2.4 Data processing and analysis
2.4.1 The classification of alluvial fans.
The quantity of alluvial fans was large, and the area had lots of sizes in our study area. In order to know the distribution of them, they were classified into five sizes by the area of alluvial fans, i.e., 1st: area≤0.1 km², 2nd: 0.1<area≤1 km², 3rd:1<area≤10 km², 4th: area>10 km². And the quantity of all the sizes was counted.
2.4.2 Indicators for the land use
There were many types of land use in alluvial fans, for example cropland, forest, grassland, shrubland, wetland, water body, tundra, impervious area and bare land. In order to reflect the land use characteristics of alluvial fans in LRB, the two indicators ofRlaf and Rllrb were used.Rlaf was the ratio between the area of a land-use types in alluvial fans and the total area of alluvial fans.Rllrb was 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% (1)
Where Ali (km²) was the area of land use iof alluvial fans; and i= cropland, forest, grassland…;Aa (km²) was the total area of alluvial fans.
Rllrb = Ali /Alrbi ·100% (2)
Where Ali (km²) was the area of land use iof alluvial fans, and i= cropland, forest, grassland…;Alrbi (km²) was the area of land use i of LRB.
Meanwhile, in order to show the spatial distribution of land-use types in alluvial fans, the alluvial fans are classified into 2 types by the area of land-use type. The first type is the alluvial fans which area are occupied as one land use i not less than 50% (A50%i ), and i= cropland, forest, grassland…. The second type is the A50%i which area are occupied as another land use j not less than 30% (A50% i A30% j ), andj= cropland, forest, grassland….
2.4.3 Indicators for utilization potential.
The utilization potential is mainly depending on the distance between an area and road, place and waterway. The centroids of all alluvial fans were calculated in ArcGIS and used to analysis the distance between alluvial fan and road, place and waterway respectively. According to the fact of road in LRB and the needs of our study, the primary road, secondary road, tertiary road, county, town and village were chosen to analyses the distances. For the town and village, the places of their government departments were used to get the distance. Meanwhile, considering the background of water shortage in semi-arid monsoon climate region, all kinds of waterways are important for this basin. So, the data of stream with consistent low were added to the river data for analyzing.
Another two ratio indicators including Rq andRa are used to show the utilization potential of alluvial fans. Rq is the ratio between the quantity of alluvial fans that their distance to road, place and river are not more than some space and the total quantity of alluvial fans in LRB. Ra is the ratio between area of alluvial fans that their distance to road, place and river not more than some space and total area of alluvial fans.
Rq =Qan /Q ·100% (3)
Where Qan is the quantity of alluvial fans not more than n (km) to road, place and river, and n= 1, 5, 10, 20, 40; Q is the total quantity of alluvial fans.
Ra = Aan /Aa ·100% (4)
Where Aan (km²) is the area of alluvial fans not more than n (km) to road, place and river, and n= 1, 5, 10, 20, 40; Aa (km²) is the total area of alluvial fans in LRB.
All figures are carried out with ArcGIS10.7 and Origin 8.5 software.