Study on spatial distribution characteristics and influencing factors of karst depressions in Shilin county, Yunnan Province

It is of significance to study the distribution and the factors of karst impact on the development of karst depression in the fields of geomorphology and environmental science. This paper takes Shilin county with abundant karst depressions as the study area, which is in subtropical monsoon climate zone, the south of the karst plateau in eastern Yunnan Province. The area is sandwiched between the Shizong-Mile fault and the west branch of Xiaojiang fault, and at the transition zone between the southeastern margin of Qinghai-Tibet plateau and the Beibuwan hilly plain.Based on digital elevation model (DEM), the methods of Ripley’s K function, nuclear density, buffer analysis, and spatial superposition in ArcGIS are used to analyze the distribution characteristics of the depressions and the influence of geographical environment on them. The results show that, (1) The distribution of karst depression in study area presents an aggregate pattern on a scale of 8 km, and the area can be divided into three sub-areas, namely high-density(>6?km-2), medium-density (5~6?km-2) and low-density (<5?km-2) areas. The high-density area has a total area of 308 km2, which is concentrated in two areas, namely Weihei, Muzhujing, Yusheng, Nuoyi in the east, and Hemo station and Shilin town in the northwest, with an average depression density of 7.20?km-2. The medium-density area is in the surroundings of the high-density area, including Yinai, Douhei, Southern Haiyi, and Northern Weize, with an average density of 5.74?km-2; and the low-density area is mainly at the edge of county, with an average density of 4.16?km-2. From the aspect of morphological parameters of the depression individuals, perimeter and area of them are high-density area < medium-density area < low-density area; depth and side slope are high-density area ≈ medium-density area > low-density area. (2) The spatial distribution of the depressions is controlled by the factors such as topography, geomorphology and geology. They have different quantity and density at different locations in elevation, relief, slope, rock type and fault buffer. At the altitude of 1,850~1,950 m, the maximum number of depressions is 37.1%,while the slope of 6°~12° accounts for the most as 82.0%, the relief of 100~150 m is 31.3%, and the landform classification of karst hilly zone is 34.3%. In terms of rock type, the depression density in the Quaternary deposit area is 5.8?km-2, and in dolomite area is 5.6?km-2, which are both dominant distributions due to lithostratigraphy. There are differences among the density in the buffer zone of three fault groups, with similar trend of density variation at each fault group. Tectonic joints and fractures developed in the Devonian, Carboniferous and Permian strata and the Jiuxiang, Weize and Guishan faults in study area strongly control the development of karst depressions along their long axis directions.(3) The spatial variation of the depression density is well correlated with two comprehensive indicators of regional heterogeneous structure of potential hydrological connectivity (IC) and hypsometric integral (HI). As the IC value increases, the depression density decreases, showing a significant negative correlation (R2=0.81). While with the increase of HI value, the depression density gradually increases, showing a significant positive correlation (R2=0.90).The spatial distribution of karst depressions is affected by geographical factors, and IC and HI can comprehensively reflect the distribution characteristics of depressions in study area, however, whether this law is universal still need further study.