Differential water distribution over dune slopes as affected by slope position and microbiotic crust,Negev Desert,Israel

Runoff is one of the main water sources responsible for water redistribution within a given ecosystem. Water redistribution is especially important in arid regions, and may be of great importance on sandy dunes, where the likelihood of runoff is low owing to the high infiltration rates of sand. Redistribution of water may significantly affect plant and animal distribution, and may explain vegetation patterns within an ecosystem. Runoff yield over sandy dune slopes in the western Negev Desert was measured under natural conditions during 1990–1994. The magnitude of runoff yield on different slope sections and on north and south exposures was established. The results demonstrate that while slope position controlled the microbiotic crust cover, crust cover and crust biomass controlled the amounts of runoff obtained. Whereas no runoff was measured on the upper dune sections devoid of crust, only meagre quantities were measured on the midslope sections, characterized by discontinuous crust cover. Substantially larger amounts were, however, obtained at the bottoms of the slopes, characterized by continuous crust cover. North‐facing slopes, usually characterized by a chlorophyll a content of 29–41 mg m⁻², yielded on average 3·2 times more runoff than south‐facing footslopes, characterized by a 17 mg m⁻² chlorophyll a content. Whereas microbiotic crust was found to be responsible for runoff generation, additional water supply owing to runoff may also explain the occurrence of a high biomass crust and the dense vegetation belt at the dune–interdune interface of the northern exposure, where runoff tends to collect. Thus, whereas crust may reduce infiltration in certain habitats, runoff generated by crust may also be responsible for the promotion of crust growth in other habitats. Runoff may also be used to promote vegetation growth at the dune footslopes. The possibility of using runoff to facilitate agroforestry is discussed. Copyright © 1999 John Wiley & Sons, Ltd.     

沙丘动力学数值模拟研究方法综述(Ⅱ):沙丘形成演变模拟

沙丘形成与发育过程强烈依赖于初始动力条件及地表特性。沙丘动力学数值模拟方法可以充分考虑其形成所依赖的这些初始条件,实现对沙丘场形成演变及沙丘间的相互作用过程和未来发展进行定量模拟和预测,是研究沙丘的一种及其有效途径。本文主要介绍了沙丘形成发育动力学数值模拟研究工作所采用的方法及所取得的主要成就。     

旱麻岗沙漠风沙地貌发育与演化

旱麻岗沙漠位于腾格里沙漠南部,是腾格里沙漠的重要组成部分。目前对该沙漠几乎没有研究。旱麻岗沙漠最主要的地貌是风沙地貌和流水地貌,且近似南北方向平行相间排列。该沙漠主要的沙丘类型有新月形沙丘链、反向沙丘和灌丛沙丘,其中新月形沙丘链和反向沙丘分布在海拔相对高的区域,且呈条带状分布,而灌丛沙丘主要分布在冲沟内。旱麻岗沙漠的起沙风主要为西北风,其次为东南风。风能环境属于低等风能环境,合成输沙方向为282°~331°,方向变率为中等变率。流动沙丘表面平均粒径最大,其次为结皮下层和结皮表层;分选性与平均粒径有相似的规律。石羊河流域、祁连山东端的冲积-洪积物和湖相沉积为旱麻岗沙漠的形成与演化提供了重要的沙源。根据野外考察,结合以往研究成果,该沙漠的形成与演化可以概括为以下几个阶段:大湖期;湖水干涸期,在退水过程形成近似平行排列的小冲沟;沙漠形成与扩张期,流动沙丘覆盖整个区域;沙漠萎缩期,降雨量增加,沙漠萎缩,平行排列的冲沟宽度和深度增加,冲沟内的流沙随流水冲向下游,而冲沟间的沙丘仍然保留在原来的位置,形成现代的地貌格局。     

Modelling palaeosol preservation in aeolian dunes

In some of the world's desert and desert‐marginal areas (e.g. Simpson/Strzelecki, Australia) dunefields preserve well‐developed palaeosols, whereas in other regions with broadly similar climatic regimes and topography (e.g. southwest Kalahari), the dunes are characterized by very poorly developed internal stratigraphy. It has been postulated that dunes such as those in the Kalahari may never have had conditions conducive to soil formation, or that soils once formed but any evidence of palaeosols has been lost due to reworking. This study develops and applies a one‐dimensional numerical model to simulate dune development, soil formation and soil preservation. Variables in the model allowed experimentation on the influence of sediment supply, the time taken for soil to form, and the additional resistance to erosion offered by the soil. Reduced sediment supply plays a vital role in landscape development during periods of initial pedogenesis. Although re‐exhumed palaeosols influence sediment supply, the effect is minimal. Although under almost all parameterized conditions more than half (and up to 80‐90%) of those soils initially formed are lost due to reworking, evidence of their past formation remains in the large majority of profiles, and the dominant factor in controlling the preservation of palaeosols is the frequency of their formation. The implication is that where dunes are found without palaeosols, the most likely (albeit not certain) inference is that they have never formed. Counter‐intuitively, the limited sediment supply means their additional resistance to erosion becomes almost inconsequential to their preservation, at least until the unit approaches complete invulnerability. Short chronostratigraphic hiatuses around palaeosols are normal, and although long gaps can occur, they are extremely infrequent. Where such gaps are observed in field studies, external forcing factors (e.g. climatic or environmental changes) are implied, as they are highly unlikely to result from stochastic net preservation. Copyright © 2014 John Wiley & Sons, Ltd.     

Back‐barrier flooding by storm surges and overland flow

Low‐elevation areas within a sandy barrier island are subject to flooding via saturation overland flow following moderate storm surges and rainfall events. Using a high resolution topographic survey and simple hydrology models, we estimate the discharge and velocities from storm surge return flow and saturation overland flow. Results show that return flow velocities are of the same magnitude as the critical velocity necessary to mobilize sand when a hydraulic connection between the watershed and back‐barrier bay is present. Storms of moderate strength and rainfall intensity may be sufficient to keep the return channels open within the back‐barrier, thus providing natural conduits for water exchange from overwash events during extreme storm surges triggered by hurricanes. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of local topography for wind direction on Mars: two examples of dune fields in crater basins

In this work, we perform an analysis of large dark dunes within Moreux Crater and Herschel Crater on Mars using High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) data sets. These data allow us to conduct a detailed analysis of dune morphology and slip faces, concluding that the studied dune fields are influenced by topographically‐controlled complex wind directions. Our morphological analysis reveals that inside Moreux Crater in particular, the topographic setting dominates the wind flow direction, leading to the development of a sand transport pathway encircling the central peak of the crater. The dune fields in Herschel Crater are also affected by winds controlled by variable topography as suggested by the presence of complex dunes and dune fields. Our analysis indicate that the studied dune systems is not the result of paleo‐wind regimes. Furthermore, we perform thermal inertia measurements using thermal emission spectrometer (TES) data, which indicate that the studied dune fields consist of medium sand 250–500 µm in diameter. Copyright © 2012 John Wiley & Sons, Ltd.     

水下沙丘形态演化的数值模拟实验

水下沙丘在海洋、湖泊、河流等浅水砂质沉积区广泛分布。基于过程的数值实验方法探讨了沙丘形态演化问题,试图解释在沙丘形成过程中各因素的作用及它们之间的关系。模拟中考虑了以下的变量:水深、沉积物粒度、沉积层厚度以及台风作用。模拟结果显示,沙丘的空间分布控制了空间流场的参数k₂,沙丘波高受水深、沉积物粒度以及沉积层厚度等因素的影响,沉积层厚度决定了沙丘的形态是否饱满。在台风作用中,沙丘波峰的沉积物被侵蚀,高程降低,波高渐小;台风作用后,沉积物被重新输运至波峰,沙丘高度逐渐恢复。因此,沙丘的高度取决于台风作用的时间以及2次台风作用之间的间隔。沙丘形态和尺度在台风作用前后变化较小,但沙丘演化的速度却有所提高。根据台湾浅滩和北海南部地貌系统数据的验证,模拟具有较好的效果。     

腾格里沙漠西部和西南部风能环境与风沙地貌

腾格里沙漠西部和西南部的沙漠边缘地区位于石羊河流域的下游地区,该地区生态环境脆弱,成为近年来备受关注的区域之一。利用自动气象站2009年年度风况资料和Google Earth影像,对该地区的风能环境与风沙地貌进行讨论,为评价区域风沙活动强度,风沙地貌形态特征提供依据。研究表明,腾格里沙漠西部和西南部的风况、风能环境呈自北向南逐渐变化的趋势,年平均风速西部最大,中部次之,南部最小;起沙风风向在北部以西北风和东北风为主,中部以西北风和东南风为主,而南部以西北风为主,东南风很少。研究区的北部为高风能环境,中部和南部为低风能环境。研究区的沙丘类型主要为格状沙丘,在沙漠边缘的部分地区为新月形沙丘链,南部为植被线形沙垄,其主要是由地形作用形成的,沙垄之间为新月形沙丘链。风能环境、沙源和植被共同影响沙丘的形态参数,研究区中部的风能比南部大,沙源比南部多,植被比南部少,因此,格状沙丘主梁之间的间距要比南部新月形沙垄的间距大。格状沙丘的走向近似相同,均在205°~225°之间。研究区南部有范围较大的植被线形沙垄,其间距在0.8～2.0 km之间,平均间距为1.37 km;走向为近似南北（164°～176°之间）。     

Magnitude and frequency of grain flows on a desert sand dune

Preliminary studies of the magnitude and frequency of lee side avalanches (grain flows) on a Namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation; and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates. These studies indicate the potential of using a simple digital video camera technique to document the magnitude, frequency and geometry of grain flows on desert sand dunes.     

黄河源区玛多县沙漠化成因与发展过程

通过分析黄河源区玛多县沙漠化的成因和过程,结果表明,玛多县沙漠化成因类型有三种,分别为固定沙丘/古沙丘活化、滑塌陡坎及风蚀斑块,其形成都与多年冻土退化相关.在固定沙丘或古沙丘分布区,冻土退化导致热融沉陷,形成沉陷坑,沉陷坑边缘形成拉裂缝或陡坎,使下伏松散沙露出.在斜坡上,冻土退化形成滑塌陡坎,使底层土壤从侧面暴露.在平坦的冲积平原,差异性冻胀和融沉导致草皮拉裂,形成积水坑洼,冻土退化导致土壤变干,土壤的底层暴露.下伏风成沉积物暴露后,遭受风蚀,形成侧向凹槽,致使上部土体坍塌,使更多的风成沉积物暴露,这些过程的不断重复,使风蚀坑、陡坎、风蚀斑块扩大相连,最终形成流动沙丘、风蚀劣地、戈壁等地貌.