Space-for-Time Substitution And Gully Evolution In The Chinese Loess Plateau

Analogy analysis is one of the important means of geomorphological research.Not only the analogy between landforms of different regions with similar external conditions,but also between similar external conditions indicated by the same landform types provide effective means for understanding the laws of geomorphological development.Differently,space-for-time substitution in geomorphology is a method of mutual succession and prediction in the development trend of time and space sequences.With the advancement of earth observation technology and the enrichment of data acquisition methods,scholars have been working on geomorphic evolution of different landform types and proposing geomorphic evolution models using space-for-time substitution.However,as a research method that has been initially recognized by geomorphologists,there are still many scientific problems that need to be further studied.We still do not clearly understand the spatial-temporal mechanisms of landform evolution,application conditions,impact factors as well as basic research paradigms of space-for-time substitution.Thus,space-for-time substitution is currently not exhaustive or systematic enough to become a basic theory of geomorphology.In addition,due to the distribution limitation of the study area,the space-for-time substitution method of geomorphic evolution is usually applied at a small spatial scale,and its correctness at a large scale needs validation.Furthermore,geomorphological evolution models obtained from space-for-time substitution researches are of qualitative conception,generally lacking evaluation process.Firstly,this paper systematically discusses the basic concepts of theories and the framework of methods involved in the research.Secondly,the framework of the theory and method of space-for-time substitution in geomorphology are explained in detail,including the basic concepts,principles,application conditions and different types.The basic paradigm of geomorphological evolution research when applying space-for-time substitution in geomorphology is discussed as well.Based on the digital terrain analysis method,the multi-scale typical watershed of the Loess Plateau is used as the experimental sample area,with the high-precision multi-scale digital elevation model(DEM)acting as the basic information source.Finally,the innovative method of geomorphological evolution of the loess gully based on space-for-time substitution in geomorphology is studied and a new understanding of the geomorphological evolution model of the loess landform is obtained.The main contents and conclusions of the thesis are listed as follows:1.The study has established a research framework for the theory and methodology of space-for-time substitution in geomorphology,including basic concepts,principles,application conditions and type division.The basic paradigm for geomorphological evolution studies applying space-for-time substitution in geomorphology is then discussed,which provides a methodological basis for subsequent research.2.The gully network of the study sample area is extracted as the analysis level and the object level with different thresholds of flow accumulation,and the main gully length is employed as the ergodic indicator to select 20 small watersheds from the Beiluo River Basin and 10 from the Qiyuangou Basin respectively.By calculating and analyzing selected indexes,i.e.,gully density,hypsometric integral,surface roughness and the ratio of main gully and branch,the loess watersheds with different lengths of main gullies can reflect the developmental evolution of the loess watershed in time on the regional scale,indicating temporal and spatial succession characteristics.The temporal and spatial succession characteristics of the loess watershed are more obvious on a smaller regional scale.The larger the spatial scale of the study area,the less obvious the temporal and spatial succession characteristics.3.Taking the gully evolution process during the development of the loess watershed as the research object,the cross-section of the gully as the entry point,main cross-sections from different spatial positions of the gully are extracted.The height-to-width ratio of the gully is calculated for the analysis of gully horizontal variation characteristics.On the basis,the results are compared with the existing river evolution model and the simulated small watersheds under the artificial rainfall.It is found that the time and space succession processes can replace each other,thereby confirming the existence of the space-for-time substitution and providing new evidence for the space-for-time substitution in geomorphology.4.This paper introduces the special gully morphology of the spoon-like gully,including morphological characteristics,confluence relations,regional differences and erosion processes.Selecting the channel length as the ergodic indicator,eight spoon-like gullies from two watersheds distributed in parallel on the slope are selected as the research object based on the high-precision remote sensing image and DEM.Then,the relevant morphological and erosive indexes of the eight spoon-like gullies are calculated.By analyzing the variation characteristics and comparing with the existing gully evolution model,the correctness of the space-for-time substitution in geomorphology applied to the development of the spoon-like gully is determined.Finally,combined with the field investigation,the development model of the loess spoon-like gully is proposed based on the space-for-time substitution in geomorphology.That is from the beginning of the loess tunnel,the length,width and depth of the spoon-like gully are continuously increasing after the water erosion,gravity collapse and erosive action,resulting in its final connection with the main gully.In conclusion,the loess spoon-like gully can be considered as a special initial form of bank gully.The innovations of the paper are summarized as:The theory and method framework of geomorphology evolution based on space-for-time substitution in geomorphology is introduced for the first time,including basic concepts,geomorphological foundations,application conditions and type divisions.On the basis,the corresponding research paradigms are proposed.The results of these studies have positive significance for a deeper understanding of the laws of geomorphological development and evolution,and is an innovative exploration of geomorphological theories and analytical methods.The geomorphological evolution of the Loess Plateau based on space-for-time substitution in geomorphology is a new perspective to examine the geomorphic features and developmental evolution of the Loess Plateau.The development and evolution of the geomorphology of the loess watershed is of great significance for revealing the external manifestation and internal mechanism of the formation of the Loess Plateau.