基于DEM的黄土地貌逼近度因子构建及特征分析

沟谷源点作为沟沿线上最为活跃的部位,其分别到上游分水线、下游沟谷线的流线空间比对关系,是表征三线空间结构的重要突破口。它在空间上向流域分水线逼近的程度,是量化黄土流域地貌系统发育程度的重要切入点。为探究黄土地貌区流域沟谷源点向流域分水线逼近的程度,揭示黄土流域地貌发育进程及该进程所表现的主要侵蚀方式,本文从水平和垂直2个维度,构建量化三线空间结构关系的核心因子--逼近度(PI),其中包括水平逼近度(HPI)和垂直逼近度(VPI),基于5 m分辨率数字高程模型,在陕北黄土高原遴选了包含16种地貌类型的42个样区,利用数字地形分析方法,探讨其平均值(MHPI, MVPI)的空间分异规律。另外,选择其中南北序列分别代表黄土塬、残塬、梁状丘陵沟壑和峁状丘陵沟壑区的淳化、宜君、甘泉和绥德4个地区,完备包含一至五级别沟谷的典型流域为重点实验样区,探讨陕北黄土高原流域尺度平均逼近度变异指数(MPIV)序列性特征。实验结果表明:①陕北黄土高原平均逼近度(MPI)存在强烈的空间自相关,MHPI在南北序列上先增大后减小,东西方向上逐渐减小,在黄河沿岸峡谷丘陵区MHPI达到最大;MVPI由西南-东北先减小后增加,由西北-东南逐渐减小,在渭北黄土台塬区达到最小;②在流域尺度上,MPIV值的正负与黄土塬区、丘陵沟壑区敏感相关;③4个重点样区MHPI、MVPI与其它地形因子存在南北序列上的一致性。104个外部汇流区平均水平逼近度与平均坡度相关性较好(P=0.43, a<0.001),平均垂直逼近度与面积高程积分强烈相关(P=0.75, a<0.001)。平均逼近度指标综合考量了黄土高原地区最典型的三条具有结构控制意义特征线的空间关系,对黄土地貌的发育程度有明显的指示性意义。

Establishment and Feature Analysis of Loess Geomorphology Proximity Indexes based on DEM

The gully source point is the most active part on the shoulder line. The spatial length of streamlines from the gully source point to the upstream watershed boundary line and the downstream gully line is an important identifications for spatial structure of the loess shoulder line, watershed boundary line and gully line. Proximity degree of gully source point to watershed boundary line spatially is a key point for quantifying the geomorphological development process in loess basin. To explore the loess landform areas watershed gully source point to approach watershed boundary line, reveal the loess watershed landscape development process and the method of main erosion processes. In this paper, A core factor of quantifying the three line spatial structural: Proximity Index (PI) was established from horizontal and vertical dimensions, including Horizontal Proximity Index (HPI) and Vertical Proximity Index(VPI). 42 study sites of 16 geomorphic types in the Loess Plateau of northern Shaanxi based on the digital elevation model data with 5 m resolution were selected and the spatial differentiation of their mean values (MHPI, MVPI) in the Loess Plateau of northern Shaanxi was discussed by using of the digital terrain analysis method. Besides, 4 typical watersheds including 5 whole gully levels were selected from 42 sites as the key study areas in Chunhua, Yijun, Ganquan, and Suide which located in loess tableland, loess residual tableland, loess ridge, and loess hill respectively in the north-south sequence. The Mean Proximity Index Variability (MPIV) of key study sites was calculated in the watershed scale. Results show that: ① There is a strong spatial autocorrelation of the Mean Proximity Index(MPI) in the Loess Plateau of northern Shaanxi. MHPI presents a trend of increasing firstly and then decreasing in the north-south sequence, and gradually decreasing in the east-west direction. MHPI reaches the maximum in the valley hilly area along the Yellow River. MHPI decreases firstly and then increases from southwest to northeast, and gradually decreases from northwest to southeast, and reaches the minimum in the Loess tableland area of Weibei region; ② The positive and negative MHPI values in watershed scale are sensitively related to loess tableland and hilly gully regions; ③ MHPI and MVPI of the 4 key sample areas are consistent with other terrain indexes in the north-south sequence. The MHPI of 104 external confluence region has a good correlation with the average slope (P=0.43, a<0.001),while the MVPI has a strong correlation with the hypsometric integral (P=0.75, a<0.001).The Mean Proximity Indexes (MPI) comprehensively take into consideration the spatial relationship of the three typical structural control lines in the Loess Plateau and have obvious indication significance for the development degree of the loess landform.