| With the rapid social and economic development,quick population growth and urbanization in Tibet,the demand for land is also increasing.However,the available land resources are very limited,and the land available for large-scale development and construction is basically concentrated in the valley areas,in which terraces and fluvial fans are already highly utilized,and mountainous areas are difficult to utilize because of the high slope.At present,the utilization degree of alluvial fans is relatively low,which has become a precious reserve land resource in Tibet and has great utilization potential.Enhance,clarifying the distribution,causes,land use,soil quality and agricultural suitability of alluvial fans in Tibet,is a theoretical support and scientific basis for the reasonable utilization and protection of alluvial fans.Therefore,this study takes the alluvial fan in Lhasa River basin as the research object,through remote sensing interpretation and field investigation,combined with laboratory test and Arc GIS analysis,the quantity,area and spatial distribution characteristics of alluvial fan in Lhasa River basin are defined.On this basis,the main influencing factors of alluvial fan development are determined by machine learning modeling,the temporal and spatial changes of land use of alluvial fan are analyzed,the soil quality of typical alluvial fan is evaluated,and the agricultural suitability of alluvial fan is evaluated,and the following results are obtained:1.There are 826 alluvial fans in Lhasa River Basin with the total area of 1166.03 km~2,the number of which is mainly concentrated in the middle and east of Lhasa River Basin,and its area is mainly concentrated in the west of Lhasa River Basin.Alluvial fans have different shapes.According to the connection between them and other alluvial fans,they can be divided into two types alluvial fans including independent and combined alluvial fan.According to their morphological characteristics,these two types of alluvial fans can be divided into three types:independent alluvial fans can be divided into fan-shaped,semi-fan-shaped,and trapezoidal alluvial fans;the combined alluvial fans can be divided into three types:fan-top-connected,fan-edge-connected and series-connected alluvial fan.2.The geomorphic factors of catchment area are the most important factors affecting the development of alluvial fans in Lhasa River Basin.The factors of geomorphic factors of catchment area include catchment area,slope,sunny slope area,half-sunny slope area,drainage density,shape coefficient and relief ratio,etc.The sum of relative feature importance value in the established optimal machine learning model can reach 74.60%.3.The alluvial fan provides more cultivated land and artificial surface for Lhasa River basin.In 2000,2010 and 2020,the alluvial fan provided 17.72%,21.84%and 24.17%of cultivated land and 7.89%,7.51%and 25.24%of artificial land for Lhasa River Basin,respectively.Those increasing area main coming from the region at altitudes of≤4 000 m or at slope of≤8°,and the cultivated land and artificial surface tended to expand to the regions with higher altitude and larger slope.4.There is no significant difference in soil physiochemical properties and soil quality index(SQI)between alluvial fan and river terrace farmland.The soil quality of alluvial fan is affected by vegetation coverage,land use types,annual average precipitation,altitude and annual average temperature.5.There are 420 suitable agricultural alluvial fans with an area of 683.58 km~2,mainly distributed in the middle and lower reaches of Lhasa River,including Lhundup,Medro Gongkar,Taktse and Tolung Dechen Counties.Under the scenario of comprehensive development of facility agriculture,the suitable level of alluvial fan can be increased to1140.63 km~2;Under the condition that all alluvial fans were passed through the roads,the suitable alluvial fan can be increased to 744.46 km~2.Agricultural suitable,semi-suitable and unsuitable alluvial fans correspond to suitable tillage,semi-suitable tillage and suitable grazing alluvial fans.Grazing alluvial fans are mainly distributed in the upper reaches of Lhasa River,including Chali and Seni Counties.The alluvial fans suitable for tillage and semi-tillage are distributed in the lower reaches of Lhasa River,including Chengguan,Taktse,Tolung Dechen,Chushur,Medro Gongkar,Lhundup and Dumshung Counties. |
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