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Journal of Geographical Sciences >

2021 , Vol. 31 >Issue 10: 1437 - 1452

DOI: https://doi.org/10.1007/s11442-021-1905-1

Research Articles

Distribution and land use characteristics of alluvial fans in the Lhasa River Basin, Tibet

  • CHEN Tongde , 1 ,
  • JIAO Juying , 1, 2, * ,
  • CHEN Yixian 2 ,
  • LIN Hong 1 ,
  • WANG Haolin 1 ,
  • BAI Leichao 1
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  • 1. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, China
  • 2. Institute of Soil and Water Conservation, CAS and Ministry of Water Resources, Yangling, Shaanxi 712100, China
*Jiao Juying (1965-), PhD and Professor, specialized in soil erosion and vegetation restoration. E-mail:

Chen Tongde (1993-), PhD Candidate, specialized in soil erosion and land quality evaluation.E-mail:

Received date: 2020-10-30

  Accepted date: 2021-08-03

  Online published: 2021-12-25

Supported by

The Strategic Priority Research Program of Chinese Academy of Sciences(XDA20040202)

The Second Tibetan Plateau Scientific Expedition and Research Program(STEP)(No.2019QZKK0603)

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Abstract

In Lhasa River Basin (LRB), land suitable for settlement or living is experiencing a shortage of resources. Alluvial fans have the potential to alleviate this problem. However, basic information, such as the distribution and land use types of alluvial fans, is rarely studied. In this study, Google Earth, ArcGIS and visual interpretation were used to obtain the outlines, areas, quantities and distribution of alluvial fans. Meanwhile, to show the utilisation potential of alluvial fans, we analysed the land use, their distance from the roads, places (town and village) and rivers. The results showed 826 alluvial fans exist in LRB, with a total area of 1166.03 km². The number of alluvial fans with areas between 0.1 and 1 km2 is 517, accounting for 62.59% of the total number of alluvial fans. Grassland is the dominant land use type, accounting for 68.70% of the total area of alluvial fans. The cropland area accounted for 2.16% of alluvial fans and accounted for 18.98% of the total cropland area in LRB. Exactly 93.70%, 53.63% and 61.86% of the total number of alluvial fans were located within 5 km from the tertiary road, village, and river, respectively. To sum up, our survey results showed that alluvial fans are important land resources in LRB and may have huge utilisation potential.

Key words： alluvial fan; Lhasa River Basin; distribution; land use; Tibet

Cite this article

CHEN Tongde , JIAO Juying , CHEN Yixian , LIN Hong , WANG Haolin , BAI Leichao . Distribution and land use characteristics of alluvial fans in the Lhasa River Basin, Tibet[J]. Journal of Geographical Sciences, 2021 , 31(10) : 1437 -1452 . DOI: 10.1007/s11442-021-1905-1

1 Introduction

Alluvial fans are fan-shaped landforms with a surface forming a cone deposit that radiates downslope (Bull, 1977). Alluvial fans can be developed in various types of terrestrial settings, such as alpine, humid tropical, humid mid-latitude, periglacial and different paraglacial settings (Dorn, 1994). Although the development settings of alluvial fans are different, the factors affecting their landform and material are similar. Bedrock types, basin area, slope gradient, and geological characteristics of catchment directly affect the material and size of alluvial fans (Blair, 2002; Stock et al., 2008). Alluvial fans consist of solid materials eroded from an upland catchment (Ashworth, 2006). Normally, the bedrock types with less resistance for erosion are classified in accordance with an alluvial fan of increasing size (Hooke, 1968). Furthermore, the characteristics of alluvial-fan deposit are affected by different bedrock types, although other environmental conditions are similar. Two adjoining alluvial fans exist in Death Valley, California. The bedrocks of one catchment are andesite and granite rocks with good anti-erodibility and the other catchment includes Precambrian to Cambrian sedimentary rocks with less anti-erodibility. The first alluvial fan was formed through flooding processes, and another through debris flow processes. In addition, the average slope of flooding-processed fans is lower than that of debris-processed fans. However, the area and radius show opposite trends (Blair, 2002). Regional tectonics and climate affect the deposition process and geomorphology of alluvial fans (Bull, 1977). Tectonic forces affect morphology by controlling the scale of accommodation space and organisational types of the deposited material (Sarp, 2015). The increased aridity in the Late Pleniglacial resulted in small-scale alluvial fans in Central Europe, and the scale was less than that in the humid periods (Sarp, 2015). Although numerous factors affect the development of alluvial fans, they are a product created by erosion and sediment processes ensuing from a significant runoff flows (Hartley et al., 2010). Therefore, a more direct factor for alluvial fan development is runoff flow, which includes river, flood and debris flows. The three main types of formation process of alluvial fans include fluvial, flooding, and debris flow processes (Santangelo et al., 2012; Dickerson et al., 2013; Birch et al., 2016). The main difference between alluvial fans formed by fluvial and flooding processes and that formed by debris flow process is whether their surface has a constant flow and fixed flow channel (Chen et al., 2020). The fluvial-process alluvial fan type generally has a constant flow, and the flow channel is mostly fixed. However, the other alluvial fan types present the opposite results. Meanwhile, the slope of debris-flow-process alluvial fans is steeper than that formed by flooding process in the same region. Combining the deposition, geomorphology and surface environmental characteristics of the three types of alluvial fans, numerous fluvial-process alluvial fans have been developed into cities due to their wide area, flat terrain, and adequate water resources. However, alluvial fans formed by flooding or debris flows have been commonly developed into towns or villages due to the limitation of their area, water resources, morphology and threat of flooding or debris flows (Chen et al., 2020).
In most mountain regions, a shortage of land suitable for human settlement or cultivation occurs due to harsh climate and natural hazards. Alluvial fans, usually featuring desirable hydrological conditions, fertile soil and a relatively flat terrain (Khan et al., 2013; Rahaman, 2016; Telbisz et al., 2016; Mazzorana et al., 2020), can be developed into settlement areas or cultivation lands in numerous mountain regions (Ma et al., 2004; Mazzorana et al., 2020). Several cities or towns are developed based on large-scale alluvial fans (Santangelo et al., 2012; Maghsoudi et al., 2014; Chen et al., 2017). In addition, solid materials from alluvial fans, such as clay, silt, sand, gravel, and large boulders can be used as construction resources (Bahrami et al., 2015). Moreover, alluvial fans are threatened by various types of natural hazards because of special formation processes and environments. Land subsidence, water shortage and pollution are commonly natural hazards in alluvial fans because of long-term overutilisation and contamination of water resources (Bull, 1973; Chen et al., 2017; Alkinani et al., 2019). Alluvial fans formed by flooding and debris flows are vulnerable to floods (Santangelo et al., 2012; Dickerson et al., 2013), debris flows (Okunishi and Suwa, 2001; Khan et al., 2013) or gully erosion (Deng et al., 2019). The two types of alluvial fans formed by debris flows or flooding have their surfaces threatened by hazards due to these processes. Alluvial fans are prone to the above natural hazards, but they are still used in different kinds of ways. The contradiction also reflects the shortage in the available lands in several mountain regions. Therefore, alluvial fans are an important land resource although limited research has been focused on this point. More studies about the survey, protection, planning and reasonable utilisation of alluvial fans should be conducted.
The Tibetan Plateau is the highest region on the earth. Given its extreme and vulnerable environment, the majority of land in Tibet is unsuitable for settlement or living (Liang and Hui, 2019). The good habitability of mountain regions is based on hydrological, geomorphological, biogeographical properties (Telbisz et al., 2016). Only river terraces, alluvial fans, and gentle-slope maintain areas can satisfy these properties in Tibet, but river terraces have been well utilised due to gentle terrain, fertile soil and hydrological conditions. For such reasons, in Tibet, the area suitable for local humans to live is limited. Meanwhile, the population is dramatically increasing, and Tibet has been in an accelerated urbanisation stage in recent years (Fan et al., 2010). Therefore, given the shortage of land for cultivation and settlement, the human-environment interaction is gradually becoming uncoordinated (Liang and Hui, 2019). Numerous farmlands and villages are distributed in alluvial fans, although they are threatened by floods and gully erosion (Ma et al., 2004). To relieve this problem, researchers should focus on the alluvial fans located in Tibet. However, studies about alluvial fans are rare, although alluvial fans are an important land resource in Tibet. The quantity, distribution and land use of alluvial fans in this region are insufficiently understood, but important information and how to use and protect this land resource must be determined. Lhasa is the capital city of Tibet in China and is located in Lhasa River Basin (LRB), where the need for land is urgent. Thus, we selected this basin to investigate the amount, distribution and land use of alluvial fans. Meanwhile, the distance from alluvial fans to roads, places (town and village) and rivers were analysed to explore the value, function and utilisation potential of this land resource in LRB.

2 Data and methodology

2.1 Study area

LRB, with an area of 32 471 km², is located in the south of Tibet Autonomous Region, southwest of China, lying from 29°20′N-31°15′N to 90°05′E-93°20′E (Figure 1), and is the largest drainage area of the Yarlung Zangbo River (You et al., 2007). This area consists of Chenguan, Tolung Dechen, Taktse, Medro Gongkar, Lhundup, Dumshung and a part of Seni, Chali and Chushur counties (Figure 1). The altitude of the study area varies from 3500 m to 7162 m (Zhang et al., 2010). The majority of the area is over 4000 m, whereas the middle and downstream of the Lhasa River valley are less than 4000 m (Wei et al., 2012). The climate of LRB belongs to plateau temperate semi-arid monsoon climate, and the average temperature is about -1.7 to 9.7°C (Zhang et al., 2010). The annual precipitation is about 340-600 mm and concentrated from June to September (Wei et al., 2012). The vegetations are characterised by alpine steppe, alpine meadows and alpine shrubs, cushion vegetation, etc. (Lin et al., 2008). Seven soil types exist in the study area: alluvial, meadow, subalpine steppe, subalpine meadow, alpine steppe, alpine meadow and alpine frozen soils. These soil types correspond to Fluvaquents, Haplumbrepts, Calcic cryoboroll, Typic cryaquept, Cryuborolls, Cryaquoll and Aridisols, respectively, in the USA taxonomy. The Fluvaquents, Haplumbrepts, Calcic cryoboroll and Typic cryaquept soils are arable land soil which can be planted with winter barley (Hordeum vulgare L), winter wheat (Triticum aestivum L), and maize (Zea mays L) (Wei et al., 2012).
Figure 1 Location of the Lhasa River Basin
The landform of LRB is composed of mountains, valleys, river terraces, alluvial fans, etc. (Wei et al., 2012; Dai et al., 2018). River terraces and alluvial fans are lower than mountains, and most of the arable land is located in these lower areas (Wei et al., 2012). In addition, according to an investigation in 2018 (Ma et al., 2018): 1) alluvial fans are a common landform in the valleys of LRB; 2) numerous villages and farmlands are located in alluvial fans; 3) alluvial fans are less developed than the river terraces that have been sufficiently developed into towns and cities.

2.2 Extraction of alluvial fans

Alluvial fans were drawn by visual interoperation in Google Earth Pro (7.3.3.7692). We used three evident characteristics to determine the alluvial fans formed by flooding process LRB (Chen et al., 2020): (1) fan-shaped landform, (2) braided flow channels, and (3) non-consistent flow (Figure 2). Based on these characteristics, the outline data were obtained using Google Earth Pro. Then, we added the outline data to ArcGIS to obtain the area, position and distribution of alluvial fans.
Figure 2 A typical alluvial fan located in the Lhasa River Basin

2.3 Data sources

The 10-m resolution land use type in 2017 was obtained from the previous study (Gong et al., 2019). We selected this land use information because of its high spatial resolution and quality (72.76% in overall accuracy). The land use types were classified into cropland, forest, grassland, shrubland, wetland, water body, tundra, impervious area, bare land, and snow/ice (Gong et al., 2019).
The position characteristics, such as road, place and waterway, were obtained from Openstreetmap. In this website, the roads were classified into trunk road, primary road, secondary road, tertiary road, residential road, cycleway, footway, etc.. The places were classified into country, province, county, town, village, etc. The waterways were classified into river, stream, drain, ditch, etc.

2.4 Data processing and analysis

2.4.1 Classification of alluvial fans

A huge quantity of alluvial fans was observed, and the area exhibited various sizes in our study location. To determine the distribution of alluvial fans, we classified them into four sizes in accordance with their area, i.e., 1st: area≤0.1 km², 2nd: 0.1<area≤1 km², 3rd: 1<area≤10 km², and 4th: area >10 km². The quantity of alluvial fans of each size was counted.

2.4.2 Indicators of land use

Numerous types of land use exist in alluvial fans; these types comprise cropland, forest, grassland, shrubland, wetland, water body, tundra, impervious area and bare land. Two indicators, namely, Rlaf and Rllrb, were used to reflect the land use characteristics of alluvial fans in LRB. Rlaf is the ratio between the area of a land use type and the total area of alluvial fans. Rllrb is the ratio between the area of a land use type in alluvial fans and the area of the same land use type in LRB.
Rlaf = Ali/Aa·100%
where Ali (km²) is the area of land use i of alluvial fans, and i= cropland, forest, grassland…, Aa (km²) is the total area of alluvial fans.
Rllrb = Ali/Alrbi·100%
where Ali (km²) is the area of land use i of alluvial fans, and i= cropland, forest, grassland…, Alrbi (km²) is the area of the ith land use of LRB.
Meanwhile, to show the spatial distribution of land use types in alluvial fans, we classified the alluvial fans into three types based on the area of land use type. The first type was an alluvial fan with an area dominated by one land use (area of ith land use ≥ 50% (A50%i), and i= cropland, forest, grassland…). The second type was A50%i whose area is occupied by another land use j, being no less than 30% (A50%i A30%j, and j = cropland, forest, grassland….). The third type was an alluvial fan with an area not dominated by any land use, which means that the area of ith land use cannot occupy half of the alluvial fan (Ai<50% and i= cropland, forest, grassland….).

2.4.3 Indicators for utilisation potential

The utilisation potential mainly depends on the distance between an alluvial fan and a road, place or waterway. The centroids of all alluvial fans were calculated in ArcGIS, and the distance between an alluvial fan and road, place and waterway were calculated based on the centroid. Given our study goal, the primary road, tertiary road, town and village were selected to analyse the distances. For the town and village, the locations of government departments were used to obtain the distance. Meanwhile, considering the background of water shortage in semi-arid monsoon climate zone, all kinds of waterways are important for this basin. As a result, streams with consistent flow were added to the river data for analysis.
Another two indicators, including Rq and Rd, were used to show the utilisation potential of alluvial fans. Rq is the ratio between the quantity of alluvial fans such that their distance from the road, place and river is not more than a certain space and the total number of alluvial fans in LRB. Rd is the ratio between area of alluvial fans such that their distance from the road, place and river is no more than a certain space and the total area of alluvial fans.
Rq = Qan/Q·100%
where Qan is the number of alluvial fans not more than n (km) from the road, place and river, and n=1, 5, 10, 20, 40; Q is the total number of alluvial fans.
Rd =Aan/Aa·100%
where Aan (km²) is the area of alluvial fans no more than n (km) from the road, place and river, and n=1, 5, 10, 20, 40; Aa (km²) is the total area of alluvial fans in LRB.
All figures and analyses were carried out with ArcGIS10.7 and Origin 8.5 software.

3 Results

3.1 Number of alluvial fans

Figure 3 shows the size distribution of alluvial fans. A total of 826 alluvial fans were counted in LRB, with 517 (62.59%) having areas between 0.1 km2<area≤1 km² (2nd size). A substantial part of the alluvial fans area was observed between 1 and 10 km2 (n=205, 24.82%, 3rd size). Several alluvial fans had an area larger than 10 km² (n=17, 2.06%, 4th size). Thus, alluvial fans with an area less than 1 km² are dominant, and those with an area more than 10 km² are rare in LRB.
Figure 3 Wumber of alluvial fans
Furthermore, the alluvial fans were ranked by ascending area size, and an exponential relationship (the equation: y=3.93e0.067x, R2=0.92) was observed between the accumulating area and the quantity of alluvial fans (Figure 4). When the quantity accounted for 50% of the total number of alluvial fans, the accumul ating area of alluvial fans was 83.39 km2. However, when the area accounted for 50% of the total area of 1166.03 km2, the quantity of alluvial fans was 746.27, which indicates that the area was occupied by 26 large-size alluvial fans (4th).
Figure 4 Relationship between quantity and accumulating area of alluvial fans

3.2 Spatial distribution of alluvial fans

3.2.1 Quantity and area distribution

Alluvial fans are concentrated in central and eastern LRB, especially in the Lhundup Medro Gongkar and south of Chali county (n=452, 54.72% of total) (Figure 5a and Table 1). However, the distribution trend of area is contrary to the quantity of alluvial fans (Figure 5b). Compared with the eastern part, the large areas are in the west, especially in Damshung county, and the area proportion of all alluvial fans accounted for 41.61% of the total area. In addition, a great number of alluvial fans were detected in Medro Gongkar and Chali counties (156 and 122, respectively) (Figure 5a and Table 1). However, the areas were small, being (89.95 km2 and 60.42 km2 (7.71% and 5.18%, respectively) (Figure 5b and Table 1).
Figure 5 Quantity and area distribution of alluvial fans in the Lhasa River Basin
Table 1 Quantity and area of alluvial fans in the Lhasa River Basin
County Quantity (No.) Area (km2) County Quantity (No.) Area (km2)
Lhundup 174 191.74 Taktse 87 61.32
Medro Gongkar 156 89.95 Seni 47 94.90
Chali 122 60.42 Chenguan 36 25.98
Tolung Dechen 90 94.61 Chushur 26 61.91
Dumshung 88 485.20

3.2.2 Spatial distribution of alluvial fans of different sizes

Figure 6 shows the spatial distribution of alluvial fans of different sizes. The 1st- to 3rd-size alluvial fans almost exist in all counties in LRB, whereas 4th-size alluvial fans exist in four counties. The 1st-size alluvial fans were mainly distributed in the east (Medro Gongkar and Chali County) but were rarely detected in the west and south (Figure 6a and Table 2). The 2nd-size alluvial fans were largely distributed in Lhundup, Modero Gongkar and Chali (Figure 6b and Table 2). In addition, the quantity of 2nd-size alluvial fans was relatively higher in these three counties compared with the other sizes. The 3rd-size alluvial fans were mainly distributed in Damshung and Lhundup counties (Figure 6c and Table 2). Several alluvial fans were characterised as 4th-size alluvial fans and sporadically distributed in Damshung and Lhundup counties (Figure 6d and Table 2). Especially, ten 4th-size alluvial fans were detected in Damshung county with an area of 361.61 km², accounting for 31.01% of the total area of alluvial fans.
Figure 6 Spatial distribution of different size alluvial fans in the Lhasa River Basin
Table 2 Quantity and area of different size alluvial fans in the Lhasa River Basin
County First size Second size Third size Fourth size
Quantity (No.) Area (km2) Quantity (No.) Area (km2) Quantity (No.) Area (km2) Quantity (No.) Area (km2)
Medro Gongkar 27 2.12 100 33.41 29 54.42 0 0.00
Chali 23 1.81 88 34.25 23 57.62 0 0.00
Lhundup 11 0.85 113 45.89 48 117.73 2 27.27
Taktse 10 0.79 59 22.58 34 107.22 0 0.00
Seni 6 0.44 40 16.08 29 54.42 4 48.61
Dumshung 4 0.29 40 16.08 34 107.22 10 361.61
Tolung Dechen 3 0.22 63 25.63 29 54.42 1 11.15
Chenguan 2 0.17 59 22.58 23 57.62 0 0.00
Chushur 1 0.07 40 16.08 17 58.85 0 0.00

3.3 Land use of alluvial fans

Table 3 shows the area proportion of different land use types of alluvial fans in LRB. Grassland is the major land use type (Rlaf = 68.70%), and the second largest land use is bare land (Rlaf =24.1%). The large area of grassland illustrates the great contribution of alluvial fans in providing grass for livestock. Several areas were developed into impervious areas covered by artificial buildings (Rlaf = 4.52%, and Rllrb = 9.18%). A similar proportion of the impervious area was developed into cropland (characterised by both Rlaf and Rllrb indicators) (Table 3). The proportions of forest, shrubland, wetland, and water body are considerably smaller than those of cropland, impervious area and bare land. The area of snow/ice can almost be ignored.
Table 3 Area percentage of different land use types in alluvial fans in the Lhasa River Basin
Land use types Rlaf (%) Rllrb (%) Land use types Rlaf (%) Rllrb (%)
Grassland 68.70 3.97 Wetland 0.13 0.29
Bare land 24.10 2.86 Shrubland 0.03 0.16
Impervious area 4.52 9.18 Forest 1.76×10-3 0.01
Cropland 2.16 18.98 Snow/Ice 5.86×10-5 6.39×10-6
Water 0.37 0.59
Figure 7 shows the spatial distribution of land use types of alluvial fans in LRB. Grassland is the most common land use type, especially in Damshung county. Croplands are mainly distributed in the south of LRB, including Taktse, Tolung Dechen, Chushur and the south part of Lhundup county. The distribution of impervious area was the same as that of croplands and mainly distributed in Taktse, Chenguan and Tolung Dechen counties. Bare lands were mainly distributed in Damshung and Chushur counties. Through verification by Google Earth, the bare land of alluvial fans is covered by sand and thus can hardly grow vegetation.
Figure 7 Distribution of land use types in alluvial fans in the Lhasa River Basin
We considered four scenarios, including A50%grassland, A50%bare land, A50%impervious area, and A50%cropland (Figure 8), in which one land use type occupied more than 50% of the total (A50%). The A50%grassland (n = 665 and area = 903.17 km2) and A50%bare land (n = 74 and area=52.59 km2) exist in all counties (Table 4). The A50%impervious area only exists in Tolung Dechen, Chenguan, and Taktsc counties. One A50%cropland was found in LRB with an area of 0.07 km²(Table 4). Eight types of A50% with their area occupied more than 30% of another land use type (A50% A30%), and four types are A50%grassland (Figure 9 and Table 4). The quantity of A50%A30% is far less than that of A50%. A50%grassland A30%bare land is the dominant type in A50%A30% (n = 38 and area = 109.03 km2, 48.10% and 80.11%) and distributed in all counties except Chushur (Table 4). A<50% was the least land use type observed (n= 64, area = 91.56 km2) (Figure 10 and Table 4), and its distribution is concentrated in the range of urban and adjacent areas.
Figure 8 Distribution of alluvial fans whose area is accounted for by one land use type of more than 50% (A50%) in the Lhasa River Basin
Figure 9 Distribution of A50% whose area is accounted for by another land use type of more than 30% (A50% A30%) in the Lhasa River Basin
Table 4 Quantity and area of different types of alluvial fans
Types Quantity (No.) Area (km²) Types Quantity (No.) Area (km²)
A50%grassland 665 903.17 A50%grassland A30%shrubland 1 0.06
A50%bare land 74 52.59 A50%impervious area A30%garssland 6 2.35
A50%impervious area 22 10.98 A50%impervious area A30%bare land 4 1.47
A50%grassland A30%bare land 38 109.03 A50%bare land A30%grassland 15 13.01
A50%grassland A30%impervious area 8 5.10 A50%bare land A30%impervious area 6 3.58
A50%grassland A30%water body 1 1.50 A<50% 64 91.56
Figure 10 Distribution of A<50% whose area is not dominated by any land use type in the Lhasa River Basin

3.4 Distance of alluvial fans from a road, place and river

Figure 11 shows the trend of the quantity and area proportion of alluvial fans with a distance from roads of different grades. For the primary road, Rq1 was 29.66%, and the Rd1 was 21.83%. When the distance was no more than 5 km, Rq and Rd were dramatically increasing. The Rq5 reached 75.91%, and the area proportion was 89.87 %. Furthermore, for tertiary roads, the regulation is the same as the primary road. Rq1, Rd1, Rq5, and Rd5 were 66.71%, 44.97%, 93.70% and 97.50% respectively. Therefore, the majority of alluvial fans are adjacent to the road with a distance of not more than 5 km.
Figure 11 Distance between alluvial fans and roads
Figure 12 shows the trend of the quantity and area proportion of alluvial fans with a distance to towns and villages. The Rq1 and Rd1 to towns were 1.94% and 1.59%, respectively. The Rq5 was 34.62%, and Rd5 was 36.30%. After the distance recached 10 km, the Rq and Rd exceeded 50%. Thus, the majority of alluvial fans were mainly distributed in the regions with the distance to town over 5 km. However, the Rq5 and Rd5 to village were 53.63% and 46.14%, respectively, indicating that about half of the alluvial fans were near villages.
Figure 12 Distance between alluvial fans with distance to places
Figure 13 shows the trend of the quantity and area proportion of alluvial fans with the distance to a river. The Rq1 and Rd1 were 23.66% and 21.83%, respectively. The Rq5 was 61.86%, and Rd5 was 77.38%, indicating that the majority of alluvial fans have a distance near water resources.
Figure 13 Distance between alluvial fans and a river

4 Discussion

4.1 Land use of alluvial fans

Alluvial fans are an important land resource in LRB, Tibet. Four main types of landforms exist in LRB: mountains, valleys, river terraces and alluvial fans (Dai et al., 2018; Wei et al., 2012). The river terrace and alluvial fan lands are easier to be utilised than mountain land because of the high altitude and severe ecological environment in LRB. Compared with alluvial fans, the river terrace lands have usually been fully utilised because they are usually located near rivers with gentle slopes and fertile soils. Therefore, the majority of towns and farmlands are located in river terraces. Alluvial fans are formed by sediments exiting a confined valley after flooding (Stock et al., 2008), forming a natural slope. The soil of several alluvial fans is less fertile due to repeated flooding and depositional processes (Bahrami and Ghahraman, 2019). Cropland and impervious areas accounted for 18.98% and 9.18% of the total area of alluvial fans in LRB (Table 3). Moreover, although the ratio between the grasslands of alluvial fans and LRB is 3.97%, the grassland area of alluvial fans can reach 790.57 km2. These characteristics reflect that the land for living and production is in shortage in LRB, and alluvial fans play an important role in the study areas. A dramatical gap exists between the land requirement of local people and the available land area in Lhasa, especially near the major towns and Lhasa River valley (Chu et al., 2010). However, the available land, especially river terrace land, is limited in LRB, in which river terraces have been adequately utilised. Therefore, alluvial fans can be an important land resource for future development. The rapid population growth and fast economic development in recent years intensified the importance of alluvial fans in the future (Wei et al., 2012).

4.2 Utilisation potential of alluvial fans

The distances between alluvial fans and road, human settlement and water systems are important factors that affect the utilisation potential of alluvial fans. Although building and maintaining roads can drive the growth of local economy and population aggregation (Ding et al., 2006), Tibet has the lowest transport network density among all the provinces of China (Jin et al., 2010), which indicates that road transportation networks are relatively sparse in LRB. However, 79.51% and 99.72% of alluvial fans are near primary or tertiary roads, usually within a 5-km distance (Figure 11). Thus, most of alluvial fans are easy to access. LRB has a typical semi-arid frigid climate (Lin et al., 2008), experiencing a high level of drought hazard (Chen et al., 2019). Thus, water resource is an important factor for regional development. A total of 61.86% of alluvial fans have a distance of 5 km from the river in this semi-arid basin (Figure 13). We thus inferred that the majority of alluvial fans are appropriate for agricultural or residential development from the water perspective. Several rivers are located between roads and alluvial fans. In this situation, rivers cause difficulty in the entry to alluvial fans. Several bridges can be built in the future to adequately use these alluvial fans.
The area of grassland and bare land in alluvial fans totals 802.59 km2, and the area of alluvial fans occupied by grassland accounts for no less than 50% (A50%grassland), reaching 903.17 km². Grassland and bare lands are less heavily used than cropland and imperious areas (Zhuang and Liu, 1997). The three types of alluvial fans, namely, A50%grassland, A50%bare land and A50%grassland A30%bare land, have utilisation potential with reasonable explorations. More attention should be given to alluvial fans located in the west and south of the LRB because numerous A50%grassland and A50%bare land are located there (Figure 8), near rivers in that region (Figure 1). Therefore, alluvial fans in Damshung and Chushur may have more potential in the future. Chenguan is an urban area of Lhasa city, which is the capital city of Tibet and is located in the south of LRB. The government residences of Tolung Dechen, Chushur and Taktse are near Chenguan. Therefore, the south of LRB, especially near Chenguan, should need more available land than another region. Alluvial fans in this region have an area of 243 km², with several of them having several bare lands. Considering that the land use dramatically intensified since the 1950s in the LRB (Li et al., 2017), alluvial fans in this region can be utilised adequately in the future.

5 Conclusions

In this study, we confirmed that alluvial fans are important land resources and have utilisation potential in RRB. The main conclusions are as follows: (1) A total of 826 alluvial fans with a total area of 1166.03 km²were detected in LRB. The area of the majority alluvial fans is less than 1 km2. However, large alluvial fans with an area of more than 10 km2 dominate the total area. (2) The distribution of alluvial fans is uniform in LRB. The alluvial fans are concentrated in the central and eastern parts of the LRB, whereas the area of alluvial fans is mainly distributed in the west of LRB, especially in Damshung county where the area of alluvial fans is 485.20 km2 (41.61% of total). (3) Alluvial fans provide essential materials and sites for local people and livestocks to survive. The area ratios of cropland and impervious area in alluvial fans were 2.16% and 4.52%, respectively, but the values in LRB were 18.98% and 9.18%. Meanwhile, the sites of grassland (68.70% of all alluvial fans) are most commonly seen in all land use types. (4) Three types of alluvial fans will have a great development potential in the future due to their considerable area and low land use degree. The first and second types of alluvial fans are occupied by grassland and bare land, being no less than 50%, respectively. The third type of alluvial fans occupied by grassland and bare land, being no less than 50% and 30%, respectively. Meanwhile, 93.70%, 53.63%, and 61.86% of the total number of alluvial fans are located within a 5-km distance to the tertiary roads, villages, and rivers. Therefore, alluvial fans have the potential to alleviate the land resource shortage in LRB.

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