Underlying Paleotopography-constrained Loess Landform Evolution Modelling

Landform is the general term for various natural or artificial feature of the solid surface on the earth's surface.Geomorphology is a discipline that studies the structural form,formation mechanisms and spatial patterns of earth's surface.Landform evolution is a long-term and continuous geological process so that an important research method for studying landform evolution is to use landform evolution models to simulate the process of geomorphic evolution.In the early geomorphological evolution research,the qualitative description of the geomorphic evolution process was mainly through the related geological theoretical models.With the continuous development of computer technology,some quantitative landform evolution models gradually emerged.Numerical landform evolution model is to describe the landform evolution process by mathematical and physical methods,so as to simulate the transformation process of the surface morphology under the effect of various forces.Although the numerical landform evolution model has been developed,the use of the evolution model to simulate the landform evolution process still faces a problem,that is,the historical topography in the sample area is difficult to obtain.Therefore,this kind of landform evolution model is unable to achieve good evolution simulation results.During the landform evolution process of the loess area,loess dust gradually deposits on the original underlying topography,forming the present loess landform.The morphological structure and distribution characteristics of the underlying topography have a significant control effect on the present loess landform.The loess layer and the underlying topography have distinct physical and chemical properties.This special geological structure makes it possible to identify and reconstruct the underlying topography using geophysical prospecting technology.This thesis combines the study of loess geomorphological inheritance with the study of landform evolution modeling,comprehensively considers the different driving factors of the loess landform evolution process,analyzes the control effect of the underlying topography on the loess landform evolution,and embeds the underlying topographic features of the loess landform.Based on this,a landform evolution model suitable for loess regions has constructed.The contents and conclusions of this thesis are as follows:(1)Using scientific theories and methods,the underlying topography of the sample area was reconstructed and used as input data to be fused into the landform evolution model.Secondly,different modules that conform to the loess landform evolution process were selected and the parameters were set.In addition,in order to scientifically simulate the loess landform evolution process,the loess dust deposition module was designed based on the loess plateau environmental data.At last,this thesis realizes the simulation of the loess landform evolution and analyzes the characteristics of the loess morphology in different evolution stages.(2)This thesis designed two landform evolution process simulations that take into account the control of the underlying topography and when there is no underlying topographic control effect.The results show that when considering the control effect of the underlying topography,the evolution result can not only retain the main gully morphology and structure of the underlying topography,but also develop some new landform features.Without the control of the underlying topography,the landform evolution process can only represent a typical trend of the loess landform evolution due to the lack of preservation of regional characteristics.(3)The landform evolution model is composed of different modules and different input parameters.Therefore,the combination of different module parameters will bring uncertainty to the evolution process.From the perspective of model uncertainty and data uncertainty,this thesis constructs the evaluation index system of loess landform evolution model.The model uncertainty mainly includes the uncertainty of the evolution result caused by the parameter setting of the evolution module.During the loess landform evolution process,the change of the rate of loess deposition of loess has a greater impact on the evolution process than the rate of loess deposition remains stable.In the erosion process,the greater the erosion intensity,the more obvious the erosion effect.Similarly,precipitation frequency will also affect the intensity of the erosion effect,but the number of precipitation events have a certain randomness.The data uncertainty refers to the uncertainty impact of the reconstructed underlying topography on the evolution result under different data resolutions.According to the comparative analysis of this thesis,with the support of 5 m resolution of the underlying topography data,the loess landform evolution model can obtain good landform evolution results.This thesis simulates the loess landform evolution process through numerical evolution model,and explores the characteristics and evolution mechanism of the loess landform process.The theory and method of modeling and simulating the loess landform evolution constrained by underlying topographic features is a useful and innovative exploration for the study of loess landform formation and evolution mechanisms.