Grain-size characteristics of linear dunes on the northern margin of Qarhan Salt Lake,northwestern China

In order to clarify the formation mechanism of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China, we analyzed the grain-size and sorting parameters of the dune and interdune sands. The surface sands(0–30 mm) from the dune base to the crest of both flanks and interdune corridors were sampled along transects from upwind to downwind through the dune field. The results indicated that the grain-size distribution differed at different positions between and within the dunes. The frequency curve for dune sands mainly showed a bimodal distribution, while the interdune sediments showed a trimodal distribution. The grain size distribution of the linear dunes showed a finer crest pattern, i.e. the crests were composed of sands that were generally finer, better sorted than those of base sands. In addition, at the dune field scale, the dune crest sands were tending to become much finer but sorting became worse along the downwind transects. However, the grain-size parameters of sediments in the interdune corridors showed no clear pattern. The results demonstrated that the grain size and sorting parameters exhibited a systematic change not only at the individual scale but also at the dune field scale. Our results quantitatively estimate the limited role of cohesive sediments on the formation of linear dune under unidirectional wind regime. More attention should be paid to a long-term wind regime observation, internal sedimentary structures and their formation ages.     

Aeolian transport over a developing transverse dune

The spatial and temporal changes in aeolian transport over a dune are fundamental factors that control the morphology of the dune.In the present study,we obtained direct field observations of aeolian transport over a developing transverse dune at the Shapotou Aeolian Experiment Site in the southeastern part of China’s Tengger Desert.The transport rate versus wind speed relationship is complicated over a developing dune compared with the relationships over flat surfaces and over dunes that are in equilibrium with the wind.We obtained trend lines for transport rate over the transverse dune versus distance.The transport rate generally increased from the toe to the crest above the stoss slope,but the difference in transport rate between the crest and the toe was smaller than those that have been proposed for taller dunes.The crest/toe ratio for transport rates therefore seems to depend greatly on dune height.Flux density profiles for different points above the dune at different wind speeds were well described by the exponential decay law,as has been proposed for saltation flux density profiles.Coefficients in the flux density profile function can be defined in terms of the transport rate and wind speed.However,the dependence of relative decay rate with height and average saltation height on wind speed was weaker than that observed in a wind tunnel and above a flat surface.The preliminary results obtained in this study require more evidence from field observations to fully describe aeolian transport above developing dunes.     

Grain size distribution at four developmental stages of crescent dunes in the hinterland of the Taklimakan Desert,China

Although scientists have performed many studies on crescent (barchan) dunes in the Taklimakan Desert,few studies reported the changes in grain size at different development stages of crescent dunes.In order to evaluate the changing trends of surface sediment grain size with dune development,we investigated the grain size characteristics at four developmental stages (oval sand pile,shield dune,incipient crescent dune and mature crescent dune) of crescent dunes by measuring the morphology of sand dune and observing the near-surface wind regime.The dunes have developed in a wide inter-dune corridor between high compound longitudinal ridges in China's Taklimakan Desert.The surface sediments at four developmental stages of the crescent dunes were primarily composed of fine sands,followed by very fine and medium sands.Mean grain sizes ranged from 2.8 to 3.1 φ,with a unimodal distribution.The sands were moderately well-sorted,their distribution varied from platykurtic to very platykurtic,and symmetrical or skewed towards the fine particles.From oval sand piles through shield and incipient crescent dunes to mature crescent dunes,incipient grain size gradually increased,particles became finer,sorting became better,kurtosis and skewness increased.Grain sizes on the surface layer became coarser upwards from the toe of the windward slope and then became finer towards the bottom of the leeward slope.We found that the coarsest particles at different positions at the four developmental stages were different.The coarsest particles were distributed at the top of the oval sand piles and shield dunes,versus at the middle of the windward slope of the incipient and mature crescent dunes.Correlations between the mean grain size and other grain size parameters showed that as mean grain size became finer,sorting became better and kurtosis became wider,but skewness changed only slightly.In addition,grain size variation in the surface sediments correlated with the movement speed of the dunes in the study area.In the open ground among tall-complex longitudinal ridges in the hinterland of the Taklimakan Desert where aeolian environment is characterized by comparatively strong wind and unsaturated sand flow,faster dune movement corresponded to coarser grain size.     

简单横向沙丘表面物质组成、气流分布及其在动力学过程中的意义

对简单横向沙丘的形态特征、气流、表面以及被输移的沙物质组成的观测和分析表明 ,沙丘形态特征和表面物质组成的分异是导致沙物质在迎风坡坡脚不能沉积并向下风向输送的主要原因。对沙丘背风坡气流观测表明 ,在沙丘背风坡贴近地面的一定高度气流仍能保持一定的强度 ,其输沙方向与沙丘走向近似于垂直 ,并有相当强的输沙能力 ,但其作用被限制在背风坡范围内 ,这对保持沙丘形态有重要的作用。在野外风沙观测中气流的湍流和波动被忽略 ,平均风速并不能完全反映野外气流的性质。在低 -中强度风沙活动过程中 ,被输送的沙物质的粒度组成与下垫面物质有明显的差异 ,被输送沙物质的平均粒径小于下垫面沙平均粒径 ,输沙过程是一典型的随机过程。特定区域内 ,一定粒径范围内的沙物质不断被堆积而形成沙丘 ,经过一系列的动力学过程 ,最终形成处于动态平衡的区域沙丘地貌分布格局     

Morphology and formation mechanism of sand shadow dunes on the Qinghai-Tibet Plateau

The formation and development of dunes depend on wind-blown sand movement which is affected by the characteristics of sand material, topography, wind regimes and other factors. In this paper, we investigated two sand shadow dune groups in Shigatse and Za’gya Zangbo of Tibet and an individual dune in Da Qaidam of Qinghai, and analyzed their topographies and morphologies, and the physical characteristics of the sand, wind regime and sand transport. Formed under harsh conditions behind hills, these mature sand shadow dunes are hundreds of meters long, have significant ridges and crescent dunes downwind, and have a hill pass on one or both sides. Wind tunnel experiments revealed that the hill gap and wind velocity are important factors in the formation of these dunes. Sand shadow dunes formed only when the gap spacing is two-thirds of the hill height. When wind velocities are 20 m/s, the sand body is divided into two parts. The hill pass allows the transport of sand by wind, creating a "narrow-pipe effect", which causes the transported material to gradually accumulate in the center of the shadow zone. We observed that the following are needed for sand shadow dunes to form:(1) strong winds, sufficient sand, suitable obstacles and a dry climate;(2) one or both sides of the obstacle forming the shadow zone must have a hill pass; and(3) the windward side of the obstacle must have a wide, flat area, providing adequate spacing for wind flow and transport of material and the leeward side must have a sufficiently broad, flat area to allow the release of the transported material. Research results on these newly discovered dunes on the Qinghai-Tibet Plateau could contribute to the understanding of dune geomorphology.     

Quantitative analysis on the dynamic characteristics of megadunes around the Crescent Moon Spring,China

The Crescent Moon Spring is a precious natural heritage. However, the dynamic characteristics of megadunes around the Crescent Moon Spring are not well known. This paper quantitatively studied the character- istics and changes of megadunes around the Crescent Moon Spring by interpreting aerial photographs taken in 1985 and 2004 and analysing the dune crestlines and the wind data collected from 2011 to 2012. Results revealed that pyramid dunes were formed by a complex wind regime. The Crescent Moon Spring was not buried by shifting sands because of the stable wind regime and relative stability of pyramid dunes. The crestlines of the dunes around the spring moved northward between 1985 and 2004. The south-facing slip faces were also exposed to wind ero- sion, whereas the other faces were under deposition, thus indicating that the southerly wind was relatively en- hanced. Limiting the scale of tall windbreaks and architectures in the Dunhuang oasis at the north of the spring was necessary to maintain the dynamic equilibrium of the wind regime and sand transport.     

Sand source and formation mechanism of riverine sand dunes: a case study in Xiangshui River,China

Riverine sand dunes develop as a result of fluvial-aeolian interactions. The primarily barchan dune chains along the Xiangshui River (a branch of the Xar Moron River in the western part of the Horqin Sandy Land of China) form a typical riverine dune field. We collected a series of samples from the riverine sand dunes parallel to the direction of the prevailing wind and investigated the sand sources and formation mechanisms of these dunes by determining the grain size, heavy mineral content and optically stimulated luminescence (OSL) of the samples. The sand of the near-river dunes was coarser than the sand of the dunes distant from the river, indicating that coarse sand of the valley mainly deposited on near-river dunes. The heavy mineral analysis suggested that wind-sand activity levels were intense on the upwind dunes, but relatively weak on the downwind dunes. This indicated that the sand sources for the near-river dunes were more abundant than those of the distant dunes. Our OSL analysis of samples suggested that the deposition rates on dunes near the river were greater than the deposition rates on dunes distant from the river. The development of dunes along the river indicated that the river played an important role in dune formation and development. In addition, airflow fluctuation and the formation of the waveform dunes had a type of feedback relationship. Grain size, heavy mineral and OSL analyses are widely used methods in wind-sand research. Sand dune grain size characteristics reflect the effects of airflow on the transport and separation of sand materials, as well as the physical characteristics of the sand sources. Heavy mineral characteristics are often used to investigate the relationships between sediments and sand sources. OSL indicates dune age, revealing formation of dunes. Therefore, it is useful to explore dune sand sources, as well as the mechanisms underlying dune formation, by determining grain size, heavy mineral content and OSL. This study investigated the sand sources of riverine dunes and provided new information about riverine dune formation and development.     

Optically stimulated luminescence dating of sandy deposits from Gulang county at the southern margin of the Tengger Desert,China

Aeolian deposits from the deserts in northern China have been used for palaeoenvironmental research to understand aeolian sedimentology and its dynamic connection to past climate conditions. The Tengger Desert in China is sensitive to the waxing and waning of the monsoonal system. In response to past climate change, the southern margin of the Tengger Desert has evolved significantly since the last glacial period. However, previous attempts to date aeolian deposits in this region were mainly based on radiocarbon dating, which has problems when applied to aeolian deposits. Moreover, sedimentary records are limited. Accordingly, past aeolian activity in this desert remains poorly understood. In the present study, we dated sand samples from Gulang county at the southern margin of the Tengger Desert using optically stimulated luminescence(OSL) to understand the history of aeolian activity in this region. Our samples represented well-sorted aeolian sands and sandy loess. Aeolian sands are evidence of dune field buildup and sparse vegetation cover whereas sandy loess is evidence of improved stabilization of sand dunes resulting from ameliorated vegetation cover. Certain samples showed a decline in the equivalent dose(De) values when successive integration intervals were applied, which resulted from unstable OSL signals from non-fast components in the initial part of the decay curve. In order to obtain reliable De estimates, we investigated component-resolved and different background subtraction approaches, and compared the resultant De estimates. We adopted the early background subtraction method to derive De values. Luminescence chronologies and sedimentary records indicated that sand dunes accumulation occurred before 10 ka, and sandy loess developed between 9.5 and 7.6 ka when sand dunes were stabilized as a result of increased effective moisture levels. The transition between sand dune mobilization and stabilization emphasizes the significance of an effective moisture threshold in controlling aeolian activity. Mobilization of sand dunes at ~2.3 ka might be related to an increased aridity during the Late Holocene.     

Influence of salinity and moisture on the threshold shear velocity of saline sand in the Qarhan Desert,Qaidam Basin of China: A wind tunnel experiment

Determination of the threshold shear velocity is essential for predicting sand transport, dust release and desertification. In this study, a wind tunnel experiment was conducted to evaluate the influence of salinity and moisture on the threshold shear velocity of saline sand. Saline sand samples (mean particle size of 164.50-186.08 μm and the total silt, clay and salt content of 0.80%-8.25%) were collected from three saline sand dunes (one barchan dune and two linear dunes) in the Qarhan Desert, Qaidam Basin of China. Original saline sand samples were placed in two experimental trays for wet and dry processing to simulate deliquescence and desiccation, respectively. Surface moisture content ranging from 0.30% to 1.90% was generated by the steam method so that the saline sand can absorb water in a saturated water vapor environment. The motion of sand particles was determined by the observers with a solid laser. The laser sheet (0.80 cm thick), which was emitted by the solid laser, horizontally covered the sand surface and was bound to the sand. Results show that the cohesion of saline sand results from a combination of salt and water. The threshold shear velocity increases exponentially with the increase in crust thickness for the linear sand dunes. There is a positive linear correlation between the original moisture content and relative threshold shear velocity. The threshold shear velocity of dewatered sand is greater than that of wet sand with the same original moisture content. Our results will provide valuable information about the sand transport of highly saline soil in the desert.     

Pattern analysis of a linear dune field on the northern margin of Qarhan Salt Lake,northwestern China

In terms of formation mechanisms of linear dunes,there are open arguments for their widespread distribution and multi-morphological diversities.In order to clarify the formation mechanism of linear dunes of Qarhan Salt Lake,we used pattern analysis method to analyze the statistical characteristics and spatial variation of their pattern parameters.Except at the west-northwest margin,the pattern parameters showed regular spatial variation from the up-middle part towards the downwind end of the dune field.Based on the cumulative probability plots for inter-crest spacing and crest length,we divided the linear dunes into three groups,which corresponding to the three evolution stages of these dunes.The first group comprises erosional relics,with shorter crests,smaller inter-crest spacing and more random dune orientation.The second group comprises dunes whose sand supply is just sufficient to maintain stability and these dunes are approaching the net erosion stage.The crest length and inter-crest spacing of these dunes are much larger than those of the first group,and dune orientation is closer to the resultant drift direction (RDD) .The last group comprises linear dunes that are still undergoing vertical accretion and longitudinal elongation,which follows the RDD of the modern wind regime.The presence of regular spatial variation of pattern parameters and a similar geometry with the vegetated linear dunes suggest that deposition and erosion coexist in the development and evolution of linear dunes of Qarhan Salt Lake,i.e.deposition predominates at the downwind end of linear dunes in the vertical accretion and longitudinal elongation stage,whereas erosion mainly occurs at the upwind end of linear dunes in the degradation stage.Therefore,the formation mechanism of linear dunes in Qarhan Salt Lake can be reasonably explained by the combination of depositional and erosional theories.     

沙障工程防治格状沙丘前移的风洞实验研究

分析研究表明，格状沙丘主梁是在主风作用下形成的横向新月形沙丘链；而副梁是在两个次主风的共同作用下，在横向沙丘链迎风坡上逆向形成的纵向沙丘。风洞实验和野外观测结果证明，延缓格状沙丘前移，可以在其主、副梁上采用高立式加半隐蔽式沙障，构成复合沙障，利用其抬高阻滞作用而达到目的。     

塔克拉玛干沙漠腹地复合纵向沙垄间地沙丘空间分形及稳定性

塔克拉玛干沙漠的大地貌格局是高大复合沙垄和宽广垄间地相间分布,而垄间地是研究简单沙丘发育过程的良好试验场地。基于Google Earth影像,以分形理论为基础,对塔克拉玛干沙漠腹地复合纵向沙垄区垄间地上覆沙丘在主导风方向上的分形特征进行了分析,结果表明：① 垄间地沙丘形状指数（面积-周长关系）的标度-频度双对数关系非常显著,分形关系客观存在,并且不受统计时所用标度的影响;② 垄间地各类沙丘的分维值存在差异,其中,饼状沙堆和沙片分维值最大（1.337）,新月形沙丘次之（1.170）,简单线性沙丘最小（1.087）;③ 垄间地沙丘分维值的空间差异较大,在1.187~1.656,其分维数：背风坡脚（1.24）＜垄间地中部（1.40）＜迎风坡脚（1.50）,变异系数为0.106;④ 垄间地沙丘稳定性指数与分维值的空间分布相反,即背风坡脚（0.26）＞垄间地中部（0.16）＞迎风坡脚（0.10）;⑤ 垄间地沙丘分维值空间差异主要与沙丘发育程度有关,其中,风信是动力基础,沙源是物质基础。在流动沙漠,沙丘分形维数可以作为反映其风沙环境特征的定量指标。     

Species composition and diversity,and carbon stock in a dune ecosystem in the Horqin Sandy Land of northern China

In this study, we determined carbon allocation and carbon stocks in the plant-soil system of different dune ecosystems in northeastern China. We quantified the species composition, above- and below-ground biomasses, and carbon stocks of three dune types(i.e. active dunes, semi-stabilized dunes and stabilized dunes) and their corresponding inter-dune lowlands(i.e. interdune lowlands of active dunes, interdune lowlands of semi-stabilized dunes and interdune lowlands of stabilized dunes) in the Horqin Sandy Land. The results showed that the succession series on interdune lowlands of the Horqin Sandy Land confirmed differences in species composition of the various dune types. Aboveground carbon(AGC) on the interdune lowlands of semi-stabilized dunes(33.04 g C/m2) was greater(P<0.05) than that on the interdune lowlands of active dunes(10.73 g C/m2). At the same time, the different dune types did not show any significant differences(P>0.05) in belowground plant carbon(BGC). However, the percentage of plant BGC in interdune lowlands of active dunes(81.5%) was significantly higher(P<0.05) than that in the interdune lowlands of semi-stabilized dunes(58.9%). The predominant carbon pool in the study dune ecosystem was in the soil. It accounted for 95% to 99% of total carbon storage. Soil organic carbon(SOC) was at least 55% greater(P<0.05) in the interdunes than in the dunes. Stabilized dunes showed at least a 37% greater(P<0.05) SOC content than active dunes up to a 1-m soil depth. Meanwhile, SOC content of interdune lowlands of semi-stabilized dunes was greater(P<0.05) than that of interdune lowlands of active dunes only up to a 20-cm soil depth. The dune ecosystem showed a great potential to store carbon when interdune lowlands of active dunes were conversed to interdune lowlands of semi-stabilized dunes, which stored up to twice as much carbon per unit volume as interdune lowlands of active dunes.     

Morphological characteristics and dynamic changes of seif dunes in the eastern margin of the Kumtagh Desert,China

The seif dune field over the gravel desert surface in the eastern margin of the Kumtagh Desert is a valuable experimental site for the observation of dune formation and dynamics. We used high-resolution remote sensing and station observation approaches, combined with wind and grain size data, to study the characteristics of the aeolian environment and the morphologies of and dynamic changes in seif dunes. We observed the ratio of the resultant drift potential (RDP) to the drift potential (DP), which was 0.37, associated with an obtuse bimodal wind regime. The drift potentials in the west-northwest (WNW) and east-northeast (ENE) directions were dominant, and the angle between the two primary DP directions was 135.00°. The dune orientations ranged from 168.75°-213.75°, which were parallel to the resultant drift direction (186.15°). The dune lengths ranged from 51.68 to 1932.11 m with a mean value of 344.91 m. The spacings of the dunes ranged from 32.34 to 319.77 m with a mean value of 93.39 m. The mean grain size of the sediments became finer, and the sorting became better from upwind tail to downwind tip, which indicated that the sediment of the seif dunes in the study region may be transported from northward to southward. The rate of increase in the length, the mean longitudinal migration rate of the dune tail, and the mean longitudinal extension rate of the dune tip (also called elongation rate) were 4.93, 4.63, and 9.55 m/a, respectively. The mean lateral migration vector of the seif dunes was approximately 0.11 m/a towards the west (-0.11 m/a), while the mean amplitude of lateral migration was 0.53 m/a, ignoring the direction of lateral migration. We found that the seif dune field formed first beside seasonal rivers, which can provide sediment, and then expanded downwind.     

Effect of sand-fixing vegetation on the hydrological regulation function of sand dunes and its practical significance

Soil water content is a key controlling factor for vegetation restoration in sand dunes. The deep seepage and lateral migration of water in dunes affect the recharge process of deep soil water and groundwater in sand dune ecosystems. To determine the influence of vegetation on the hydrological regulation function of sand dunes, we examined the deep seepage and lateral migration of dune water with different vegetation coverages during the growing season in the Horqin Sandy Land, China. The results showed that the deep seepage and lateral migration of water decreased with the increase in vegetation coverage on the dunes. The accumulated deep seepage water of mobile dunes (vegetation coverage<5%) and dunes with vegetation coverage of 18.03%, 27.12%, and 50.65% accounted for 56.53%, 51.82%, 18.98%, and 0.26%, respectively, of the rainfall in the same period. The accumulated lateral migration of water in these dunes accounted for 12.39%, 6.33%, 2.23%, and 7.61% of the rainfall in the same period. The direction and position of the dune slope affected the soil water deep seepage and lateral migration process. The amounts of deep seepage and lateral migration of water on the windward slope were lower than those on the leeward slope. The amounts of deep seepage and lateral migration of water showed a decreasing trend from the bottom to the middle and to the top of the dune slope. According to the above results, during the construction of sand-control projects in sandy regions, we suggest that a certain area of mobile dunes (>13.75%) should be retained as a water resource reservoir to maintain the water balance of artificial fixed dune ecosystems. These findings provide reliable evidence for the accurate assessment of water resources within the sand dune ecosystem and guide the construction of desertification control projects.     

古尔班通古特沙漠边缘春秋季沙丘水分状况初步研究

古尔班通古特沙漠边缘植被破坏严重,沙丘活化明显,是新疆干旱区植被恢复的重点地区之一,当地降水稀少,其沙丘的水分状况便成为植被生存和恢复过程的关键条件,为掌握古尔班通古特沙漠边缘流动沙丘和固定沙丘不同部位的水分状况,从2003年3月到10月植物生长期内利用中子仪进行连续监测的结果表明:沙丘含水量月变化与月降水量分布相一致;流动沙丘0-30cm的含水量低于固定沙丘,而其它各层都高于固定沙丘;流动沙丘含水量从上部到下部依次升高,而固定沙丘则并无此规律;流动沙丘中部和下部稳定湿沙层处在距沙丘表面60cm以下,且厚度超过100cm,固定沙丘稳定湿沙层只存在于沙丘中部距沙丘表面60~150cm深度范围内,其厚度不超过100cm。     

Determining the spatial distribution of soil properties using the environmental covariates and multivariate statistical analysis: a case study in semi-arid regions of Iran

Natural soil-forming factors such as landforms, parent materials or biota lead to high variability in soil properties. However, there is not enough research quantifying which environmental factor(s) can be the most relevant to predicting soil properties at the catchment scale in semi-arid areas. Thus, this research aims to investigate the ability of multivariate statistical analyses to distinguish which soil properties follow a clear spatial pattern conditioned by specific environmental characteristics in a semi-arid region of Iran. To achieve this goal, we digitized parent materials and landforms by recent orthophotography. Also, we extracted ten topographical attributes and five remote sensing variables from a digital elevation model (DEM) and the Landsat Enhanced Thematic Mapper (ETM), respectively. These factors were contrasted for 334 soil samples (depth of 0-30 cm). Cluster analysis and soil maps reveal that Cluster 1 comprises of limestones, massive limestones and mixed deposits of conglomerates with low soil organic carbon (SOC) and clay contents, and Cluster 2 is composed of soils that originated from quaternary and early quaternary parent materials such as terraces, alluvial fans, lake deposits, and marls or conglomerates that register the highest SOC content and the lowest sand and silt contents. Further, it is confirmed that soils with the highest SOC and clay contents are located in wetlands, lagoons, alluvial fans and piedmonts, while soils with the lowest SOC and clay contents are located in dissected alluvial fans, eroded hills, rock outcrops and steep hills. The results of principal component analysis using the remote sensing data and topographical attributes identify five main components, which explain 73.3% of the total variability of soil properties. Environmental factors such as hillslope morphology and all of the remote sensing variables can largely explain SOC variability, but no significant correlation is found for soil texture and calcium carbonate equivalent contents. Therefore, we conclude that SOC can be considered as the best-predicted soil property in semi-arid regions.     

Wind tunnel experiments on dust emissions from different landform types

The measurement and assessment of dust emissions from different landforms are important to understand the atmospheric loading of PM10(particulate matter ≤10 μm aerodynamic diameter) and to assess natural sources of dust; however, the methodology and technique for determining the dust still present significant research challenges. In the past, specialized field observation and field wind tunnel studies have been used to understand the dust emission. A series of wind tunnel tests were carried out to identify natural sources of dust and measure the magnitudes of dust emissions from different landforms. The method used in this study allowed the measurement of the PM10 emission rate using a laboratory based environmental boundary layer wind tunnel. Results indicated that PM10 emissions demonstrated strong temporal variation and were primarily driven by aerodynamic entrainment. Sand dunes, playa, and alluvial fans had the largest dust emission rates(0.8–5.4 mg/(m~2·s)) while sandy gravel, Gobi desert and abandoned lands had the lowest emission rates(0.003–0.126 mg/(m~2·s)). Dust emissions were heavily dependent on the surface conditions, especially the availability of loose surface dust. High dust emissions were a result of the availability of dustparticle materials for entrainment while low dust emissions were a result of surface crusts and gravel cover. Soil surface property(surface crusts and gravel cover) plays an important role in controlling the availability of dust-sized particles for entrainment. The dust emission rate depended not only on the surface conditions but also on the friction velocity. The emission rate of PM10 varies as a power function of the friction velocity. Although dynamic abrasion processes have a strong influence on the amount of dust entrainment, aerodynamic entrainment may provide an important mechanism for dust emissions. Large volumes of dust entrained by aerodynamic entrainment cannot only occur at low shear velocity without saltation, but may dominate the entrainment process in many arid and semi-arid environments. So it may also be responsible for large magnitude dust storms. Playa and alluvial fan landforms, prior to developing a surface crust, may be the main sources of dust storms in Qinghai Province.     

塔克拉玛干沙漠腹地纵向沙垄的粒度分布特征

塔克拉玛干沙漠东经８４°附近、北纬３９°－４０°之间垄间低地中纵向沙垄的粒度分布特征表现为：平均粒径３．１４５，以极细沙为主，其次为细沙和中沙，三者含量达８０％～９０％以上。两坡下部较细、丘顶较粗。标准高差平均值为０．５９，分选程度中等，两坡中部分选较好，丘顶分选较差。偏度平均值为０．２９，大部分沙样为正偏态，粗组分含量较高。上述粒度特征在沙垄两翼坡面上呈不完全对称分布，并随着不同方向起沙，风的作用呈现一定程度的周期性变化。     

塔克拉玛干沙漠腹地纵向沙垅表面沙物质粒度特征

本文对塔克拉玛干沙漠腹地纵向沙垅表面沙粒、丘间地沙粒进行了粒度分析。结果表明,沙垅以极细沙为主,平均粒径３．０６Φ,沙粒从沙垅迎风侧到背风侧逐渐变细。沙丘沙比丘间地沙分选好,沙垅背风侧比迎风侧分选好。沙粒属极负偏,偏度－０．２２,丘间地沙比沙丘沙更趋向负偏。峰度１．３８,属尖峰态。粒径与偏度和峰度成正相关,分选和偏度成良好负相关。沙丘沙以单峰态为主,累积频率曲线为２～３段式,丘间地以双峰态为主,累积频率曲线为３～４段。大尺度地形对沙粒段式没有影响,对各段组分含量相对大小有影响。