| The total area of deserts in China is more than one tenth of the total area of the country,thus,the study of aeolian geomorphology is an important part of geomorphology research in China.Sand dune morphology and its evolution is one of the major parts of aeolian geomorphology research.The formation mechanism of different types of sand dunes has been relatively well studied,but the research on the factors influencing dune height growth,the formation and limitation of the maximum dune height is still insufficient.The Badain Jaran Sand Sea is considered to have the tallest dune in the world,and about 110 perennial lakes locate in the interdunal area of the mega-dunes.Various hypotheses have been proposed about the causes of these mega-dunes,each one partially explains the formation mechanism of the mega-dunes from different perspectives,but a unified understanding has not yet been formed,especially for the special spatial distribution pattern of the tallest dunes concentrated in the lake group area.Some researchers have recognized that mega-dune formation may be related to moisture conditions,but the understanding of the mechanisms and pathways of lake-dune interactions is still highly controversial.Numerical simulation is an important way to study dune evolution,but existing models are mostly focused on the relationship between the formation of different types of dunes and the related wind regimes and sand supply.The evolution of dune height is not sufficiently studied,and there are few models that can both simulate the evolution of complex dune fields and provide spatial and temporal linkages to real dunes.Models which have a real time and length are either too complex,which will bring unnecessary complexity to the dune height evolution researches,or too simple to simulate a complex dune field.So,a suitable model for dune height evolution remains to be developed.In terms of the formation mechanism of the existing mega-dunes,we developed a dune height and slope algorithm based on digital elevation model(DEM),and obtained the maximum dune heights of 32 major sand seas worldwide.Then,we focused on the spatial distribution of dune heights and slopes in the Badain Jaran Sand Sea.Based on the high-resolution images from Google Earth,the location and area of the lakes in the Badian Jaran Sand Sea were obtained.Based on the in situ meteorological observation of the Badain Jaran Sand Sea and the shallow water content samples at different time and in different heights,the spatial and temporal distribution of shallow water content in the lake group area of the Badain Jaran Sand Sea were discovered and its influencing factors were studied.Combining previous studies on deeper water content and water isotope results,a new viewpoint of lake-dune water interaction is proposed,and the spatial distribution of lake area,dune height and dune slope is combined to propose a "lake-dune" pattern model to explain the coexist of the tallest dunes in the lake group area.In order to further investigate the evolution pattern and growth limitation of dune height,a two-dimensional cellular automata model was developed with reference to the Werner model,that can well reproduce the two-dimensional dune evolution process under different wind conditions and has advantages in matching spatial and temporal scales to the reality.The main conclusions obtained in this study are as follows.(1)In the case of coexistence of lakes and dunes,the presence and the size of lakes have significant influence on the relative humidity of air within the lake basin,and the water vapor in the air can exchange with water in the shallow layer of dunes within the lake basin in the form of non-rainfall water,increasing the water content of shallow layer of dunes,but the depth and magnitude of this influence need to be further studied.(2)The coexistence of lakes and dunes in the Badain Jaran Sand Sea is conducive to the increase of its dune heights.The long-term coexistence of lakes and dunes maintains the influence of lakes on the shallow water content of dunes in the lake basin.The increasing of water content of shallow sand layer causes the increase of dune slope and the slowdown of dune movement,the former increases the height of dunes while the dune volumes remain unchanged,and the latter increases the ability of dunes to capture aeolian sand,i.e.,increases the volume of dunes.This mechanism is associated with the fact that the maximum dune height in the Badain Jaran Sand Sea is among the highest in the world's major deserts.(3)The study of the dune height change during the evolution of dunes conducted by the cellular automata model shows that the evolution of the dune field gradually tends to stabilize over time,when the acceleration rate of dune height and the deceleration rate of dune height-space ratio gradually slow down.At this quasi-steady state,the dune heights and the dune height-space ratios are linearly related to the equivalent sand thickness of the initial dune fields.The underlying topographic barrier facilitates aeolian sand trapping,but the final height of the topography after the dune overtops the barrier does not exceed the height of the barrier.When the dune is in the process of overtopping the barrier,the underlying terrain has a significant effect on increasing the dune height,in the way equivalent to increasing the amount of immovable sand inside the dune.Based on the spatial distribution of dune heights in the Badain Jaran Sand Sea,shallow water content observations and meteorological observations,this study provided a new perspective on the water circulation between lakes and dunes.The dune growth model with "lake-dune" pattern proposed in this study may be an important factor affecting dune height that has been neglected before,which contributes to the final solution of the formation of the mega-dunes in the Badain Jaran Sand Sea.The two-dimensional dune evolution model established in this study is simple and effective,and this model has outstanding advantages in matching spatial and temporal scales to the reality,and its three-dimensional extension is worth expecting. |
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