Saltation Activity Research Articles

Saltation Activity on Non-Dust Days in the Taklimakan Desert, China

Dust aerosols persistently affect nearly all landscapes worldwide, and the saltation activity caused by dusty weather (e.g., dust days) releases considerable amounts of aerosol into the atmosphere. Nevertheless, dust-induced saltation activity may also occur on non-dust days. To date, few studies have investigated the saltation activity characteristics on non-dust days. Moreover, the contribution of non-dust days to the total saltation activity remains ambiguous. This study comprehensively investigates the characteristics of saltation activity on non-dust days. Specifically, we analyze the influence of the saltation activity of non-dust days on dust aerosols by utilizing saltation, atmospheric, soil, dust aerosol (i.e., PM10 and aerosol optical depth), and weather record data obtained from the Taklimakan Desert, China, between 2008 and 2010. Our results show that lower temperature, vapor pressure, and soil moisture on non-dust days reduces the saltation threshold velocity (5.9 m/s) more compared to on dust days (6.5 m/s). Furthermore, regarding wind speed, relatively strong monthly saltation activity occurs from March to August, and daily saltation activity occurs from 9:00 to 16:00. Although non-dust days only contribute 18.5% and 7.7% to saltation time and saltation count, respectively, both significantly influence the dust aerosols. Therefore, the effect of saltation activity on non-dust days cannot be undervalued, particularly while performing dust aerosol studies.

Improved parameterization for effect of soil moisture on threshold friction velocity for saltation activity based on observations in the Taklimakan Desert

Soil moisture is an important factor in saltation activity as it affects the threshold friction velocity. Based on observed data of wind speed, soil moisture, and saltation from March 2008 to February 2018 in the Taklimakan Desert, the effects of soil moisture on the threshold friction velocity for saltation activity are analyzed, and an improved parameterization for the effect of soil moisture on threshold friction velocity is developed. The results illustrate that soil moisture significantly impacted threshold friction velocity when the soil moisture was greater than 0.025 m3/m3 in the study area. The corrected parameterization of the threshold friction velocity of moist soil can effectively lower the sensitivity of threshold friction velocity to soil moisture. The threshold friction velocities were 48.7%, 49.5%, and 32.1% lower, and the horizontal dust fluxes were 66.6%, 69.2%, and 65.8% greater for soil moistures of 1.0%, 2.0%, and 5.0%, respectively. Compared with the observed horizontal dust fluxes, the error is reduced by 80.2% when the corrected parameterization was used, and the correlation coefficient between the calculated result and the observations increased from 0.87 to 0.91. The results demonstrate that the effects of soil moisture on dust emissions are nonnegligible.

Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

AbstractWind‐blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind‐driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field‐based estimates of distinct fluid and impact thresholds from high‐frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near‐continuous transport conditions. Although both thresholds are thus important for high‐frequency saltation prediction, our results suggest that the time‐averaged saltation flux is primarily governed by the impact threshold.

Saltation activity and its threshold velocity in the Gurbantunggut Desert, China

A field experiment was conducted from 2 May 2010 to 1 May 2012 in the Gurbantunggut Desert, the second largest desert in China, to investigate saltation activity and its threshold velocity, and their relations with atmospheric and soil conditions. The results showed that saltation activity occurred more frequently during 08:00–20:00 Local Standard Time in spring and summer, with air temperatures between 20.0 and 29.0 °C, water vapor pressures between 0.6 and 0.9 kPa, soil temperatures between 25.0 and 30.0 °C, and a soil moisture lower than 0.04 m3/m3. At 2 m height, the saltation threshold velocity varied between 11.1 and 13.9 m/s, with a mean of 12.5 m/s. Threshold velocity showed clear seasonal variations in the following sequence: spring (11.7 m/s) < autumn (12.7 m/s) < summer (13.6 m/s). Affected by soil conditions, aeolian sand transport was weak, with an average annual aeolian sand that transported across a section (1.0 m × 2.0 m) of less than 6.0 kg.

Threshold Velocity for Saltation Activity in the Taklimakan Desert

The threshold velocity is an indicator of a soil’s susceptibility to saltation activity and is also an important parameter in dust emission models. In this study, the saltation activity, atmospheric conditions, and soil conditions were measured from 1 August 2008 to 31 July 2009 in the Taklimakan Desert, China. the threshold velocity was estimated using the Gaussian time fraction equivalence method. At 2 m height, the 1-min averaged threshold velocity varied between 3.5 and 10.9 m/s, with a mean of 5.9 m/s. Threshold velocities varying between 4.5 and 7.5 m/s accounted for about 91.4% of all measurements. The average threshold velocity displayed clear seasonal variations in the following sequence: winter (5.1 m/s) < autumn (5.8 m/s) < spring (6.1 m/s) < summer (6.5 m/s). A regression equation of threshold velocity was established based on the relations between daily mean threshold velocity and air temperature, specific humidity, and soil volumetric moisture content. High or moderate positive correlations were found between threshold velocity and air temperature, specific humidity, and soil volumetric moisture content (air temperature r = 0.75; specific humidity r = 0.59; and soil volumetric moisture content r = 0.55; sample size = 251). In the study area, the observed horizontal dust flux was 4198.0 kg/m during the whole period of observation, while the horizontal dust flux calculated using the threshold velocity from the regression equation was 4675.6 kg/m. The correlation coefficient between the calculated result and the observations was 0.91. These results indicate that atmospheric and soil conditions should not be neglected in parameterization schemes for threshold velocity.

Soil Wind Erodibility Based on Dry Aggregate‐Size Distribution in the Tarim Basin

The Tarim Basin is an important source of airborne particulate matter that contributes to poor air quality in China. However, little attention has been given to estimating wind erodibility of soils in the region. The objective of this study was to determine the soil wind erodibility for four land use types in the Tarim Basin. Wind erodibility was determined from the dry aggregate size distribution of soils collected from desert, farmland, forest, and shrubland environments in spring 2013. Our analysis revealed that the percentage of soil mass composed of aggregates &lt;840 μm in diameter (erodible fraction) ranged from 5.2 to 99.9% across sites with a mean of 58.9% across land use types. The four land use types had a high content of erodible aggregates. The mean percentage of soil mass which was comprised of saltation‐size (500–100 μm in diameter) and suspension‐size material (&lt;100 μm in diameter) was 15.0 and 39.0%, respectively, across land use types. The desert had the highest potential for saltation activity and suspension. Our results suggest that all land use types examined in this study have potential to contribute to the atmospheric dust load in the Tarim Basin and to transcontinental and transoceanic transport of dust.

Application of a satellite-based aridity index in dust source regions of northeast Asia

A modified temperature-vegetation dryness index based on land-surface temperature was used to represent land-surface aridity in northeast Asia. Past results indicated that saltation activity and dust emission did not often occur in northeast Asia when soil water content was greater than 4% and normalized soil water content was greater than 0.2. The threshold value of the modified temperature-vegetation dryness index for dust emissions was assumed to be 0.8 when the normalized soil water content was 0.2. However, a detailed grid of meteorological data is needed to predict the modified temperature-vegetation dryness index; therefore, large-scale use is presently difficult. A new satellite-based aridity index using day/night land-surface temperature differences has been introduced and tested to represent the modified temperature-vegetation dryness index. The satellite-based aridity index was 0.03 when the modified temperature-vegetation dryness index was 0.8, which is the threshold for dust emissions. The relationship between coverage of satellite-based aridity index values greater than 0.03 in the target area (35°N–45°N and 100°E–115°E) and Asian dust events over Japan was analyzed for 2000–2011. Results indicated that coverage of satellite-based aridity-index values greater than 0.03 significantly affects Asian dust events over Japan (R2 = 0.4117, p < 0.05).

Frequency, magnitude, and characteristics of aeolian sediment transport: McMurdo Dry Valleys, Antarctica

Due, in part, to the challenging environment of Earth's high‐latitude regions, available information on cold climate effects on aeolian processes in these areas remains limited. Data from these areas, however, provide insight into the physics of sediment transport by wind and the controls on erosive winds in proximity to ice caps and topographic influences. This study presents a 2 year record of meteorological, saltation activity, horizontal saltation flux, and particle size distribution data from four sites in the McMurdo Dry Valleys of Antarctica, 2008 to 2010. Saltation measurements revealed daily and seasonal patterns with spring and summer sediment transport events occurring between 09:00 and 24:00 hours due to thermally generated winds. Fall and winter events occur at any time of day with the strongest associated with foehn winds. Threshold wind speed at 4.2 m in all seasons for all locations was ≈10 m s−1. Saltation occurred in the temperature range −40°C to +5°C. Westerly winds in the fall/winter and easterly winds in spring/summer are associated with the majority of transport events. The sand in transport is mainly 250 to 500 µm in diameter and poorly sorted. The integrated saltation flux varies over three orders of magnitude among the sites, with the lowest mean flux recorded in the Taylor Valley (2.9 kg m−1 day−1) and the highest in the eastern Victoria Valley (2271 kg m−1 day−1) for 24 hours of continuous saltation. The percentage of time saltation active at these locations annually is ≈2%, ≈4%, and ≈13%, respectively, for the Victoria, Taylor, and Wright Valleys.

Observation of saltation activity at Tazhong area in Taklimakan Desert, China

A two-year field observation of saltation activity was carried out at Tazhong area, the hinterland area of the Taklimakan Desert with highly frequent dust storms. From 1 September 2008 to 31 August 2010, a piezoelectric saltation sensor (Sensit) was used to continuously collect the data on saltation activity at a level sand surface. Analysis on the data suggests that saltation activity can occur at any time of the year when conditions are favorable; however, the necessary conditions are rarely satisfied in most time. In the daytime of spring or summer, saltation activity can persist even over a continuous one-hour-or-so period. It is found that, from 1 September 2008 to 31 August 2010, saltation activity accounts for more than 3% of the total yearly time, and it tends to peak in spring and summer months with strong winds. During winter months when winds are weak, however, it is often at a minimum. It seems that precipitation does not appear to be significant in reducing saltation activity in arid regions like Tazhong.

Diurnal variations of saltation activity at Tazhong: the hinterland of Taklimakan Desert

The Taklimakan Desert of China is a region of frequent sandstorms and, thus, is a major sand and dust source area. Tazhong, a small mining village, is located near the center of the Taklimakan Desert at a distance of 220 km from the desert margins. Near Tazhong, we conducted a 2-year field investigation designed to monitor the diurnal variation of saltation activity using fast-responding piezoelectric saltation sensors (Sensits). Results suggest that saltation activity tends to occur more frequently during daytime in all seasons, relatively high levels of saltation activity are maintained from around 11:30 to around 16:30 local standard time (LST), because of stronger wind speed, higher soil temperature and lower relative humidity. During the spring and summer seasons, the saltation activity can occur at any time of the day, while there are some periods with zero saltation seconds at night and in the early morning during autumn and winter seasons. The results confirm that sandstorms tend to occur more frequently during daylight hours, so it may be helpful to forecast and guard against the occurrence of blowing sand or sandstorms in the Taklimakan Desert.

A field experiment on dust emission by wind erosion in the Taklimakan Desert

Dust emission by wind erosion in surface is a serious problem in many arid regions around the world, and it is harmful to the ecological environment, human health, and social economy. To monitor the characteristics of saltation activity and to calculate the threshold wind velocity and sediment discharge under field conditions have significance on the research of dust emission by wind erosion. Therefore, a field experiment was conducted over the flat sand in the hinterland of the Taklimakan Desert. One sampling system was installed on the flat sand surface at Tazhong, consisting of a meteorological tower with a height of 2 m, a piezoelectric saltation sensor (Sensit), and a Big Spring Number Eight (BSNE) sampler station. Occurrence of saltation activity was recorded every second using the Sensit. Each BSNE station consisted of five BSNE samplers with the lowest sampler at 0.05 m and the highest sampler at 1.0 m above the soil surface. Sediment was collected from the samplers every 24 h. It is found that saltation activity was detected for only 21.5% of the hours measured, and the longest period of saltation activity occurring continuously was not longer than 5 min under the field conditions. The threshold wind velocity was variable, its minimum value was 4.9 m s−1, the maximum value was 9.2 m s−1, and the average value was 7.0 m s−1. The threshold wind velocity presented a positive linear increase during the measurement period. The observation site had a sediment discharge of 82.1 kg m−1 over a period of 24 h. Based on hourly saltation counts, hourly sediment discharge was estimated. Overall, there was no obvious linear or other functional relationship between the hourly sediment discharge and wind velocity. The results show that the changes of sediment discharge do not quite depend on wind velocity.

The response of saltation to wind speed fluctuations

ABSTRACTThe response time of saltation to spatial or temporal wind speed fluctuations constitutes an important control parameter for aeolian sediment transport and deposition. In this paper, we present direct measurements of the response time obtained from several field experiments.The sand transport was studied using six small microphones arranged in a vertical profile and collocated with a sonic anemometer, a webcam and a cup anemometer tower. The webcam was coupled with the sonic anemometer via a personal computer and provides information on creeping and saltating grains with a sampling rate of 10 Hz. Sediment transport measurements were obtained over four periods.The Wiener filter, a signal processing technique, is used to obtain a discrete transfer function that relates the horizontal wind speed and the non‐intermittent sand transport. The transfer function can be established using an exponential function with a time constant or characteristic response time τ without time shift. The response time fluctuated between zero and 1·5 seconds depending on the turbulence intensity, the saltation activity, the measuring height and sampling rates. The Wiener filter coefficients suggest that the response of saltation to wind speed alterations is determined by more than one process. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A wind tunnel study of aeolian sand transport on a wetted sand surface using sands from tropical humid coastal southern China

This article reported a wind tunnel test of sediment transport related to surface moisture content and wind velocity using sands from tropical humid coastal area. A 1 mm-thick portion of surface sand was scraped using a self-made sediment sampler, and the gravimetric moisture content was determined. Sand transport was measured via a standard vertical sand trap with a 60 cm height. The result shows that the sand transport profile above the wet surface can be expressed with an exponential equation. In general, the influence of moisture content on sand transport profile mainly focuses on the bottom of the blowing sand cloud. Meanwhile, with moisture content increased, total sand transport dropped, and a relatively larger proportion is transported at greater heights. The vertical movement of particles on higher moisture surface (0.587% 1.448%. The total sand transport rate varying with moisture content is divided into three regions of differing gradient at the moisture contents of 0.587 and 1.448%. The gradient of the curve reflected the different influences of the various water forms in surface sediments. The higher moisture surface (M > 1.448%) merely functions as a transport plain for the saltation material. Surface moisture content was the dominant control factor for saltation activity between the moisture contents of 0.587 and 1.448%, wind velocity could resume control saltation after the surface dried to the extent (M < 0.587%).

Threshold wind velocities for sand movement in the Mescalero Sands of southeastern New Mexico

Saltation activity and threshold velocities were measured at two Chihuahuan Desert sites located in the Mescalero Sands to the east of Carlsbad, New Mexico. Sand movement was detected with piezoelectric saltation sensors, and these data were used to calculate saltation activity and threshold wind speeds for the two sites. Significant blowing events were observed on nine days of this 112-day study, and it was only during these active periods that threshold values could be calculated. During periods of sand transport, threshold values were calculated every 5 min. Average threshold values typically exceeded 10.5 m/s at both sites; these are about twice those measured previously at a less vegetated dune site in West Texas. Less saltation activity occurred at the Gnome site compared with the Near Field site, especially during April; and while this can be partially attributed to differences in wind speed, additional studies are needed to evaluate other important factors such as the influence of undulating terrain and complex vegetation. Nevertheless, the results demonstrate the feasibility of long-term unattended monitoring of wind erosion activity in remote locations and that reliable and consistent estimates of threshold velocity can be obtained in complex desert environments using the time-fraction-equivalence principle.

Aeolian sediment transport following wildfire in sagebrush steppe

Wind erosion of soil is an appreciable but unstudied event following fires in cold desert. We examined aeolian transport of sediment for 1 year following fire in semi-arid shrub steppe on loess soils in southern Idaho, USA. Sediment collectors were used to determine horizontal mass transport of soil and saltation sensors and anemometers were used to determine saltation activity (fraction of time having saltation) and threshold wind speed in an area burned in August and an unburned control site. Horizontal mass transport (per 30-day period) was negligible in the unburned area, but in the burned area was 5.40 kg m −1 in October and decreased to 2.80 kg m −1 in November and 0.32 kg m −1 in December. Saltation activity was high enough to determine threshold wind speeds only in the burn site during fall, when values ranged from 10.0 to 10.6 m s −1. Sediment flux and saltation activity in the burned site became much less pronounced following the emergence of herbaceous vegetation in the spring. Post-fire sediment flux in the shrub steppe we examined was of greater magnitude but shorter duration than post-fire fluxes in warm deserts or sandier regions that experience more frequent wind erosion.

Threshold wind velocity as an index of soil susceptibility to wind erosion under variable climatic conditions

AbstractWind erosion starts when the threshold wind velocity (µt) is exceeded. We evaluated the sensitivity of µt to determine the wind erosion susceptibility of soils under variable climatic conditions. Three years field data were used to calculate µt by means of the equation µt = ū ‐ σ Φ−1 (γ), where ū is the mean wind speed (m s−1), σ the ū standard deviation (m s−1), γ the saltation activity and Φ the standard normal distribution function of γ. Saltation activity was measured with a piezoelectric sensor (Sensit). Results showed that ū of the whole studied period (3·41 m s−1) was lower than µt (7·53 m s−1), therefore, wind erosion was produced mainly by wind gusts. The µt values ordered in the sequence: Winter (6·10 m s−1) &lt; Spring (8·22 m s−1) = Summer (8·28 m s−1) &lt; Autumn (26·48 m s−1). Higher µt values were related to higher air humidity and lower wind speeds and temperatures. The µt values did not agree with the erosion amounts of each season, which ordered as follows: Summer (12·88 t ha−1) &gt; Spring (3·11 t ha−1) = Winter (0·17 t ha−1) = Autumn (no erosion). Low µt and erosion amounts of Winter were produced by a scarce number of gusts during eroding storms. We concluded that µt is useful as an index of soil susceptibility to wind erosion of different climatic periods. The use of a unique µt value in wind erosion prediction models can lead to erroneous wind erosion calculations. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The role of wind and splash erosion in inland drift-sand areas in the Netherlands

European drift-sand areas reached their maximum size in the 19th century. From the late 19th until the mid-20th century most drift-sand areas in Europe were successfully reforested. Today drift-sand formation can still be studied in the Netherlands. However, due to change in land-use, their limited size, eutrophication and colonisation by invasive exotics, the area occupied by active drift sand is rapidly declining and with it its unique fauna and vegetation. Between 2001 and 2005, experiments and observations were carried out at the inland drift-sand area Kootwijkerzand (central Netherlands) to know more about the extent, the intensity and the role of wind and splash erosion in a drift-sand ecosystem. Measurements were carried out on the horizontal sediment flux, the vertical accumulation flux, the raising and lowering of the surface, the organic matter content of the topsoil, the saltation activity and the weather conditions. An attempt was also made to evaluate the effect of various measures to re-activate erosion. Without sufficient erosion the scattered drift-sand vegetation will rapidly change into closed, grass-dominated vegetation. The role of erosion in the drift-sand ecosystem is therefore crucial: erosion slows down the succession rate because it causes the regular deposition of nutrient-poor sediment, and facilitates the regular renewal of the vegetation by creating new locations with extremely poor conditions for colonisation (blow-outs). On bare deflation plains wind erosion is the dominant process. Here the transport generated by splash drift can be of the same magnitude as during a moderate wind erosion event, but splash drift events are less frequent and also shorter than wind erosion events. They normally represent less than 10% of the total sediment transport. In the scattered drift-sand vegetation areas, on the other hand, splash erosion and splash drift dominate. Here the organic matter content of the transported (and later deposited) sediment is equal to the organic matter content of the parent soil, showing that splash (drift) has been the main transport mechanism. Biological crusts developed in these areas may later reduce the influence of splash. High transport rates are needed to reactivate wind erosion in vegetated areas. Under the present conditions the erosion activity in most drift-sand areas is small. Heavy wind erosion is only observed during extreme storms.

Simultaneous observations of the critical aeolian threshold of two surfaces

Threshold is an important parameter in wind erosion research and in the field of aeolian research in general. A new technique was recently developed that provides a means of determining threshold with a sampling system that continuously collects wind data along with critical information regarding saltation activity. By employing two identical sampling systems, it was possible to monitor the threshold of a highly mobile sand surface while simultaneously monitoring the threshold of a less mobile playa surface. Results indicate that threshold could be measured at both locations with enough precision to establish clear differences between the surfaces. The sandy surface at the Morgenstern Dunes site was considerably more active than the Yellow Lake playa site over the 113-day sampling period because of its consistently lower threshold. The Morgenstern site tended to maintain a fairly constant threshold of around 5.4 to 5.5 m/s whereas the threshold of the Yellow Lake playa surface varied from a low of 6.4 m/s to values greater than 13.3 m/s. Limitations of this method lie in the fact that threshold can be determined only when winds are blowing sufficiently strongly to cause sediment transport.

A method for establishing the critical threshold for aeolian transport in the field

AbstractA basic feature of any wind‐eroding surface is its threshold – the wind speed at which sediment transport is initiated. A new method was developed and tested that allows for the rapid determination of threshold under natural wind conditions in the field. A mathematical expression that relates saltation activity and relative wind strength was reformulated so that threshold may be calculated from measurements of saltation activity and the mean and standard deviation of wind speed. To test the new method and determine its usefulness, a field experiment was performed within a region of low‐relief dunes on the Southern High Plains of West Texas. The experimental system consisted of a 2‐m meteorological tower and a piezoelectric saltation sensor. It was found that during periods of active aeolian activity, threshold values could be calculated every 5 minutes. This new method allows for routine monitoring of surface threshold conditions in the field. Example threshold calculations are presented and they demonstrate that the method works well. Published in 2004 by John Wiley &amp; Sons, Ltd.

Thresholds of aeolian sand transport: establishing suitable values

AbstractThis paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout &amp; Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.