关键词: 乌兰布和沙漠北部, 历史时期, 环境变迁

Abstract: Based on hydrography and historic geography, the paper studied the environmental evolution of northern Ulan Buh Desert in historical time. According to historic and natural conditions, sandy land in this area had been formed in Pre-Qin Dynasty. But on the plain of Yellow River and round the TuShenZe lake, some ancient oases still existed resulting from plentiful water supply from the river and the lake. Three cities were just founded on oases in western Han Dynasty. Recorded by historic volumes, the above cities belonged to military settlements whose scales were small and the output of cereals couldn't meet the demand of local people, let along economic prosperity. Moreover, natural conditions which the three cities depended on lay in four aspects as follows: (1) During the period from Western Han Dynasty to Northern Wei Dynasty about 600 years, the TuShenZe lake was formed by overflowing of the Yellow River, and its area reached 400 km². Every year, the lake conveyed 400 million m 3 vapor to atmosphere and an amount of ground water to the land round the lake. (2) At the same time, the Yellow River flowed across hinterland desert, then water surface evaporation and river seepage flow prevented soil desertification. (3) Yellow River diversion works located near northern Linrong, they watered the crops of Linrong and Woye counties, thus crops coverage also prevented soil desertification. (4) Small lake groups near the Sanfen city was the natural condition maintaining ancient oases. In spite of these, the danger led to oases desertification was still existed. Based on investigation by Li Dao yuan in 494 AD, the landscape along Wuja river, Yellow River and in Urad Front Banner were all dunes. There were seven pieces of shifting dunes in northern Suofan prefecture, namely today's west point of Hobq Desert and northern Ulan Buh Desert. After Northern Han Dynasty, because of the natural course change of the Yellow River, TuShenZe lake lost its water supply, which resulted in the water level of the lake decreased 1 m per year and dried up soon. The area of ancient oases shrank and the area of desert expanded, influenced predominantly by the climate changing towords a severe aridity and coldness with frequent winds. In comparison of oases in northern Ulan Buh desert with that in Taklimakan desert, it is indicated that ancient oases dispeared because of water source drying up led to desertification. However, the Yellow River, with 300 000 km² water collection surface upper the Linrong, was influenced slightly by human activities in historical time. So that its water amount is plentiful and stable. The possibility of drying up or interruption is faint. In summary, the desertification of northern Ulan Buh Desert shows: (1) When develop water resources of arid and semi arid regions, the lowest limit water balance should be maintain. (2) The wetland such as lake is the huge sustain system to keep ecological balance, especially in arid and semi arid regions. (3) Viewed from thousand year scale, Mu Us desert, Hobq desert and Northern Ulan Buh desert from western Han Dynasty to northern Wei Dynasty were in same climate dry grassland to desert grassland climate. More proofs about the three deserts were recorded by historic volumes. So the view of "man made desert" is untenable.

Key words: northern Ulan Buh desert, historical time, environmental evolution