基于DEM的黄土塬形态特征分析及发育阶段划分

Morphological characteristics and developmental stages of loess tablelands based on DEM

基于DEM的地形分析方法已被广泛应用于地貌学研究中,但当前研究以定性或半定量居多,利用各类参数对大面积区域的量化分析研究较少。本文以黄土高原为研究区,利用GIS及DEM地形分析方法,将坡度小于15°的正地貌提取出来,然后利用坡度与起伏度结合分析得到黄土塬顶面,两者相减得到黄土塬坡面,最终提取黄土塬1106个;选取黄土塬核心区的106个样本,分别计算坡面平均坡度、顶面长短轴比、沟壑密度、顶面与底面比、地面裂度5个指标,然后对其分别进行等级划分,计算5个指标的几何平均值,作为评价研究区黄土塬相对年龄的指标;对组合指标(I)进行分析后,将其划分为3级,分别为早期(I<1.74)、中期(1.74≤I<2.12)和晚期(I≥2.12)。研究结果显示,在黄土塬的不同发育阶段,各个形态指标整体上存在差异,各子类型内部也存在发育先后不一致的情况。在对黄土塬各子类型计算发育阶段指数之后发现,从黄土塬、黄土残塬到黄土梁塬,I值逐渐增大。形态指标的有序变化与前人研究所得的黄土地貌发育过程十分一致。

DEM-based terrain analysis has been widely used in geomorphology. Currently most researches are qualitative or semi-quantitative, whereas quantitative analyses of large areas based on a variety of parameters are few and far between, and most studies use artificial field measurement with a small scale in the aspect of the rela- tive age of the landscape entity. So, in this article, by using 30 m ASTER-GDEM, GIS and digital terrain analy- sis method, we extracted morphological parameters of a wide range of the areas of loess tablelands and deter- mined their relative ages. We tried to provide basic knowledge of multi scale representation of the loess table- lands and fine landforms, achieved quantitative descriptions of the similarities and differences among the loess tablelands, and calibrated their developmental stages. First, we extracted positive landforms less than 15° in the Loess Plateau. Then, we acquired the top surface of the loess tableland using two parameters： gradient and wavi- ness. We determined 1106 loess tablelands based on the subtraction between positive landform layer and the top surface layer. We selected 106 loess tablelands as samples to calculate average slope, axial ratio of top surface, gully density, top and bottom area ratio, percentage of negative terrain. The geometric mean of these five param- eters was used as an index to estimate the relative age of the loess tableland. The index （abbreviated toI） was cat- egorized into three levels： early development （≤1.74）, middle development （1.74≤I〈2.12） and late develop- ment （I≥ 2.12）. The results showed that： （1） the morphological parameters of loess tablelands are different from one another in different developmental stages; （2） developmental stages are also inconsistent for the subtypes; （3） transitional type exhibits the characteristics more similar to the type of the previous stage. After calculating I of each subtype, we found that from early to late stages of the loess tablelands, I value increases gradually, which is consistent with other researchers＇ conclusions on the developmental stages of loess landforms.