沙丘形态及表沙粒度特征对风况和地表植被变化的响应

以毛乌素沙地3种沙丘（新月形沙丘、抛物线形沙丘和反向沙丘）为研究对象，对其形态、表沙粒度特征和区域风况进行了量化分析，探讨了沙丘表沙物理运动过程及其形态对外界条件（风况和地表植被）变化的反馈，揭示了沙丘表沙粒度特征对不同沙丘形态的响应机制。结果表明：新月形沙丘表沙平均粒径由迎风坡底部向顶部逐渐变小，分选呈现逐渐变好趋势， 但粒径较小和分选较差的表沙样出现在沙丘迎风坡中部。随着地表植被覆盖度的增加，新月形沙丘逐渐向抛物线形沙丘转变，近地表输沙能力和沙丘上风向沙源的供应同样受到限制，致使抛物线形沙丘由迎风坡底部向顶部呈现表沙平均粒径变大，而分选逐渐变好的趋势。毛乌素沙地内季节性风况（春季盛行强劲西北风，夏季盛行较弱东南风）的变化不仅促进了反向沙丘的发育，并且重组了西北盛行风影响下的表沙粒度特征。在夏季反向风风蚀的作用下，沙丘落沙坡顶部出现反向堆积和脊线反向移动的现象，同时其顶部呈现平均粒径由小变大、分选逐渐变好的趋势。

Response of dune morphology and grain-size characteristics to the change of wind regimes and vegetation cover

Based on the analysis of the morphology， grain-size characteristics and wind regimes among three different dunes （barchan， parabolic dune and reversing dune） in Mu Us Sandy Land， we investigated the response of dune forms and their surface grain-size parameters to the change of external conditions （i.e.， wind regimes and vegetation cover）. The results show that the mean grain-size （M_{\mathrm{Z}}） of barchan decreases gradually from the bottom to the top of the stoss face， and the sorting becomes better， but the fine grain-size and poor sorting appear in the middle of the stoss face. With the increase of vegetation cover， barchan gradually transforms into the parabolic dune， and the near-surface sand transport and incoming sand supply are also limited. As a result， the mean grain size of the parabolic dune becomes coarser from the bottom to the top of the stoss face， and the sorting gradually becomes better. Seasonal wind regimes in Mu Us Sandy Land promoted the development of reversing dunes and reorganized sand grain-size characteristics. Under the influence of wind erosion in summer， eroded sand deposited on the upper of the stoss face and dune crestline flip. Thus， the mean grain-size and sorting are getting coarser and better in the dune crestal area， respectively.