GISdevelopment.net ---> AARS ---> ACRS 1990 ---> Poster Session Q Remote Sensing Analysis of Soil Erosion in Loess Plateau of North Shaanxi Province Chen Zhengyi, Yu Tianxu Institute of Remote Sensing Application, Academia Sinica Abstract There is the most serious soil erosion in gully and small gully area of loess plateau of north Shaanxi Province , the dissected density of this area can reach 30km/km2. it is of great and practical importance for the study of soil erosion intensity and erosion type and for the regional management of soil erosion in north Shaanxi Province to divide correctly the development stages of gully and erosion cycle, meanwhile, the wind-drift and action in north Shaanxi province has greatly increased the amount of soil erosion and amount of sediments in river. Introduction The loess plateau of north Shaanxi Province is famous for its special loess landform and its serious soil erosion, with countless valleysand gullies on it due to high dissected density and severe erosion. A great amount of sand has been sent to the Yellow River every year because of serious soil erosion , it had greatly effected the production and living standard of local people and endangered the life and property of people in low reaches. The problem of soil erosion and its regional management in loess plateau of north Shaanxi Province have long been concerned by the people. In recent years, in carrying out the major national scientific project, we have made microcosmically analysis of invading effect of Maowusu desert to the loess plateu of north Shaanxi province on the basis of comprehensive remote sensing survey and regional differences of soil erosion through analysing the development of loess landscape by using the large scale aerial photography. Analysis of Dissected Density in Loess Gullied Hill Area The valleys and gullies are formed by the erosion of running water. The dissected density reports not only the dissection of landsurface, but also the loss of soil and water. Generally speaking, the higher the dissected density, the stronger the soil and water erosion. There-fore, it is the proper dissected dencity that shows exactly the loes geomorphic feature and erosion intensity. The dissected density written in the previous documentation is mostly 6 km/km2, no more than 8km/km2 [1], according to the result of remote sensing analysis, we now conclude that the value of dissected density mentioned above is obviously low. it does not reflect the geomorphicfeature of loess gullied-hilly area density not loss of soil and water . The deviation is caused of the complicated geomorphic condition in loess area, where the valleys and gullies can be divided into six types : brook, dry gully, small gully, shadow gully, and rill and the ordinary topographic map can only report the gullies and the small gullies, which are very important in showing the soil erosion can not be represented clearly due to the limitation of map scale. So, the dissected density of 8km/km2 can only represent the distribution and dissection of gullies. According to the analysis of geomorp0hic feature non the remotely sensed image, we can conclude that it is small gullies on which the rock slides, avalanches and other serious erosions happen reflect the dissection of gullies and loess of soil and water more sufficiently. We can say that the dissection of small gully is of great and practical value. TM image of north Shaanxi province clearly show the gully system, the dissected density on it is about 8 km/km2 (Fig.1) . Meanwhile small gullies and rills can be seen clearly on the large scale aerial photograph of north Shaanxi Province, and their dissected density can be measured accurately as a about km/km2 the highest of them can reach to 35km/km2 , four or six time higher than the previous dissected density in gullied-hilly area of loess plateau. These values are a new index of dissected density of gullied-hilly area, and they reflect appropriately the real condition of dissection and loss of soil and water in these areas. Figure 1 Gully system, broken geomorphy and loess covered with wind-drift sand (1:100000 TM image) The Development Stages of Gullies and Erosion Intensity There are two major geomorphic types in loess area , i.e. gullies and interfluves. They can be distinguished clearly by the gully boundary between them, there are differences in geomorphic feature, erosion type and erosion intensity between gullies and interfives . The running water erosion, avalanches and other gravitational erosions often happened in the gullies where the erosion intensities are stronger, while the slope lamimar-flow erosion mostly exist in interfluves there had slight erosion . Compared with interfluves erosions, that of gullies play an important role in the development of the loess landform. of all the gullies, which dissected the land surface in the loess plateau, stream, dry gullies and gullies have their definite shape and position, while the small gullies and other tiny gullies do not have definite shape and position, they will mostly disappear with the later erosion, few of them will become new gullies. however, these indefinite gullies distributed widely, and they represent the development stages and position of strong erosion . In gullied –hilly area, gullies and small gullies are main part of gullies system (Fig.2) their. length takes 80% total length of all gullies and play the major role in the development of soil erosion. Figure 2 Dissected density of loess gullied-hilly area (by 1:10000 aerial photograph) With the lowering erosion basis, the dissection of running water become strong. Meanwhile the gravitational erosion, such as rock slides and avalanches happened there the small gullies taking the shape of plume formed on both side of gullies .the development of small gullies determined directly the erosion intensity of gullies and their further development . According to the information of valley and gully shape on remotely sensed images, following three development stages images, following three development stages are divided . The first development stages are divided. The first development stage. It is the young period of loess gullies . In this stage, the gullies take the shape of line channel and the valleys are narrow , with great dissected depth and steep gully banks, running water erosion is the main erosion type combining with small gravitational erosion. In the second stage, the sawtooth shaped small gullies developed on both banks of gullies, loess interfluves are narrow and unstable, rock slides and avalanches developed widely, gravitational erosion become strong. The loss of soil and water comes into its peak period. In the third stage, with the stability of erosion basis, rock slides, avalanches and other gravitational erosion are slight and gradually disappeared . The slopes on both side of gullies mostly developed into wide and stable escarpments and interfluves. At this time, the slope erosion becomes the main type of erosion in gullies, the intensity of erosion becomes lower than that of the previous stage. The three stages of gully development mentioned above formed erosion cycle of an entire process of loess geomorphic development. This research subject is of practical value for forecasting the tendency of soil erosion and making appropriate management measures in the gullied-hilly area. The Erosion Intensity of Interfluves and Division of Geomorphic Erosion Cycle The modern geomorphic features of interfluves are not produced by one time of erosion, but many erosion cycles. In every erosion cycle, there formed a remarkable boundary between gullies and interfluves (i.e. gully boundary line) . hence the erosion cycles in the development of gullies can be determined or distinguished . Four or five erosion cycles are shown on the image of north Shaanxi Province. There are distinct terraces on the slope of interfluves formed in many erosion cycles, that is to say, one erosion terrace on the slope of interfluves formed in many erosion in deferent erosion terraces on the slope of interfluves formed in many erosion cycles, that is to say , one erosion terrace on the slope of interfluves formed in many erosion cycles, that is to say, one erosion terrace on the slope of interfluves formed in many erosion cycles, that is to say, one erosion terrace formed in one erosion cycle. The intensity of erosion cycles, that is to say, one erosion terrace on the slope of interfluves formed in many erosion cycles, that is to say, one erosion terrace formed in one erosion cycle. The intensity of erosion in different erosion terraces in quite different because of different geomorphic positions, slopes and different developing digress of tiny gullies. On the erosion terraces formed in earlier erosion cycles in high geomorphic position, there is at present a low intensity of soil erosion because of ill-developed tiny gullies and rock slides planes flatted by erosions. Mean while, on the terrace formed in later erosion cycles in the low geomorphic positions, there are high erosion intensities because of well-developed tiny gullies and step slopes. The impact of Wind-Drift Sand Action on Soil Erosion The loess plateau of north Shaanxi Province is located in the south-eastern part of Erduos Plateau and maowusu desert, and is influenced by arid airflows from Mongolia plateau all the year round. It is distinct on remotely sensed image that the tendency of Maowusu desert continuously moving eastward and its invasions to the north Shaanxi Province. At present, a few strip sand dunned are formed in the original Maowusu desert, while the main part of Maowusu has been in the west region of loess plateau, and the invasion tendency of wind-drift sand id continuous. A wind-drift sand interim region of 500 km long and 10 to 20 km width has been formed between baiyu Mountain in South Zenger area of inner Mongolia in the north. with the process of the desert taking the place of original loess area. It can be clearly see on the image, under the control of north-western wind, there are about ten tongue shaped longitudinal dunes, which are 40-50km long stretching into the center of the loess plateau of north Shaanxi Province, and their front parts have approached to Yellow River. On the other hand, the fact that much cultivated land of 1950’s has been decertified or partly wasted has shown that a large amount of wind-drift sand has been transferred on the surface of loess plateau of north Shaanxi Province, hence increased the amount of erosion and sediments in rivers of this area. According to the information concerned, many attributaries of Yellow River in north Shanxi Province, such as Huangpu River, Gushan River, Tuwei River, Kuyie River, Jialu River and Wuding River have the erosion modulus larger that 20000t/km2 . More 50% of them are coarse sand [2], of which the diameter is greater than 0.05mm. This area is not only with highest erosion modulus but also with highest content of coarse sand in rivers. Remote sensing information shows these areas with high erosion modulus and rivers with great amount of sediments are all located in the extent of Maowusu desert invasion region . &0% of these rivers basins is loess area covered with wind-drift sand, and there are different amount of wind-drift and sedimented on other 30% . These wind-drift invasion sand and wind-draft sedimentary sand are one of the principal origin of coarse sand in these rivers. Conclusions In the gullied area of loess plateau, gullies and small gullies have serious soil erosion. The dissected density of this area can reach to 30 km/km2 . It is of great and practical importance to divide a appropriately the stages of development of gullies and soil erosion cycle for the research work of soil erosion type and intensity and for the regional management in gullied-hilly area of north Shaanxi Province. The wind-draft sand action to the north Shaanxi Province has increased the amount of soil erosion and sediments in rivers of this area, therefore , the wind-drift and action and its further development must be considered in the management of loss of soil and water in this area. References Physical Geography of China (Geomorgraphy) “ compliation committee: Physical Geography of China, Academia Sinica. Science Press, 1981, In Chinese. Chen yongzong, Collected Works on source of Course sand of Yellow River and Mechanism of Erosion and Sand Production, Meteorological Publish House, 1989, in Chinese.