SOIL LOSS AND RESTORE IN JINSHA RIVER BASIN

Cao Shuyou, Liu Xingnian and Deng Xiangui

Sichuan University, Chengdu, Sichuan 610065, China

Phone: 028-5402614, Fax: 028-5405148, E-mail: caosy@mail.sc.cninfo.net

Abstract: Based on the data of runoff and sediment yield from main control hydrographic stations on the trunk stream and the main tributaries of Jinsha River in South-Western China, the characteristics of soil erosion and sediment yield in key soil erosion areas are studied. The analysis shows that The Lower Jinsha Basin, between Longjie and Pingshan, is the major erosion district of Jinsha basin, and the temporal trend of soil erosion in Jinsha River Basin is serious. Comparing the periods of 1954-1982 with 1983-1992, with the almost equal annual average runoffs of 142 billion cubic meters, the averaged annual sediment yields for two periods are 238.0 and 267.9 million tons respectively. This means that the annual average sediment yield in 1983-1992 increased by 12.2% comparing that in 1954-1982. The reasons influenced the soil erosion and main approaches for sediment management are described. 

Keywords: jinsha river, sediment yield, soil erosion, china

1   INTRODUCTION

The main sediment resources flow into the Three Gorges Reservoir is from Jinsha River and Jialing River. Based on the data of Chuntan Hydrographic Station, the control station of the Three Gorges Reservoir’s entrance sediment, 52% of the averaged annual suspended load into the reservoir comes from Jinsha River. The effects of sediment problem of Jinsha River is serious for the flood control, navigation, irrigation, water supply and environment not only in local area but also in Middle and Lower Course of Yangtze River, especially to the Three Gorge Project. Therefore it is necessary to study the characteristics of soil erosion and sediment yield in Jinsha River Basin.

Jinsha River is originated from Tanggula Mountain in Qinghai Province, and flows through Sichuan and Yunnan Provinces. Main tributaries from upper stream to down stream are Yalung River, Huichuan River et al on the left bank and Lungchuan River, Niulang River et al on the right bank. Jinsha is divided into upper stream reach and down stream reach at Panzhihuao City. The down stream reach covers the period from Panzhihua to Yibing City. The location of Jinsha River Basin is shown in Fig.1. Jinsha River covers a drainage area of 500,000 km², or about 50% of the total drainage area of upper Yangtze River (from the source to Yichang). The investigation indicated that the eroded area amounted to about 135,000 km.². The annual total gross erosion was 557x10⁶ ton, with a mean erosion rate of 1114 t/km²/year (Dai and Tan, 1994).

Based on the data of runoff and sediment from main control hydrographic stations on the trunk stream and the main tributaries of Jinsha River, the characteristics of soil erosion and sediment yield in the catchment basin are studied. The analysis shows that the trend of soil erosion in Jinsha River Basin is serious. Comparing the periods of 1954-1982 and 1983-1992, with the almost equal average annual runoff of 142 billion cubic meters, the annual average sediment yields for two periods are 238.0 and 267.9 million tons respectively. This means that the averaged annual sediment yield in 1983-1992 increased by 12.2% to that in 1954-1982. A wide ranging investigation of soil erosion and sediment yield in the Jinsha River Basin has been taken (Dai and Tan, 1996，Deng, 1997). A large amount data has been obtained, and a general assessment of status and trend of soil erosion and sediment yield has been derived. Some of the research results are briefly reviewed in this paper.

2    SPATIAL DISTRIBUTION OF EROSION

The Jinsha River is the first reach of Upper Yangtze River. It flows through the Qinghai-Tibet Plateau (with an elevation of 4000 m), the Parallel Ridges and Valleys (2000-4000m), the Yunan-Guizhou Plateau (1000-2000m), and to the Sichuan Plain (500-1000 m) at Yibing City. Jinsha Basin is located at the geological contact zone between the rising geosynclinal area to the west and the stable plateau to the east. The landscape undulates drastically, river valleys are deeply incised, and intense tectogenesis has resulted in large amount of folded, fractured and fragmented rocks. These contribute a rich potential source for surface erosion and sediment load. Jinsha River is climatically located in a subtropical zone. It can be divided into two climatic sub-areas, reflecting the different topography. The western plateau climate sub-area receives little rain whereas the eastern subtropical monsoon sub-area receives large amounts of rain. Both temporal and spatial distributions of precipitation and runoff are uneven in the basin. The average annual precipitation varies from less than 400 mm in the Qinghai-Tibet plateau in the west to more than 1200 mm and even 2000 mm in the Sichuan Plain in the east (Dai and Tan, 1996). In most areas, 70% of the annual rainfall takes place in the flood season, especially in July and August. Fifty percent of the annual runoff is concentrated in the period from July to September. The main composition of sediment in Jinsha River is suspended load. Therefore the spatial distribution, the season distribution, and the annual variation of suspended load are analyzed only. The average annual runoff and sediment yield at control hydrographic stations in main stream are given in Table 1.

To describe the spatial distribution of sediment resources, the whole basin is derived into 6 areas based upon the hydrographic stations in main river are shown in Table 2. The observed data in the same period of 1966-1975 are used. The percentages of averaged annual sediment yield in each area to that of the Pingshan station are shown in Table 2. We can see in Table 2 that the averaged annual sediment yield rate in the total basin is 505 t/km³/year. The sediment yield rates in 4 upper four areas, Longjie upward, are less than the basin-averaged value. The population of these four areas is sparse, and natural erosion mainly takes place. However, in the two lower basin areas, Longjie- Huadan and Huadan- Pingshan, the averaged annual sediment yield rate as high as 2440 t/km²/year, nearly 5 times higher than basin-averaged value. The area of these two regions is only 12.8% of the whole basin, but the sediment yield is as high as 61.6%. It is clear that the reach between Longjie and Pingshan is the main erosion district of Jinsha River Basin.

Sediment yield in some area is more serious based on the data of hydrographic station observations. Table 3 gives the statistics results of observed data from the control hydrographic stations of 12 main tributaries. The control area of these stations is 59.6% of the total area of Longjie-Pingshan region comparing with the sediment yield of 21.2%. However in some small tributaries and the valley area along the trunk stream other than these 12 main tributaries in Longjie-Pingshan region, the sediment yield contrbution is 78.8%, but only 40.4% controling watershed area. The conclusion is that the both banks along the trunk stream and some small tributaries between Longjie and Pingshan are the main soil erosion area. The most serious soil erosion area is the Xiaojiang River watershed where the averaged annual erosion rate is as high as about 2958 t/km²/year, even up to 3420 t/km²/year. In this region, landslides, avalanches, debris flows, and gully erosion occur extensively.

3    TRENDS OF THE SEDIMENT YIELD

The annual variation of runoff and sediment yield is induced not only by natural factors, such as rainfall (rainfall amount, intensity, and local distribution) and the conditions of underlying surface (morphologic types, geologic types, and soil types et al), but also by the effects of human activities. Data analysis shows (Fig. 2) that the sediment yield in Jinsha River between 1954 and 1992 can be derived into two periods based on the variation trend of data, one from 1954 to 1982, another from 1983 to 1992. The averaged annual runoffs for the two periods close to the average annual runoff , 142.8 billion cubic meters. However there are obviously difference in sediment yield The analysis results is shown in Table 4.

Table 4 shows that in spite of the averaged annual runoff in two periods of time is almost equal, the averaged annual sediment yield in the second period increased 29.9 million tons to the first period. In other words, the averaged increase of sediment yield is 12.2% each year. This phenomenon obviously indicates that the sediment yield in Jinsha River trend to increase after 1982. This trend is more obvious in some of tributaries. Fig. 3 gives the relationships between the cumulative annual runoff and sediment yield in four main tributaries, named Annin, Longchuan, Heshui, and Meigu. The reasons may be mainly human activities. As pointed as Dai and Tan (1996) that human activities introduce important complications and may have dual effects in both accelerate and mitigating soil erosion. The expansion of cultivated slope land, especially steep land, deforestation for land reclamation under the pressure of increasing population and economy, large scale mining, road building, rock quarrying and other industrial and civil construction activities etc. may greatly increase the supply of loose erodible material and reduce the erosion resistance of the surface soil and the stability of mountain side and lead to severe soil loss.

4    SEDIMENT CONSERVANCE

To control the serious erosion in Jinsha River Basin, attention has paid since 1988. An overall strategy for soil and water management has been formulated. The stress is laid on comprehensive control of soil erosion in watershed, taking into account both the principle of ecological protection and feature of socio-economic development, and combining the reduction of soil loss with improving environmental quality and increasing the income of local people. The plan will be implemented progressively and hierarchically within the basin. At the national level, the State Council decided in1988 to include four regions consisting of 61 counties in the Upper Yangtze River Basin as the national key areas for priority control of soil loss with special funds provided by the state. The Lower Jinsha Basin is one of the four key areas. The completed erosion control works will include: bench terracing of sloping farmland, afforestation on waste sloping land, grassed areas, and a great number of engineering works such as check dams, retaining walls, slide-resistant piles, horizontal screens for debris flow breaking, division works etc. At province and local levels, comprehensive improvement of small watersheds will be implemented extensively (Dai and Tan, 1996). For example, the efforts have been made in the Xiaohe basin and the Dianwei basin of Puduhe River, one of the tributaries of Jinsha River. The sediment yields are obviously decrease as shown in Fig.4. However, the further work must be done continuously in large areas, until the soil erosion in whole basin has been controlled 

5    CONCLUSIONS

The averaged annual runoff and suspended sediment yield of the Jinsha River Basin are 142x10⁹ m³ and 245x10⁶ t respectively. The Lower region, between Longjie and Pingshan, is the main erosion district of Jinsha Basin where the averaged annual sediment yield rate as high as 2440 t/km³/year, nearly 5 times higher than basin-averaged value. This area contributes 61.6% sediment yield, but only only 12.8% area and 16.9% of runoff of the whole basin. The soil erosion in Jinsha River basin concentrates highly in flood season. The sediment yield in flood season is as high as 84.1% - 97.5% of annual amount. Owing to the influencing of extensive human activity, the soil erosion in Jinsha River trend to increase after 1982. The averaged annual sediment yield in 1983-1992 increased 29.9 million tons to the period of 1954-1982. The averaged increase of sediment yield is 12.2% each year. Soil erosion control has been achieved in some small basin since 1988, but the further work must be done continuously in large areas, until the soil erosion completely is well controlled in whole basin.

Acknowledgements

This study is granted by the joint major project of the National Natural Science Foundation and the Water Resources Ministry of China (No. 59890200), and by the Educational Ministry of China (No. 1996-145).

References

DAI, D. and TAN, Y., 1996, Soil erosion and sediment yield in the Upper Yangtze River basin, Erosion and sediment yield and regional perspective (Proceedings of the Exeter Symposium, July 1996), IAHR Pub. No. 236, pp.191-203.

DENG, X., 1997, Characteristic analysis of sediment yield and transport of Jinsha River basin, Sichuan Hydropower (in Chinese), Vol.16, No.1, 23-25.

Table 1    The averaged annual runoff and sediment yield in main hydrographic stations

Table 2    Averaged annual runoff and sediment yield in each region

Table 3    Averaged annual runoff and sediment yield in 12 tributaries basins

 Table 4    Averaged annual runoff, sediment concentration and sediment yield for two periods of time at Pingshan Hydrographic Station

Fig. 1    Location of jinsha river basin

Fig. 2    Relationship between cumulated annual runoff and sediment yield at the outlet station, Pingshan

Fig. 3    The relationship between cumulated annual runoff and sediment yield in main tributaries of the Jinsha River