Spatial distribution and characteristics of the gullies in Caijiachuan watershed in loess region of Western Shanxi Province, China

Abstract: A gully is one of the erosion geomorphic types in the Loess Plateau. Taking the Caijiachuan watershed in Ji County, the loess region of Western Shanxi in China as the object, this study aims to determine the spatial distribution and characteristics of cutting ditches using the QuickBird images and topographic mapping. A field investigation of 40 gullies was conducted to translate the 370 gullies in the watershed. An analysis was also made on the effects of topographic factors (such as slope, aspect, and elevation) on the spatial differentiation of the gully. The number, density, and frequency of erosion gullies were then calculated on the different topographic factors. The spatial variations of morphological and development indicators were determined, such as the erosion gullies area, length, morphological elements, gradient, Horizontal Proximity Index (HPI), Vertical Proximity Index (VPI), and Proximity Index of Variation (PIV). The main results are as follows: 1) The kernel density of the gully reached more than 8.23 in the northeast and southeast of the watershed, indicating a high concentration area. The kernel density of the gully was 4.11-8.23 in the middle dense area, which was located in the upper-middle reaches of the watershed; 2) The largest erosion was found in the gullies at the slope of 15-25°. The erosion gullies presented the maximum density and frequency when the slope range was between 5-15°. As such, the slope range of 5-25° was the most serious erosion for the drainage channel. According to the average of PIV, the vertical proximity index of the erosion gullies on the slope was greater than the horizontal proximity index as the slope level between 0-5°, 15-25°, 25-35°, 35-45°and more than 45°. The main type of erosion was the undercutting in these slope levels. Moreover, there were similar morphological characteristics of gullies on the slope of 25°-45°. 3) There were 66 erosion gullies in the northeast, which was the largest number in the study area. A minimum, 34 erosion gullies were located in the south. The maximum density of erosion gullies was 3.03 km/km2 in the north, whereas, the minimum density was 0.87 km/km2 in the east. The frequency of erosion gullies was significantly greater on the west side of the boundary line of 67.5°-247.50° in the slope direction than that on the east side. There were similar morphological characteristics of each gully on the west slope. 4) The altitude between 1 000 to 1 200 m was the most serious erosion of gully, indicating the highest density and frequency of erosion gullies. The PIV gradually approached zero, as the altitude increased from the altitude level of 1 000-1 100 m, but there were the negative average PIVs of all the altitude levels, where the main erosion type was undercutting. There were also similar morphological characteristics of the trenches below 1 100 m. Therefore, the key features were revealed in the mutual topography for the erosion gullies in the watershed. The finding can also provide a strong basis to manage the spatial distribution of erosion gullies in the small watersheds of the Loess areas.