宁夏河东沙地沙丘冻融过程的时空差异

中国的沙漠和沙地部分或全部分布在季节冻土区， 研究沙丘的冻融过程是讨论季节冻结期间沙丘风蚀和形态演变规律的条件之一。以宁夏河东沙地流动沙丘和沙障固定沙丘为研究对象， 通过野外观测和室内控制实验， 分析了沙丘的冻融过程及其控制因素。结果显示： 沙丘的冻结期在11月中旬至3月上旬， 流动沙丘各地貌部位的冻结时长和冻结层厚度均存在较大差异（背风坡面>迎风坡面>丘顶）， 背风坡脚的冻深最大。在季节冻结期内沙丘表层始终不发生冻结， 未冻层厚度的阈值约为10 cm且具有保护冻结层的作用， 流动沙丘迎风坡中在未冻层风蚀后， 地表冻结层融化再被风蚀， 如此循环过程造成其冻结层厚度远小于沙障固定沙丘的冻结层厚度。流动沙丘丘顶和背风坡面的冻结层厚度分别受短时（32 h）和较长历时（15 d）平均气温的影响。野外观测和室内控制实验均证明水分含量低于1.6%的沙丘沙不发生冻结， 冻结层硬度随含水率的增加呈幂函数递增（P<0.001）， 随温度降低呈缓慢递增。

Spatial and temporal differences of freeze-thaw processes on dunes in the East Desert of the Yellow River in Ningxia

The deserts and sandy lands in China totally or partly located in seasonally frozen soil regions. The aim of this study on freeze-thaw processes of dunes was to further elucidate wind erosion and morphological evolution of dunes during seasonal freezing. The freeze-thaw processes and its controlling factors of mobile dunes and fixed dunes by barrier located in the East Desert of the Yellow River in Ningxia were analyzed through field observation and laboratory experiment. The results show： The freezing period of dunes was from mid-November to early March next year， there were distinct differences in freezing duration and thickness of frozen layers among geomorphic sites of mobile dunes （leeward slope > windward slope > top of dune）. During the seasonal freezing period， there was a surface unfrozen layer on dune which protected the frozen layer， and the thickness of it was about 10 cm. The frozen layer thickness of mobile dune was much smaller than that of fixed sand dune by barriers， which caused by the cyclic process of thawing and wind erosion on frozen layer surface in the windward slope of mobile dune. The thickness of frozen layer of dune top and leeward slope on mobile dune affected by short-term （32 h） and long-term （15 d） average temperature， respectively. The results from field observation and laboratory experiment indicated that the dune could not be frozen with sand moisture content under 1.6%； there was a power function relation between the hardness of frozen soil and moisture content （P<0.001）， and the hardness increased slowly with temperature decreasing.