Analysis of soil erosion and karst rocky desertification in the Mahuangtian small watershed based on RapidEye remote sensing images

The Mahuangtian small watershed, located at Huajiang town of Guanling county in Guizhou Province, covers an area of 16.51 km2which extends about 5.78 km in north-south and 6.63 km in east-west directions, respectively. As a developed karst region where limestone outcrops widely, its karst area is 12.68 km2while the rest is 3.83 km2. Based on field spectral reflectance measurements in 22 study regions by FieldSpec@3 spectroradiometer, spectral features of typical ground objects (e.g. rock, dry soil, wet soil, forest, grassland, crop, water) were analyzed and a linear distribution of soil and rock spectral was found in the red-nir dimensional feature space. We defined the linear relationship as the rock-soil index and further derived the formula of this index to get the exposed bedrock fraction of the karst area using remote sensing images. According to Techniques standard for comprehensive control of soil erosion and water loss in the karst region (SL461-2009), the technical process was designed to evaluate the soil erosion (SE for short) and karst rocky desertification(KRD for short) in the karst area. Applying this method, we carried out the intensity classification of SE and KRD on the basis of index factors, such as land use, vegetation coverage, exposed bedrock fractions and gradients, generated from RapidEye images, geographical map (1∶200,000) and topographic map (1∶10,000). Results demonstrate that the correct ratios of SE and KRD are respectively 86% and 89%, which can satisfy the specification requirement. By analyzing the interpretation, the SE area is 857.27 hm2, accounting for 51.92% of the research region. Among them, the area of SE in the karst is 695.60 hm2 which accounts for a higher proportion of 54.85% in the Mahuangtian watershed while the non-karst area is 161.67 hm2 which accounts for 42.23% and has a major part of the severe SE. For the 1,268.30 hm2 KRD area, the constituent ratio decreases by 38.89% of inconspicuous KRD, 31.51% of potential KRD, 16.73% of slight KRD, 10.21% of moderate KRD and 2.67% of intense KRD. And their areas are, in order, 493.26 hm2, 399.58 hm2, 212.20 hm2, 129.44 hm2 and 33.83 hm2. Obviously, the slight and moderate KRD are predominant in this karst region. In addition, our study indicates that the SE is inversely associated with vegetation coverage but has no common bond with gradients and exposed bedrock fractions in this area. And the KRD is negatively related with vegetation coverage and positively correlated with gradients and the rate of exposed bedrock.