This paper analyzes the reasons why there are many dry-hot valleys in Yunnan Province.

Dry-hot valley refers to the valley zone with high temperature and low humidity, which is mostly distributed in tropical or subtropical areas. The region is rich in light and heat resources, with hot climate and little rain, serious soil erosion and fragile ecology, especially natural disasters such as cold, drought, wind, insects, grass and fire. China's dry-hot valleys are mainly distributed along the Jinsha River, Yuanjiang River, Nujiang River and Nanpanjiang River, such as Panzhihua, Yunnan and Guizhou in Sichuan. The Yunnan-Guizhou Plateau has gentle mountains and deep soil layers, but the vegetation is scarce, and the forest coverage rate is really less than 5%. Looking around, it is all bare red soil.

The dry-hot valley area was like this tens of thousands of years ago. The deep and special landform of Hengduan Mountains on both sides of the great river in the low latitude plateau makes it barren, which has nothing to do with man-made destruction. The drought in Yunnan is not entirely the fault of El Nino, but also the climate in the dry-hot valley. The climate of dry-hot valley is a peculiar climate formed by special landforms. Its formation is the result of complex geographical environment and local microclimate. When water vapor condenses in these areas, it will lead to heat release and moisture reduction, and increase the air temperature. In the valley with closed terrain, the forest vegetation here is difficult to recover due to excessive water loss caused by dry heat. The lack of water makes the land barren in a large area, and the topsoil on the slope of the valley loses in a large area, exposing a large area of bare soil and bare rocks.

Accompanied by the climate of dry-hot valley, there is another peculiar natural phenomenon-wind burning. As the name implies, it is the general wind that can make foreign objects burn. Some people call it the "Flame Mountain" wind. Once there is a burning wind crossing the border, the climate will become hot and dry, just like a sauna. Warming will make crops and fruits premature, and strong wind burning will lead to drought and forest fires. Foehn is caused by the sinking of airflow after passing through the mountain. When a mass of air sinks from the sky to the ground, the temperature rises by 6.5 degrees Celsius on average for every drop of 1000 meters. That is to say, when the air drops to the ground from the mountain at an altitude of 4000 to 5000 meters, the temperature will rise by more than 20 degrees, and the cool climate will suddenly become hot.

The harsh ecological conditions and serious soil erosion in the dry-hot valley have had a great negative impact on the middle and lower reaches of the Yangtze River, especially on the Three Gorges Dam. The important source of harnessing the Yangtze River basin and protecting the Three Gorges Dam is the ecological environment construction in the dry-hot valley of Jinsha River. The ecological environment of wood growth in Siemund is very similar to that in the dry-hot valley of Jinsha River, and Siemund wood has extremely high economic value. Therefore, planting Siemund wood in the dry-hot valley of Jinsha River can not only improve the ecological environment, but also develop the local economy.