Dynamic Trans-scale Model To The Temporal-spatial Evolution Of Aeolian Dune Field And Simulation Research

Desertification or sand desertification has been becoming one of the severe environmental problems in China, particularly in Northwestern China. It is of a very important and practical meaning to quantitative predicting the spreading rate of desert boundary and evaluating the effectiveness of related prevention strategies. The evolution process of Aeolian dune field is a typical issue involving multiple temporal and spatial scales which is still out of accurate description through current field observation on earth and other planets like Mars or successful simulation with existing numerical models. Therefore, to establish an available method aiming for the evolution process of aeolian dune field, as an exploratory probe into multi-scale science, is of an urgent necessary and significant scientific value.This dissertation put forward a universal scale-coupled model concerning aeolian dune fields. This model incorporates underlying physical processes including the collision and saltation of sand particles, the formation and development of wind-blown sand flow, and the evolution of sand dune fields as well as the coupling among them which spans three specific tempo-spatial scales ranging 8-9 orders of magnitude. It therefore is suitable for various incoming wind and surface conditions. The main highlights of innovation in this dissertation are:1) to establish a new mode for calculating local wind velocity fields around sand dunes so as to obtain a more reasonable and realistic description to the variation of wind velocity raised by the change of surface morphology in a dune field and 2) to introduce a meso-scale "sand body element" which is much smaller than the scale of dune fields in macro but containing micro information on the movement of sufficient number of sand particles, for the first time put forward two new statistic quantities in the study of wind-blown sand movements, i.e. the coverage factor and transportation factor of "sand body element", and therefore build a "bridge" between macro quantities such as the thickness and transportation length within a given period of "sand body element" and micro quantities describing the erosion and deposition of sand surface, rebound and ejection of sand particles including the average velocity particles impacting sand bed, saltation length and time etc. The novel model proposed in this dissertation made a quantitative simulation of the whole evolution process of an aeolian dune field with an area of hundreds of square kilometers over one century.Based on the novel model, this dissertation successfully reproduced the whole formation and evolution processes of aeolian dune fields under the action of single and multiple prevailing wind directions. The simulated achieved a qualitative agreement in dune pattern and a quantitative correspondence in changing law of dunes' migration speed with dune height and that of dune height with thickness of sand supply comparing with field observations, and further revealed quantitative laws on the influence of incoming wind velocity, kinds of prevailing wind directions, thickness of sand supply and sand diameter to the evolution of dune fields, dune patterns, and collision behaviors of sand dunes. Results presented 1) the threshold thickness of sand supply determining dune patterns; 2) fitting formula governing the dune height and migration speed varying with sand diameters and incoming wind velocities; and 3) a probability distribution function of dune size etc. By extending this model to describe the evolution and spreading of dune field under various surface conditions, it can realize 1) the quantitative differences between barchans dunes on earth and Mars in evolution processes, dune patterns and migration speeds etc. and theoretical evidence for the possibility of forming dunes under the action of sand storms taking place on Mars under the contemporary atmospheric conditions; 2) boundary of desert and the quantitative laws of its spreading rates changing with wind velocities, sand diameters, vegetation coverage, laying manners of straw checkerboard barriers and grassland degradation rates etc.; 3) a fitting formula describing the grassland degradation intensity varying with overgrazing sheep units per area and grazing area. The results given out in this dissertation provided reliable quantitative analysis methods and theoretical evidences for predicting the spreading rate of the boundary of deserts, designing related prevention strategies and evaluating their efficiency.