Effect of stones on the sand saltation threshold during natural sand and dust storms in a stony desert in Tsogt-Ovoo in the Gobi Desert,Mongolia

Non-erodible elements such as stones and vegetation are key to controlling wind erosion and dust emission in drylands.Stony deserts are widely distributed in the Gobi Desert,but the effect of stones on wind erosion and dust emission have not been well studied,except under artificial conditions.In this study,we evaluated the effect of stones on wind erosion and dust emission by measuring the sand saltation threshold in a stony desert in Tsogt-Ovoo in the Gobi Desert,Mongolia,under natural surface conditions during sand and dust storms.We quantified the amount of stones by measuring the roughness density,and determined the threshold friction velocity for sand saltation by measuring wind speed and sand saltation count.Our results showed that the threshold friction velocity increased with the roughness density of stones.In the northern part of the study area,where neither a surface crust nor vegetation was observed,the roughness density of stones was 0.000 in a topographic depression(TD),0.050 on a northern slope(N.SL),and 0.160 on the northern mountain(N.MT).The mean threshold friction velocity values were 0.23,0.41,and 0.57 m/s at the TD,N.SL,and N.MT sites,respectively.In the southern part of the study area,the roughness density values of stones were 0.000 and 0.070-0.320 at the TD and southern slope sites,respectively,and the mean threshold friction velocities were 0.23 and 0.45-0.71 m/s,respectively.We further compared the observed threshold friction velocities with simulated threshold friction velocities using Raupach's theoretical roughness correction and the measured roughness density values,and found that Raupach's roughness correction worked very well in the simulation of threshold friction velocity in the stony desert.This means that the results of our stone measurement can be applied to a numerical dust model.

THE RELATION OF THE THRESHOLD VELOCITY OF COAL DUST TO ITS SIZE AND HUMIDITY

On the ground of wharf and a station where coal is piled up, the sources polluting the atmosphere are mainly the raised coal dust. According to the principle of mechanics, with the consideration of the gravities of coal dust particles and liquid droplets, coal dust size and humidity, under the condition of force balance when the coal dust was raised, the authors obtaineda theoretical formula for the threshold velocity of coal dust with humidity,Finally, the theoretical values from the formula gave a good agreement with the experimental data from some wind tunnels.

Estimation of the Threshold Friction Velocities over Various Dust Storm Source Areas in Northwest China

The emission of dust particles into the atmosphere is governed by the aerodynamic and resistant factors, which are quantified by the friction velocity u. and the threshold friction velocity u＊t, respectively. The threshold friction velocity u＊t influences the vertical dust flux and dust transport. Based on the micro-meteorological data obtained in the springs of 2004 and 2006 over Hunshandake desert area, Loess Plateau, and Gobi desert area, the relationship between dust concentration and friction velocity for the dust events that occurred over Hunshandake desert area was investigated, and the threshold friction velocities over the three different dust source areas were estimated. The results show that the value of dust concentration is low during the pre-emission stage of a dust storm event, and the rapid increase of friction velocity provides favor-able dynamic conditions for dust emission. During the dust emission stage, the dust concentration increases sharply due to mechanical and thermal turbulent mixing. At the calm-down stage, the dust concentration drops nearly linearly with the decreasing friction velocity, on account of the gravitational deposition of larger dust particles. When the dust concentration is higher than 200 μgm-3, it is considered as a dust emission process. According to the criteria, the values of threshold friction velocity over Hunshandake desert area and Gobi region are 0.6 and 0.45 m s-1, respectively. The threshold friction velocity over Loess Plateau depends on the wind direction, due to the complex terrain and inhomogeneous surface. The northwest wind shows the effects of the Mu Us desert in the northwest. The corresponding u＊t is 0.35 m s-1. The south wind exhibits the characteristics of the Loess hilly dunes in the south, and the u＊t is 0.7 m s-1. The large roughness length of the Loess hilly dunes and the large inter-particle cohesion for the clay soil texture increases the local friction velocity. Different threshold friction velocities and occurrence frequencies of strong wind account for different dust emission capabilities for various source areas.

A transport-rate model of wind-blown sand

Sand transport by wind plays an important role in environmental problems.Formulating the sand-transport rate model has been of continuing significance,because the majority of the existing models relate sand-transport rate to the wind-shear velocity.However,the wind-shear velocity readapted to blown sand is difficult to determine from the measured wind profiles when sand movement occurs,especially at high wind velocity.Detailed wind tunnel tests were carried out to reformulate the sand-transport rate model,followed by attempts to relate sand-transport rate to parameters of wind velocity,threshold shear-velocity,and grain size.Finally,we validated the model based on the data from field observations.

Are dogs good at spotting movement?Velocity thresholds of motion detection in Canis familiaris

The ability to perceive motion is a fundamental property of the visual system,and one of its most basic aspects is the ability to discriminate moving objects from motionless ones,for example,motion detection.Velocity thresholds represent the minimum rate of displacement over time unit that an animal is able to perceive as movement,any slower motion being not discriminable from a still object.Although such topic has grabbed the attention of scientists already at the beginning of the 20th century,the interest has waned in time,and velocity thresholds have thus far been assessed in only a handful of species.

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.

Experiment on Dust Flux During Duststorm Periods over Desert Area

The present study investigates the characteristics of turbulent transfer and the conditions for dust emission and transport using the dust concentration and micrometeorological data obtained during dust events occurring in the spring of 2004 over the Hunshandake desert area. The turbulent exchange coefficients and turbulent fluxes of momentum and heat are calculated. The relationships between dust flux, friction velocity, and wind speed are also explored. The results show that thermal turbulence is dominant during daytime of non-dusty days. The dynamic turbulence increases obviously and the sensible heat flux reduces by different degrees during dust events. There is an efficient downward transfer of momentum before duststorm occurrence, and both the dynamic turbulence and the thermal turbulence are important in the surface layer. The dynamic turbulence even exceeds the thermal turbulence during severe duststorm events. The values of dust flux vary in the range of -5 5, -30 30, and -200-300 μg m^-2 s^-1 during non-dusty days, blowing dust, and duststorm events, respectively. A slight upward transport of dust is observed during non-dusty days. The dust flux gradually varies from positive to negative during duststorm periods, which indicates the time evolution of dust events from dust rising to stably suspending and then deposition. The dust flux is found to be proportional to u＊^3. The threshold values of wind speed and friction velocity are about 6 and 0.4 m s^-1, respectively.

Relationships between soil moisture and dust emissions in a bare sandy soil of Mongolia

This study presents experimental results of the change in threshold friction velocity and dust emission strength as a function of soil moisture content. The dust and soil moisture data were obtained from irrigated plots of a bare, sandy soil （no vegetation） situated in the Mongolian steppe. Dust flux was measured using the PI-SWERL^（R） device, while soil moisture was measured using gravimetric methods. Our results demonstrate the strong controlling effects of soil moisture on both the threshold friction velocity and dust emission strengths. Threshold friction velocity increased from 0.44 m/s for dry soil （0.002 g/g） to 0.67 m/s for wet soil （0.06 g/g）, confirming the importance of soil moisture for controlling dust events. Dust emission strength was significantly depressed for wet soils, starting at a soil moisture value of 0.02 g/g. From these results, we developed a simple dust diagram that may be useful as part of a warning system to identify initial sensitivity to threshold conditions as well as conditions that could lead to potentially significant dust emissions. Overall, the research findings in this study could be used to provide foreknowledge of conditions that would be conducive to high dust emissions for this area of Mongolia.