belongs to the semi-arid monsoon climate in the temperate zone of the plateau. The highest temperature in history is 29.6 degrees Celsius, the lowest temperature is minus 16.5 degrees Celsius, and the annual average temperature is 7.4 degrees Celsius. Rainfall is concentrated in June, July, August and September, and the sunshine time is more than 3,000 hours. It is known as the "Sunshine City".

The annual precipitation is 200-5 10 mm, concentrated in June-September, and it rains many nights, which is called rainy season. Strong solar radiation, thin air, low temperature, large temperature difference between day and night, cold, dry and windy in winter and spring. The annual frost-free period is 100 ~ 120 days. Lhasa is located in the middle of the Qinghai-Tibet Plateau, with an altitude of 3,650 meters. It is one of the tallest cities in the world. Its terrain is high in the north and low in the south, and it slopes from east to west. The south-central part is the valley plain in the middle reaches of Lhasa River, a tributary of Yarlung Zangbo River, with flat terrain.

At 100 km north of Lhasa, Nyainqen Tang Lala Snow Mountain stands, with Namco at the northern edge, and the highest point of the mountain is 7 1 17 meters above sea level. Nyainqentanglha Mountain is located in the middle of Tibet Plateau, about 600 kilometers long from east to west. It is bordered by Gangkuka Shame in the west, Boshula Ridge in Hengduan Mountains in the southeast, and slightly protrudes to the north in the middle. It is the watershed between the Yarlung Zangbo River and the Nujiang River, and divides Xizang Autonomous Region into three regions: northern Tibet, southern Tibet and southeastern Tibet. The Great Tanggula Pass is 523 1 m above sea level, which is the natural dividing line between the two provinces of Qinghai-Tibet and the highest point of the Qinghai-Tibet Line 109 National Highway. "Nyainqentanglha" in Tibetan means "Spirit should be the god of grassland". These four peaks and their surrounding areas suffered from strong Quaternary glaciation, forming steep mountains now, especially the steep northwest slope. Lhasa River is the mother river of Lhasa, which originates from Pengcuola Kongmagou in Jia Lili at the southern foot of Nyainqentanglha Mountain. It flows through Naqu, Dangxiong, Linzhou, Mozhugongka, Dazi, Chengguan and Duilong Deqing and reaches Qushui County. It is a large tributary of the middle reaches of the Yarlung Zangbo River, with a total length of 495 kilometers and a drainage area of 3 1.760 square kilometers. The maximum flow is 2830m3/s, the minimum flow is 20m3/s, and the annual average flow is 287m3/s; It is one of the highest rivers in the world at an altitude of 5500 meters from the source to the estuary. This river belongs to the type of snow melting and long rain, and its water volume changes with temperature and precipitation.

Most of the Lhasa River basin is mountainous, with towering peaks and steep slopes, and the terrain inclines from north to south. There must be a small-scale modern glacier in Nyainqentanglha Mountain. Most tributaries on the right bank of the basin originate from glaciers, and most tributaries on the left bank originate from lakes or swamps.

The Lhasa River is V-shaped in the valley above Tanggu in Linzhou County, and widens to the valley below Mozhugongka County, with a width of about 1- 1.5 km. The floodplain began to appear, and the vegetation on the floodplain was good. This section of the river is relatively regular, and there are three continuous terraces on both banks: the third terrace is 40-50 meters higher than the river surface and the surface is thick. The second terrace is 20-30m higher than the river surface; The first terrace10-20m is higher than the river surface, and most of it has been cultivated.

Both banks of the river are valley alluvial plains with a width of 1- 10 km, and the cultivated land area is about 570,000 mu. These areas have mild climate, flat terrain, thick soil and abundant water resources, and are one of the main grain producing areas in Tibet.