Analysis Of Watersheds Structure Characters In The Loess Plateau Based On Digital Elevation Models

China's loess plateau region has thousands of hills and gullies,which formed the gully landscapes and shown various morphology.The land surface region can be decomposed into a series of watershed units with a hierarchical organizational structure.For loess landform,the watershed is a basic spatial-structural unit that can express natural landforms,surface morphology characteristics,spatial organization and developmental evolution.Meanwhile,gully erosion is the main type of soil erosion in loess plateau region,and the different erosion effect is also the main reason in forming the current watershed morphology.The systematic study for watershed shape,watershed structure and terrain feature and discussing evolution process of watershed structure both have important theoretical significance and application prospects for verifying loess plateau spatial pattern,revealing the formation and evolution mechanism of the gully geomorphic morphology and guiding the ecological restoration of loess plateau and region sustainable development.This research took the complete watershed of northern Shaanxi loess plateau region as the research object.Based on high resolution DEM data,the watershed segmentation,scale characteristics of watershed structure and node calibration model were further studied by using the relevant theory and technical methods such as digital terrain analysis,watershed geomorphology,hydrology and mathematical statistics.The main work of this paper is described as follows:(1)Firstly,flow direction matrix of watershed was extracted,and the whole watershed region was divided according to the flow direction of the grid cell.The transfer law of flow accumulation between grid elements was analyzed,and the branch watersheds contained in the complete watershed were segmented.Then,the segmentation results were calibrated according to hierarchy law of gully structure.The results of watershed segmentation and hierarchical calibration realized the spatial membership relationship among sub-watershed,which provided the research foundation for analyzing watershed structure information and the construction of watershed node model.(2)The research selected indices of gully bifurcation ratio and fractal dimension as characteristic parameters of watershed structure information.Then,the change rule of the characteristic parameters of gully structure with different scales was analyzed by setting the scale sequence of analysis at the extraction gully and data scale sequence.The gully structure of the watershed changed only with variations in the analysis scale,and the change rule was obvious when the gully level changed.Finally,the change rule of the characteristic parameters of the gully structure at different areas was analyzed.The gully fractal dimension showed a significant numerical difference in different areas,whereas the variation of the gully branch ratio was small.The change rule indicated that the development degree of the gully obviously varied in different regions,but the morphological structure was basically similar.(3)We calculated the quantitative indices,such as elevation,flow accumulation and hypsometric integral and found there are remarkably closer linear correlation between flow accumulation and elevation with increasing gully order,and the same variation tendency of hypsometric integral also presented by establishing a watershed hierarchical structure model that relies on gully structure feature points.Then,the regular characteristics of hierarchical structure and spatial differentiation of watershed structural units in loess plateau region were summarized,and the evolutionary characteristics of watershed structural units were discussed from the view of the relationship between whole and part.