Aeolian Research最新文献

Nebkhas are a type of anchored dune that forms by sediment accumulation around grasses, shrubs or trees. This paper outlines their importance, their distribution at global and local scales, summarises what is known about their morphology, discusses those plants with which they are associated, draws attention to the materials of which they are composed, and then concludes with a discussion of their ages. Nebkhas are very widespread in dry and coastal environments. They have considerable importance because they can be ‘islands of fertility’ and have major hydrological and biological effects. They can, if eroded, be major sources of dust emission. Examples of coastal nebkhas are known from both temperate areas and from lower latitude drylands, as along the Caspian shoreline in Iran, Kuwait, northern Peru, Namibia, Western Sahara, the Makran, southwest Madagascar, and Mauritania. Dryland nebkhas have been described from numerous areas, with concentrations of studies in Kuwait, the southwest USA, northwest China, and Iran. Within an area with suitable climatic conditions, nebkhas do not occur everywhere. Certain conditions need to be met for their development and these are illustrated with examples from the Tarim and Qaidam basins of China, the Lut of Iran, northwest Argentina, and the Namib. For this, Google Earth was employed. The heights of these features range from a matter of centimeters to as much as 20 m. There is a considerable quantity of data on the plants that are associated with the development of nebkhas. They have to have certain qualities for this role: ability to withstand drought, tolerate salt, thrive on sand burial, have wide shallow rooting systems to obtain water from fog and occasional rain events, extract moisture from groundwater, withstand sand abrasion, and develop adventitious roots. Although nebkhas are normally thought of as a type of sand dune, and most nebkhas are indeed composed of sand-sized material, largely consisting of quartz, there are many nebkhas which contain appreciable amounts of silt and clay, as well as salts such as gypsum. Although some nebkhas are ephemeral features, various dating techniques have shown that they can be decades, centuries and even millennia old.

Aeolian processes are fundamental to arid and semi-arid ecosystems, but modeling approaches are poorly developed for assessing impacts of management and environmental change on sediment transport rates over meaningful spatial and temporal scales. For model estimates to provide value, estimates of sediment flux that encapsulate intra- and inter-annual and spatial variability are needed. Further, it is important to quantify and communicate transparent estimates of model uncertainty to users. Here, we present a wind erosion and dust emission model parameterized for rangelands using a Generalized Likelihood Uncertainty Estimation framework. Modeled horizontal sediment flux was calibrated using data from five diverse grassland and shrubland sites from the USDA National Wind Erosion Research Network. Observations of wind speed, vegetation height, length of gaps between vegetation, and percent bare ground were used as model inputs. Horizontal sediment flux estimates from 10,000 independently selected parameter sets were compared to flux observations from 44 ∼ month-long collection periods to calculate a likelihood measure for each model. Results show good agreement for individual sampling periods across sites with few observations falling outside prediction bounds and a one-to-one relationship between median predictions and observations. Additionally, combined distributions of sediment flux estimates from all sample periods for a given site closely approximated the probability of observing a given flux at that site. These results suggest AERO effectively represents temporal variability in aeolian transport rates at rangeland sites and provides robust assessments suitable for assessing land health and better predicting changes in air quality and the impacts of land management activities.

Measurements of atmospheric dust have long influenced our understanding of dust sources and dust model calibration. However, assessing dust emission magnitude and frequency may reveal different dust source dynamics and is critical for informing land management. Here we use MODIS (500 m) albedo-based daily wind friction estimates to produce a new dust emission climatology of North America (2001–2020), calibrated by the novel use of dust point sources from optical satellite observations (rather than being tuned to dust in the atmosphere). Calibrated dust emission occurred predominantly in the biomes of the Great Plains (GP) and North American Deserts (NAD), in broad agreement with maps of aerosol optical depth and dust deposition but with considerably smaller frequency and magnitude. Combined, these biomes produced 7.2 Tg y^-1 with contributions split between biomes (59.8% NAD, 40.2% GP) due to the contrasting conditions. Dust emission is dependent on different wind friction conditions on either side of the Rocky Mountains. In general, across the deserts, aerodynamic roughness was persistently small and dust sources were activated in areas prone to large wind speeds; desert dust emissions were wind speed limited. Across the Great Plains, large winds persist, and dust emission occurred when vegetation cover was reduced; vegetated dust emissions were roughness limited. We found comparable aerodynamic roughness exists across biomes/vegetation classes demonstrating that dust emission areas are not restricted to a single biome, instead they are spread across an ‘envelope’ of conducive wind friction conditions. Wind friction dynamics, describing the interplay between changing vegetation roughness (e.g., due to climate and land management) and changing winds (stilling and its reversal), influence modelled dust emission magnitude and frequency and its current and future climatology. We confirm previous results that in the second half of the 21st century the southern Great Plains is the most vulnerable to increased dust emission and show for the first time that risk is due to increased wind friction (by decreased vegetation roughness and / or increased wind speed). Regardless of how well calibrated models are to atmospheric dust, assuming roughness is static in time and / or homogeneous over space, will not adequately represent current and future dust source dynamics.

Relationships among hydrology, sand movement, and vegetation community dynamics within the Slate Canyon alluvial fan and Keeler Dunes Complex, Owens (dry) Lake, California, were characterized. Construction of flood control berms (1954, 1967) cut off surface overflow events on the alluvial fan, resulting in subsequent very low plant cover and significant sand movement. Hydrologic modeling and analysis of plant cover and sand movement from aerial photos were combined to document and explain changes in plant cover in the study area for upland vegetation and groundwater-dependent vegetation. Where hydrologic factors dominated, upland vegetation was observed to have much lower cover (median 6%) than groundwater-dependent vegetation (median 41%) and much higher variability throughout the 1944 to 2012 study period. For upland vegetation, simulated potential plant cover was 9.2% and this declined to 4.4% when channel flow inputs were eliminated due to berm construction. Areas with minimal sand movement over the study period expressed actual plant cover similar to simulated levels. Areas with significant sand movement, however, had much lower plant cover than simulated potential values and areas with flood flow inputs had much higher cover. The decline in plant cover from 9.2 to 4.4% resulted in an estimated 4.4-fold greater sand movement (normalized sand flux increased from 14% to 62% relative to 100% for bare soil). The sensitivity of surface stability at low vegetation cover and the potential for hydrologic changes to reduce vegetation cover in this range illustrates how hydrology and sand movement interactions can destabilize sensitive sand dunes.

Wind erosion studies based on wind speed of different averaging times always weaken the effect of effective wind force and wind speed fluctuation, and produce large errors in wind erosion assessment. To quantitatively reveal the time scale effect of average wind speed on wind erosion research, this study analyzed the changes in wind speed distribution, as well as that in wind erosion event statistics and wind erosion amount calculation under different statistical time intervals, based on field observations. To further eliminate this time scale effect, wind speed conversion relationships of different averaging times were established through quantile matching of wind speed distribution function. The results show that as the averaging time increases: (1) both high and low wind speeds are filtered, making the proportion of medium wind speeds increasing, and wind speed distribution gets narrow significantly; (2) both the number and cumulative duration of wind erosion events decrease exponentially; (3) wind erosion amount corresponding to different wind speed grades differ significantly; (4) a continuous conversion equation between wind speed of different averaging times and that of one minute were established; (5) the time scale effect on different surface types are significantly different. In general, by using the statistical characteristics of wind speed distribution innovatively and considering difference in land surface properties, this study established an empirical wind speed conversion model on typical wind erosion surfaces, which will improve the evaluation accuracy of soil wind erosion intensity, and provide basis for the construction of process-based soil wind erosion model.

Coastal loess is a unique and intriguing eolian sediment on Earth, but understanding its formation time, provenance, composition and the geomorphic processes that resulted in its accumulation is very limited compared to its inland counterpart. In this paper, we focused on a loess section at Xiazhupan (XZP) Village, Penglai City in Shandong Peninsula, the only known coastal loess bluffs preserved in China. The coastal loess here not only has characteristics that are common to loess terrains such as being composed of predominantly silt-sized particles and the tendency to stand in vertical exposures, but it also reportedly preserves planktonic foraminifera, sizable pebbles and volcanic glass. In addition to its peculiar near-shore locality, these features make it more unusual compared to other loess deposits. Systematic optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL and pIRIR) and radiocarbon dating reveal it was deposited mostly during 60–20 ka, Marine Isotope Stage (MIS) 4-2, which correlates to the deposition of the Malan loess (L1) in the Chinese Loess Plateau. High-resolution grain size data indicate that the coastal loess is relatively coarse-grained, suggesting a source terrain that is at least partially proximal. Given its proximal source and the geochronological evidence, this coastal loess was sourced from the exposed continental shelf during the sea-level lowstand of the last glacial period, and this hypothesis is further supported by a core collected from the Bohai Sea in which silt was present and would have been subaerially exposed during MIS 4-2. In addition, this source area of exposed continental shelf is comparable to coastal loess in Europe. The coastal loess in Shandong can be regarded as a special type of desert or dryland loess, rather than glacial loess. Finally, future sea level rise will likely increase the erosion potential of these vulnerable coastal loess bluffs, making it urgent to study this special landform.

In this study, wind tunnel experiments were conducted to determine the relationship between the texture of road surfaces and the threshold wind velocity that causes the movement of sparsely distributed gravels. We proposed a method that uses analogous particles having lower densities compared to actual gravel, considering the threshold velocity for gravel is too large to imitate in a typical wind tunnel. We first conducted a preliminary experiment, which confirmed that the threshold velocity for denser prototype particles can be estimated from the density ratios of the analogous and prototype particles. Furthermore, a main experiment was conducted to analyze different road surface textures using pumice stones, as their particle density is less than that of gravels. The results showed that coarser road surfaces exhibited larger threshold velocities for the gravel movement. Quantitatively, the coarsest asphalt concrete exhibited a threshold velocity that was 2.5 times larger than that of the smoothest surface (cement concrete). However, surface coarseness was dependent on the particle size, whereas the effect of particles trapped by the gaps in surfaces was more dominant than the fluid force acting on the particles of a relatively coarse surface. This force balance is reversed for a smooth surface, which indicates the possibility of determining threshold friction velocity from the particle size and surface texture coarseness.

Situated in the Salawusu River Valley in the southeast of China’s Mu Us Desert, the Milanggouwan stratigraphical section MGS1 has recorded 11 sedimentary cycles, consisting of alternations between dune sands and fluvial or lacustrine facies. This paper analyzed the distribution of grain-size and Zr/Rb ratios in the MGS1, which is located in the arid and semi-arid areas of North China, and grain-size and Zr/Rb ratios are good proxy indicators of East Asian winter and summer monsoons. Results determined the age of the top of MGS1 to be 11.6 ± 0.9 ka BP, and Mz and σ values in the dune sands to be lower than those in the corresponding fluviolacustrine or paleosols. Furthermore, Zr/Rb also exhibited low values in the dune sands and a relatively high content in the fluviolacustrine and paleosols. Both grain-size and Zr/Rb ratios displayed 11 cycles similar to the sedimentation changes, while the Zr/Rb ratios were observed to correlate with Mz and σ. The results suggested that the cycles resulted from the fluctuations between cold-dry and warm-humid climates, and the MGS1 segment experienced at least 11 cold-dry and warm-humid climatic fluctuations. This type of high-frequency climatic fluctuation on a millennial time-scale is related to variations in the strength of the East Asian winter and summer monsoons during the Holocene Period. The dominant periods of the winter monsoon corresponded to the cold events of 8200, 5000, 4000, 2700 and 1400a BP reported in the North Atlantic and other regions in China.

Blowing dust events are common meteorological phenomena that can influence the atmospheric vertical electric field (E_z). In this work we examine different local blowing dust days in Lubbock, Texas in order to understand their impact on the local vertical electric field, and the relationships of the vertical E_z to horizontal wind speeds, visibility, relative humidity, temperature and PM_2.5 (particles with aerodynamic diameter <2.5 μm). All blowing dust days had an increase in E_z, but they did not generate similar E_z patterns. Several of the blowing dust days had an expected increase in E_z with a reduction of visibility and an increase in PM_2.5, wind speed, or wind gusts. But others were more complex without a direct relationship between E_z and PM_2.5. Variability of E_z at different time scales was examined, including correlation tests with wind speed, visibility, PM_2.5, temperature, or relative humidity for each blowing dust event and overall. The complexity found, including a decorrelation time scale between wind speeds and E_z at relatively high frequencies, emphasizes the sensitivity of the dust events in this region and the dependence on the analysis interval used in this study.

The fluvial-aeolian interaction field of the Guandacol valley (northwest Argentina) is studied through different methodologies that comprise satellite, drone and GPR images, bedform descriptions, and grain-size analysis. The obtained information allowed the recognition of five depositional subenvironments: 1. Dune patches; 2. Aeolian sand sheets; 3. Muddy plains; 4. Active channels; and 5. Abandoned and secondary channels. Moreover, the lithofacies pattern, sedimentary structures and geometry of the beds permitted the definition of ten architectural elements in both channel and interchannel areas. The channel architectural elements comprise active channels with intercalations of aeolian deposits (CHe), abandoned or secondary channels (CHa), aeolian sand ramp (CHsr), lateral bars (CHlb), and aeolian mesoforms (CHem). The architectural elements in the interchannel area include active dunes (Fad), fixed or low-migration rate dunes (Fsd), partially flooded interdunes (Ffd), sandy flats (Fsf), and muddy plains (Fmp). A model of the evolution of fluvial-aeolian interaction environment is proposed in which three types are recognized: dry, intermediate, and wet. Migratory dunes and sand sheets environments dominate the dry interaction systems in the floodplain (Fad, Fsf). At the same time, during the intermediate stage, Fsd and Fsf architectures prevail, together with fluvial bars with thin aeolian intercalations into the channels (CHe and CHa). The wet systems consist of flooded interdunes, muddy plains, and different types of aeolian mesoforms in the channel (Fmp, Ffd and CHem).

The analysis of present-day depositional subenvironments in the Guandacol valley and the definition of architectural elements serve as a potential analogue for studying ancient fluvial-aeolian interaction environments.