The effectiveness of check dams in controlling ...

Erosion, Debris Mows and Environment in Mountain Regions (Proceedings of the Chengdu Symposium, July 1992). IAHS Publ. no. 209, 1992.

423

The effectiveness of check dams in controlling upstream channel stability in northeastern Taiwan

CHIUN-MING LIU Associate Professor of Operations Research, Feng-Chia University, Taichung, Taiwan, China

Abstract Surveys of both sediment deposited in check dams and stream channel profiles were conducted to study the effectiveness of check dams in controlling stream channel stability in northeastern Taiwan. Results showed that the average annual sediment deposition in individual check dams ranged from 5.43 to 58.78 m3 ha"1. The gradient of the deposited surface was flatter than that of the original channel. The stream channel profile increased in width. The downstream reach might be unstable if the associated dissipator was not well designed. In the long term, most of the check dams in the study area can serve as stream power dissipators and stabilize the stream channel.

INTRODUCTION Northeastern Taiwan is characterized by an alluvial plain deposited by the Lang-Yang River. The upstream areas of this river basin are associated with precipitous slopes, tectonic disturbance and heavy rainstorms, which promote mass movements and give rise to severe channel aggradation in the lower reaches. The aggradation of stream channels has caused flooding and has also reduced the flow capacity of bridges constructed across the streams. In order to reduce these detrimental effects, intensive sediment control works have been implemented in these upstream areas since the 1970s (Taiwan Soil Conservation Service Bureau, 1979). Check dams have been the most widely used structures because it is expected that they can be used to trap sediment, to stabilize stream channels, to reduce channel slope and to prevent stream bank erosion (Yano, 1968; Lusby & Hadley, 1967). The construction of check dams in the upstream channels may, however, alter stream channel profiles and destabilize the downstream channel. Some investigations have suggested that check dams might destabilize downstream banks and bridge foundations in the downstream reaches (Liu, 1983). In this study, we examine the long-term impact of check dams on stream channels and sediment transport characteristics. The results are used to evaluate the effectiveness of check dams.

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Liu

METHODS The study area includes three contiguous headwater drainage basins in northeastern Taiwan, i.e. the Song-Lou, Yuan-Sun and Su-To basins (Fig. 1). These basins drain 1400, 750 and 220 ha, respectively and they are characterized by narrow, steep-sided valleys with average slopes of 49, 57 and 58%. Mass movements have been frequent since the 1960s, because of forest operations in these areas (Taiwan Soil Conservation Service, 1973). Check dams were therefore constructed in these areas. A typical check dam structure is shown in Fig. 2. In general, the design is such as to provide a large size and a solid shape (Liu & Lin, 1984). Five check dams within these three basins were selected for study. The dimensions of these check dams are listed in Table 1. A detailed survey was performed on the original stream channels. Bed load particle size composition was analysed. Selected channel cross sections in close proximity to the check dam sties were periodically surveyed. The cumulative volume of sediment deposits and their depositional pattern behind the check dam were obtained from periodic surveys.

RESULTS From periodic surveys of channel profiles behind the check dam sites, we obtained estimates of the accumulated sediment storage and its depositional pattern. After installation, the storage capacity of the Yuan-Sun, Su-To 1, Su-To 2 and Su-To 3 check dams was completely filled by sediment within the first three years after completion. The average annual sediment volumes deposited behind the check dams were 58.8, 5.4, 14.6 and 13.6 m3 ha'1 respectively. In 1991, the Song-Lou check dam still retained some storage capacity. During its life time, the average annual volume of sediment deposited behind the Song-Lou check dam was 26.5 m3 ha"1.

Fig. ] The study area of the Song-Lou, Yuan-Sun and Su-To basins in northeastern Taiwan.

The effectiveness

of check

425

dams

G.L

b /YOBOPROFILE

a:Dam biStilling basin c:Sill end d:Concrete block

PLAN Fig. 2 A typical check dam structure.

The surface gradients of the deposits behind the individual check dams were almost half those of the original channel. In addition, the stream reaches behind each check dam increased in width. The length of the depositional surface behind each check dam was positively related to the height of the check dam and the gradient of the depositional surface, but negatively related to the original channel gradient (Table 2). Analysis of the bed load particle-size composition showed that the particle-size distributions before and after check dam construction were very similar (Table 3). In addition, the deposited material closely resembled the coarse bed load existing in the original channel and possessed the typical particle-size distribution characteristics of recent alluvial material. The depositional surfaces behind each check dam can be divided into an upper and lower part, according to the maximum diameter of the bed load particles. The upper part mainly consists of gravel and boulders, while the lower part mainly consists of sand and gravel. In the reach downstream of each check dam, the degree of stream channel deformation was measured by cross sectional and longitudinal profiles. Data from periodic surveys of these profiles showed that downstream channel Table 1 The dimensions of check dams in the study area. Check dam

Height Width

Stilling basin: Height

Length

Sill end

Date of completion

(m)

(m)

(m)

(m)

(m)

9.5

169

16

133

1.8

July 1980

Yuan-Sun

9.5

100

10

90

1.8

June 1979

Su-To 1

3.5

32

12

20

1.2

June 1978

Su-To 2

5.8

21

10

15

1.0

July 1979

Su-To 3

7.5

47

12

20

1.2

July 1979

Song-Lou

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Liu

Table 2 Average annual sediment deposition rates (m 3 ha' 1 ). Check dam Song-Lou

Mean rate of deposition 26.49

Original channel Depositional Depositional gradient surface gradient surface length (m) 0.041 0.020 452

Yuan-Sun

58.78

0.070

0.036

268

Su-To 1

5.43

0.127

0.062

50

Su-To 2

14.60

0.147

0.067

80

Su-To-3

13.40

0.273

0.140

60

scouring occurred within the first three years after completion (see Figs 3-5). Due to channel scouring, additional concrete sill-end structures were installed downstream of the Yuan-Sun check dam in 1983. These structures provide increased energy dissipation (see Fig. 6). Subsequently, not only the channel downstream of the Yuan-Sun check dam but also the channels downstream of the Song-Lou. Su-To 1, Su-To 2, and Su-To 3 check dams became stable. Table 3 Sediment particle-size distribution (% by weight) before and after construction of the check dams. Check dam

Time of sampling Before After

Boulders >76.2 mm 33.6 30.7

Gravel >4.76 mm

Coarse sand >0.149mm

Fine sand 1.0 mm

0-10 10-20 20-30 30-40 40-50 Total

988 441 184

310 239 232 89 8 878

47 56 33 14 4 156

1613

D > 1.0 mm 46 13 7 66

D > 1.0 mm

536 317 147 63

84 36 22 10

1063

151

The physical, chemical, microbial properties of soils on abandoned land After cultivation ceases, the soil of the slope land changes in response to the vegetation recovery, but the soil still retains a few properties of the ploughed soil. The physical properties of the soil in the first year are listed in Table 6. The gravel content is 45% and considerably more than that of the shrubgrass and China fir land, because fine soil has been eroded during cultivation. Therefore the physical properties of the soils from the abandoned land differ from those of the other soils. The rapid water penetration and the greater moisture capacity of the abandoned land soils are beneficial in decreasing water and soil loss. On the other hand (see Table 7), in the first year after cultivation ceased, the chemical properties when compared with the shrubgrass demonstrate a 26.3% lower organic matter content, an equal content of easily soluble potassium, a 17.4% greater total nitrogen content, a 84.4% increase in total phosphorus, a 4033.3% increase in easily soluble phosphorus, a 282.3% increase in exchangeable calcium, a 381.8% increase in exchangeable magnesium, and a 223.7% increase in base saturation degree. Because of the application of fertilizer and manure to ploughed land, the soil nutrient content of abandoned land assists the rapid restoration of the vegetation. After cultivation ceased in the first year, the numbers of bacteria, Table 6 The physical properties of soils on various lands. Land

Structure

Compactness Moisture

Abandoned Shrubgrass China fir

Granular Granular Granular

Loose Moderate Moderate

Dry Humid Humid

Gravel (%) 45 37 20

Maximum moisture Penetration capacity (%) (mm min ) 43.07 13.806 39.25 5.353 39.76 8.212

Initial stages of natural succession on abandoned land in mountain areas

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Table 7 The chemical properties of soil from abandoned land during the first year after cultivation ceased and of shrubgrass soil. Soils

Organic Total Total phos- Easily soluble Easily soluble Exchanged Exchanged Base saturaniatter nitrogen phorus phosphorus potassium calcium magnesium tion degree (%) (%) (%) (ppm) (ppm) (mg 100 g'1) (mg 100 g 1 ) (%)

Abandoned 1.46 Shrubgrass 1.98

0.101 0.086

0.083 0.045

24.8 0.6

80 80

5.62 1.47

1.59 0.33

56.04 17.31

actinomycetes and fungi in the abandoned land were 84.5%, 67.8% and 82.9% of those in the shrubgrass land. This is because the soils of abandoned land are dryer than those of the shrubgrass and the organic matter content is lower. DISCUSSION The results obtained from this study show that, under conditions of natural recovery, on a slope of 28°, the erosion index of the abandoned land was reduced from 1784.67 t km"2 year"1 to 199.07 t m"2 year"1. In protecting the abandoned land it is important to exclude the various activities of people and livestock, otherwise the recovery of the vegetation will be restricted, the water and soil loss will remain high and the soil nutrient content will be depleted. In the experimental area, on the basis of preliminary observations, the vegetation was seen to succeed to brush five years after cultivation ceased. If seed bearers (e.g. Masson's pine) are growing near the abandoned land, it is easy for them to colonize the land. Acknowledgements Thanks are due to Wu Qixin, a research fellow, for his invaluable suggestions concerning the report. REFERENCES Lu Junpei& Zeng Qinbo (1981) A preliminary observation on the ecological consequences after "slash and burn cultivation" of the tropical semideciduous monsoon forest on the Jian Feng Ling Mountain in Hainan Island (in Chinese). Acta Phytoecologia et Geobotanica Sinica 5(4), 271280. Qian Zhengyuan (1991) Technical measures and counter strategies of land conversion into forests and grasses in the eroded areas along the Wujiang River valley (in Chinese). Bull. Soil and Wat. Conserv. 11(2), 13-18. Zhang Dayong, Wang Gang, Du Guozhen & Li Feng (1988) A quantitative study of the vegetation succession on the abandoned arablelands of the subalpine meadows in Gannan Prefecture of Gansu Province (in Chinese). Acta Phytoecologia et Geobotanica Sinica 12(4), 283-291. Zhao Xiejing, Lu Shihua, Jia Chun & Zhang Qi (1990) Soil conservation on slopes is the main way to control soil and water loss in the Sichuan hill area (in Chinese). Bull. Soil and Wat. Conserv. 10(1), 25-29. Zhen Dahao, Chen Yonggui & Wu Jinhuo (1991) Economic analysis of stopping cultivation and rebuilding forest in over reclamation./. Guizhou Agric. Sci. 108,53-58. Zhu Anguo (1985) Loss and Protection, Part II. Soil Erosion and Factors Affecting (in Chinese). Guizhou People's Publishing House, Guiyang, China.

Erosion, Debris Flows and Environment in Mountain Regions (Proceedings of the Chengdu Symposium, July 1992). IAHS Pub!, no. 209, 1992.

471

Degradation and protection of grassland on the Qinghai-Tibet plateau YANG DINGGUO Institute of Mountain Disasters and Environment, Chinese Academy of Science, Chengdu, PO Box 417, Sichuan 610015, China

Abstract The Qinghai-Tibet plateau, which has been called the roof of the world and the third pole, is a vast territory which includes a large area of grassland. It is one of the four main areas of animal husbandry in China. This paper, discusses the causes and development of grassland degradation in the region, and the countermeasures which have been proposed for protecting the grassland resource and preventing grassland degradation.

INTRODUCTION Grassland degradation is an important component of eco-environmental degradation in the world today, and grassland protection represents an important aspect of eco-environment protection. The Qinghai-Tibet plateau, which has been called the roof of the world and the third pole covers a vast territory and includes a large area of grassland. It is one of the four main areas of animal husbandry in China. Study of the degradation and protection of grassland in the region is important for the protection of the eco-environment of the Qinghai-Tibet plateau and for the assessment of global environmental change. There is evidence that the degradation of the grassland has become increasingly serious with the developing animal husbandry during recent years, but further investigations are required. This paper analyses the causes and trends of grassland degradation, and reviews the measures that have been proposed for protecting the grassland resource and preventing grassland degradation in the Qinghai-Tibet plateau. A BACKGROUND TO GRASSLAND DEGRADATION IN THE QINGHAI-TIBET PLATEAU The Qinghai-Tibet plateau is located in the southwest of China, and includes the whole of the Tibet Autonomous Region, and most of Qinghai Province, west Sichuan Province and northwest Yunnan Province. The total area of land is approximately 2.269 x 106 km2, which represents 23% of China. The area of grassland is approximately 1.33 x 108 ha, accounting for 58.8% of the Qinghai-Tibet plateau and 30% of the grassland in China respectively. Grassland degradation has become an important, widespread and growing

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problem in the Qinghai-Tibet plateau, involving both a large area and various types of degradation. The results of surveys undertaken in the grassland area of northwest Sichuan show that the area of degraded grassland accounts for between 30% and 50% of the total area of grassland in the region (Yang Dingguo, 1987). Based on these results, the total area of degraded grassland in the Qinghai-Tibet plateau is therefore estimated to be approximately 4.0 x 107 to 6.0 x 107 ha. Grassland degradation occurs throughout the region in a wide range of physiographic and climatic zones. Degraded grassland therefore occurs in the plateau areas and in the alpine and subalpine zones. It is found in areas of both agriculture and animal husbandry as well as in areas of simple pastoral farming. However, serious degradation is particularly found on the plateau surfaces of the area of smooth terrain, in the winter-spring grasslands, on the banks of rivers and alongside traffic routes. It is most serious around watering sites and the camps where animals are gathered in the winter-spring. Various types of degradation occur. The main types of degraded grassland in the Qinghai-Tibet plateau, based on underlying cause and outward characteristics, are the grasslands associated with hard soils, areas affected by rodents and insects, and areas influenced by encroaching sand and desertification. The degraded grasslands associated with hard soils, cover a large area and are widely distributed. The area involved is about 2.0 x 106 ha in the Ganzi Tibet Autonomous State accounting for 28% of the 7.15 x 106 ha of grassland in the State (Shi Yulin et al., 1985) and 43% of the total degraded grassland in the State respectively. The degraded grassland caused by rodents and insects in northwest Sichuan accounts for more than 1.33 x 106 ha. This type of degradation extends over 32.5% of the usable grassland in Zhiqu County (Zheng Yuanchang, 1990). Grassland destroyed by sand encroachment occurs mainly in the areas with a dry climate and a plentiful supply of dust and sand. However, the destruction of grassland by sand encroachment in the region has been gradually becoming more serious. For example, the area of grassland in Qinghai Province affected by sand encroachment is 2.7 x 106 ha (Xiang Lipin & Zhong Shanchang, 1990). The rapid increase in the extent of grassland degradation in the QinghaiTibet plateau should be noted. For example, the area of degraded grassland at the end of the 1970s was 3 to 4 times greater than at the beginning of the 1950s in the grasslands of northwest Sichuan (Yang Dingguo, 1987). Similarly, the area of grassland affected by sand encroachment caused by overgrazing in the Zoige region was about 3000 ha, whereas it only extended to 1100 ha at the beginning of the 1970s (Li Mingsen, 1986). THE CHARACTERISTICS OF DEGRADED GRASSLAND Grassland degradation reflects a destruction of the grassland eco-environment. The main characteristics may be summarized as follows:

Degradation and protection of grassland in China

(a)

(b)

(c)

(d)

(e)

473

Reduction in plant size and cover density. The results of typical surveys undertaken in the grasslands of northwest Sichuan show that the height of grasses is generally 50-60 cm in the natural grassland, and 70-80 cm and even over 80 or 100 cm in the good grassland. Cover density is generally from 60% to 80%. However, in the degraded grassland, the height of grasses is generally less than 30 cm, with heights of 10 to 20 cm common. Cover density is reduced to less than 50%. In the seriously degraded grassland, the height of grasses is only a few centimetres and seldom more than 10 cm, and cover density is often reduced to 20 to 30% together with a reduced yield. The yield of grasses is commonly reduced by 60 to 90%. A deterioration in grass quality. The Qinghai-Tibet plateau, with its favourable natural conditions for grass growth, has some of the richest grass resources in China. There are more than 400 species of grass, and good grasses are represented by over 100 species. Hard and impervious soils. Another obvious characteristics of degraded grassland in the region is that the soils become increasingly hard and impervious. Degraded grasslands, characterised by hard and impervious soils, are the most common type of degraded grassland in the region. The area of this type of degraded grassland in Ganzi Tibet Autonomous State is about 2.0 x 106 ha. This represents 27% of the grassland and 43% of the degraded grassland in the state respectively. Desertification. Desertification is a serious result of grassland degradation. Deterioration of the grass cover leads to exposure of the soil surface and causes soil erosion. Desertification of grassland in the Qinghai-Tibet plateau is partly a response to natural conditions and partly the result of human activities. Covering by sand. The covering of grassland by sand is the end-result of the degradation of the grassland eco-environment. It is associated with inappropriate human activity and occurs in regions with a dry climate and a plentiful supply of dust and sand. Sand-covered grassland in the Qinghai-Tibet plateau occurs mainly in the Zoige area, the Litang and Gonghe basins, the Beilu and Tuotuo River basins, as well as the longitudinal valley of the Yellow River between the Bayanhar and Anyemaqen mountains.

THE CAUSES OF GRASSLAND DEGRADATION The causes of grassland degradation in the Qinghai-Tibet plateau include inappropriate human activities and natural disasters. Inappropriate use of the grassland The main causes are, firstly, overgrazing. Overgrazing is a fundamental cause

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Yang Dingguo

of the increasing degradation of grassland in the region. Uncontrolled increase in the number of animals causes the carrying capacity of the grassland to be exceeded. In this region, the development of animal husbandry was associated primarily with increased numbers of animals. For example, in the Ganzi Tibet Autonomous State, the number of animals at the end of the 1970s was double that at the beginning of the 1950s. In the Rongma area of Shiqu County, the carrying capacity of the grassland was exceeded by 25%. Secondly, there is an imbalance between the winter-spring and summerautumn grasslands, with the area of winter-spring grassland being too small. The proportion is generally less than 30% of the usable grassland in many places. This leads to overgrazing of the winter-spring grassland, especially around the watering points and the gathering areas for animals, where the number of animals may exceed the carrying capacity of the grassland by several times and even by as much as 10 times. Thirdly, there is frequently uncontrolled and destructive grazing by animals. The former is particularly common in the boundary districts, due to uncertainties over grassland use. The latter occurs in favoured locations and due to a lack of knowledge amongst the herdsmen. Poor management of the grassland The utilization of grassland in the region is characterized by overuse and limited fertilizer input. There has also been only limited development of basic facilities, including irrigation, drinking water camps, feedgrass stores, and artificial grassland. Animal husbandry in the region still depends on the natural conditions. Rodent and insect infestations Such problems have affected large areas and have been an important factor in causing grassland degradation in the region. Rats gnaw the roots, stems, leaves and seeds of grasses, and insects eat the green leaves, tender stems and seedlings of grasses, thereby influencing the future reproduction and development of the grasses. Reclaiming of wasteland and gathering of Chinese medicinal herbs Although the Qinghai-Tibet plateau is a poor grain producing area, there has been a history of ill-planned reclamation of land for crops with an emphasis on grain production. The reclaimed land, with a low productivity due to natural conditions, was often abandoned after one to two years. Vegetation recolonization of the fallow areas is very slow leading to problems of

Degradation and protection of grassland in China

475

desertification and sand encroachment. In addition, the Qinghai-Tibet plateau, with its unique natural conditions, is the habitat of many rare Chinese medicine herbs, including Cordyceps sinensis, Fritillaria thunbergii, Gastrodia elata, Astragalus membranaceus, Anemarhen asphodeloides, Scutellaria baccalensis and Licorice root. The peasants and herdsmen collect the Chinese medicine herbs at suitable times of the year to make money. This leads to destruction of the roots of the grasses, burial of their seeds, and disturbance of the soil surface which in turn influences the reproduction, development and propagation of the grasses. Forest clearance Forest protects the grassland. Forest clearance leads to reduced precipitation, a drier climate, and soil erosion, which in turn harm the growth and development of the grasses.

SUGGESTIONS FOR THE PROTECTION OF GRASSLAND AND PREVENTION OF GRASSLAND DEGRADATION On the basis of the above analysis, the following suggestions for protecting grassland and preventing grassland degradation in the region are advanced. (a) Improvement of the level of education and the eco-environmental awareness of the peasants and herdsmen. (b) Restricting the number of animals to the carrying capacity of the grassland in order to avoid overgrazing. (c) Sensible utilization of the grassland resource. This includes the development of regional plans and the establishment of rotation grazing. The former includes dividing the usable grassland into winter-spring and summer-autumn grasslands, as well as areas for hay production. (d) Increased economic input and improved grassland management. The management of grassland in the region is poor, with overuse and little input. The input should be increased to improve the management of the grassland. Here the term input includes many aspects such as funds, manpower, materials and science-techniques etc. (e) Improvement of the degraded grassland is an important task for management of the grasslands of the region, due to the large area of degradation. Measures for improvement include the development of grass cover through closure, soil aeration, reseeding, irrigation and fertilizer application, but the measures used should be adapted to the different characteristics and underlying causes of the degraded grassland. (f) Prevention and cure of rodent-insect infestations. Such infestations have had a major influence on the degradation of grassland in the region, and

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their control is very important for protecting the grassland. Control measures include the use of pesticides and the protection of natural predators. (g) Strict scientific gathering of Chinese medicine herbs. The Qinghai-Tibet plateau, with its unique natural environment, is a region where the existing forests and animal husbandry should be preserved as the dominant occupation during the development of the economy, and reclamation of wasteland should be strictly prohibited. The rich resource of Chinese medicine herbs in the region is of great value to humanity, and since it is impossible to prohibit their gathering, scientific methods should be used. To generalize, it is obvious that relatively little is known about the degradation and protection of grasslands in the Qinghai-Tibet plateau. In this paper, the problems have only been identified. Further work on the degradation and protection of grassland in Qinghai-Tibet plateau should be undertaken. REFERENCES Li Mingsen (1986) Characteristics and utilization of the grassland soils in Zoige. Studies of the QinghaiXizang (Tibet)plateau. Special issueof the HenhduanMountainsscientificexpedition,489-502. Beijing Science and Technology Press. Shi Yulin et al. (1985) WastelandResourcesfor Agriculture in China. Beijing Science and Technology Press. Xiang Lipin & Zhong Shanchang (1990) Sandification and prevention and cure of land degradation in Qinghai Province. Agro-grass Resources and Regionalization Study no.2, 43-47. Yang Dingguo (1987) A discussion of the degradation and protection of grassland in northwest Sichuan. Mountain Research 5(2), 115-121. Zheng Yuanchang (1990) Main problems of eco-environment in the northeast Qinghai-Tibet plateau. Expedition report.

Erosion, Debris Flows and Environment in Mountain Regions (Proceedings of the Chengdu Symposium, July 1992). IAHS Publ. no. 209, 1992.

477

A study of land degradation and restoration in mountain environments in Liaoning Province ZHAO HUANCHEN, LI BAOPIN Changchun Institute of Geography, Chinese Academy of Sciences, 10 Gongnong Road, Changchun, Jilin, China

LI GUILIAN Department of Environment Science, Northeast Normal University, China

ZHAO WENJING Jilin Institute of Meteorology, China

Abstract Liaoning Provinces lies in the south of the northeastern region of China. Under the special geomorphological and geological conditions found in the mountains in this region, the local climate, coupled with irrational human activity, result in severe weathering and denudation of the land surface, and land degradation. Consequently, problems such as disastrous erosion, and debris flows occur frequently and are increasing. As the ecological environment degenerates; soil fertility declines and land productivity decreases over wide areas. Serious land degradation has restricted development of the regional economy and social improvement. In this paper, both the factors influencing the occurrence of land disasters, such as disastrous erosion and debris flows, and the feedback relationship between the effect of such disasters and the development and exploitation of the region are considered. Based on such work we present proposals for the improvement of the mountain environments in this region as a means of improving social conditions and strengthening the local economy.

REGIONAL CHARACTERISTICS Liaoning Province is located in the south of the northeastern region of China and stretches from 118°53'E to 125°46'E in longitude, for a distance of 574 km. In a north-south direction the latitude spans 38°43'N to 43°26'N, which amounts to 530 km. The relief declines from the north to the south and from the eastern and western margins to the centre. In the eastern and western areas there are mountains and hills. Based on their landforms, these can be divided into four parts, namely, the eastern Liaoning Mountains, theLiaodong PeninsulaHills, the western Liaoning Low Mountains and Hills and the northwestern Liaoning Low Hills. Overall they cover 108 722 000 ha or 60% of the total area of the province. Thirty percent of the province is occupied by plains. With reference to geological structure, most of this region belongs to the China-Korean Qiasi-platform, although the northern margin belongs to Tianshan-Xingmeng geosyncline folded zone. The crust is active and faults are

478

Zhao Huanchen et al.

distributed widely and well developed. The region experiences a monsoon climate influenced by altitude, the land-sea distribution and the atmospheric circulation. The annual average temperature is 5-10°C and the maximum range of temperature from the south to the north is no more than 5.6°C. The annual range is 30-40°C. Precipitation is influenced by the prevailing westerlies and the annual average varies from 400 to 1100 mm over the whole region, decreasing from the southeast to the northwest, with great temporal and spatial variation. In summer, high intensity rainfall occurs and surface runoff is intense. Mountains hazards in this region, which mainly include soil erosion, debris flows, rockslides and avalanches, result in serious degradation of the environment. It has become a limiting factor in the development of the regional economy and in social improvement.

A MODEL OF THE DEGRADATION OF THE MOUNTAIN ENVIRONMENT Unfavourable natural conditions and irrational human activity have caused many land disasters, including serious soil erosion and debris flows. As a result of the aggregate effects of these disasters, soil and water losses are increasing. Consequently, the ecological environment is deteriorating. Soil fertility is decreasing and floods and droughts are occurring more frequently and getting worse. Figure 1 illustrates a model of the degrading mountain environment. The distribution of soil and water losses in time and space In the time dimension, erosion and debris flows of varying intensity occur each

land

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