Chunlai Zhang

Cogitation on developing a dynamic model of soil wind erosion

Studies on soil wind erosion began with single factors affecting soil wind erosion; with increasing quantities of data being accumulated, the wind erosion equation (WEQ), the revised wind erosion equation (RWEQ), the wind erosion prediction system (WEPS), and other soil wind erosion models have been successively established, and great advances have been achieved. Here we briefly review the soil wind erosion research course and analyze the advantages and disadvantages of the current soil wind erosion models. From the perspective of the dynamics of wind erosion, we classified the factors affecting soil wind erosion into three categories, namely, wind erosivity factors (WEF), soil antierodibility factors (SAF), and roughness interference factors (RIF). We proposed the concept of a standard plot of soil wind erosion to solve the problem of uncertainty of the soil wind erosion modulus on a spatial scale, and provided methods to set similarity conditions in wind tunnel simulation experiments and to convert the spatial scale of the wind erosion modulus from the standard plot to a large scale field. We also proposed a conceptual model on the basis of the dynamics of soil wind erosion with the theoretical basis that wind produces a shear force on the soil surface. This shear force is partitioned by barely erodible soil surfaces and roughness elements on the ground, and the amount of soil loss by wind should be calculated by comparing the shear force of the wind on barely erodible soil surfaces with the anti-erosion force of the surface soil. One advantage of this conceptual model is that the calculated soil wind erosion modulus is not subject to changes of spatial scale. Finally, we recommended continual improvement of the existing models while also establishing new models.

Monitoring of aeolian desertification on the Qinghai-Tibet Plateau from the 1970s to 2015 using Landsat images

Aeolian desertification, one of the most serious environmental issues, has hampered socioeconomic development on the Qinghai-Tibet Plateau (QTP). However, research on aeolian desertification in this region has been limited. To develop a set of science-based preventive measures to mitigate desertification in this region, it is first necessary to clarify the status, evolution, and driving factors of aeolian desertification. In this study, based on extensive field investigations and a current classification system for aeolian desertification, we established a new system for interpreting aeolian desertified land (ADL) on the plateau using Landsat images from 1977, 1990, 2000, 2010, and 2015 and obtained the distribution of ADL through visual interpretation of the images. The results showed that ADL covered 392,914km2 (15.1% of the study area) in 2015, including gravel ADL, sandy ADL, and aeolian monadnocks. Controlled by climate, landforms, the type of Quaternary deposit, and human activities, ADL is scattered throughout the plateau but is concentrated mostly in the western and northern parts. Aeolian desertification on the plateau expanded from 1977 to 2000 and then began to reverse. The evolution during the study period is the result of the combined effects of natural and human factors. Irrational human activities were the dominant factor responsible for the expansion of ADL prior to 2000, whereas the subsequent reversal was mainly caused by climate change combined with large ecological restoration projects.

Sand flux and wind profiles in the saltation layer above a rounded dune top

The near-bed airflow and the movement of sand dune sediments by wind are fundamental dune geomorphological processes. This research measured the wind profiles and sand mass flux on the rounded top of a transverse dune at the southern edge of the Tengger Desert to examine how to best predict the vertical profile of sand flux. This work also tested the accuracy of previously developed models in predicting the apparent roughness length during saltation. Results show that mass flux vertical distribution over the dune top is underestimated by an exponential function, overestimated by a power function, but closely matches the predictions made using the LgstcDoseRsp function. Given suitable values of α, β and γ according to the grain size composition, Sørensen equation with the peaked shape of the mass transport curve will well predict the dimensionless mass flux qg/ρu*3 against dimensionless shear velocity u*/u*t. The modified Charnock model works best of the previously published models tested, with an R2 of 0.783 in predicting the enhanced roughness over the moving sand surface, as opposed to an R2 of 0.758 for the Owen model and an R2 of 0.547 for the Raupach model. For the rounded dune top in this study, Cm=0.446±0.016.

Sediment grain–size characteristics and relevant correlations to the aeolian environment in China's eastern desert region

To identify characteristics of aeolian activity and the aeolian environment in Chinas eastern desert region, this study collected surface sediment samples from the main desert and sandy lands in this region: the Hobq Desert and the Mu Us, Otindag, Horqin, and Hulunbuir sandy lands. We analyzed the grain-size characteristics and their relationships to three key environmental indicators: drift potential, the dune mobility index, and vegetation cover. The main sediment components are fine to medium sands, with poor (Hulunbuir) to moderate (all other areas) sorting, of unimodal to bimodal distribution. This suggests that improved sorting is accomplished by the loss of both relatively coarser and finer grains. Since 2000, Chinas eastern desert region has generally experienced low wind energy environmental conditions, resulting in decreased dune activity. In the Hobq Desert, however, the dry climate and sparse vegetation, in conjunction with the most widely distributed mobile dune area in the eastern desert region, have led to frequent and intense aeolian activity, including wind erosion, sand transport, and deposition, resulting in conditions for good sediment sorting. In the Mu Us, Otindag, and Horqin sandy lands, mosaic distribution has resulted from wind erosion-dominated and deposition-dominated aeolian environments. In the Hulunbuir Sandy Land, high precipitation, low temperatures, and steppe vegetation have resulted in well-developed soils; however, strong winds and flat terrain have created an aeolian environment dominated by wind erosion.

Unsteady aeolian saltation

Abstract.Wind velocity and saltating grain count rate in the natural unsteady aeolian sediment transport are synchronously measured on the gently inclined windward slope of one horn of a large barchan. The obtained time series of these two variables are analyzed, by using the improved complete ensemble empirical mode decomposition and wavelet coherence, to investigate the wind-saltation interactions at different timescales. It is found that the wind-saltation trend relation obeys the traditional low-order polynomial expressions, and saltation mode is roughly proportional to its corresponding wind mode if they are strongly correlated. As a conclusion, it is probable to partly predict instantaneous saltation activities near the surface by the empirical trend and effective modes of wind speed at a given height.Graphical abstract

Wind tunnel tests of the dynamic processes that control wind erosion of a sand bed: The dynamic processes responsible for wind erosion

Aeolian sand transport is a complicated process that is affected by many factors (e.g. wind velocity, sand particle size, surface microtopography). Under different experimental conditions, erosion processes will therefore produce different results. In this study, we conducted a series of wind tunnel experiments across a range of wind velocities capable of entraining sand particles (8.0, 10.0, 12.0, and 14.0m s) to study the dynamic changes of the shear velocity, aerodynamic roughness length, and sand transport. We found that the shear velocity and aerodynamic roughness length are not constant; rather, they change dynamically over time, and the rules that describe their changes depend on the free-stream air velocity. For wind tunnel experiments without feeding sand into the airflow, the sand bed elevation decreases with increasing erosion time, and this change significantly affected the values of shear velocity and aerodynamic roughness length. A Gaussian distribution function described the relationships between the sand transport rate (qT) and the duration of wind erosion (T). It is therefore necessary for modelers to consider both deflation of the bed and the time scale used when calculating sand transport or erosion rates.

Experimental Investigation on Shear-Stress Partitioning for Flexible Plants with Approximately Zero Basal-to-Frontal Area Ratio in a Wind Tunnel

Shear-stress partitioning is investigated for one type of flexible plant for very small values of the basal-to-frontal area ratio σ (0.001–0.007). The plant model is made of plastic with irregular structures, which are different from previously investigated rigid regular or flexible roughness elements with larger σ values. The distribution of the surface shear stress and the total shear stress at four plant densities with five plant heights are measured in a wind tunnel using Irwin-type sensors and a load cell, respectively. The wind-tunnel experiments prove that, for these flexible plants, the plant height and lateral cover usually decrease with increasing friction velocity, especially for taller plants, while the plant coverage generally increases. However, these characteristics may be inconsistent with flexible roughness elements with very large σ values (and usually very low aspect ratios) because these elements are less flexible. The present flexible plants generally result in lower shear-stress ratios compared with other roughness elements, which is also proven by the higher values of β (the ratio of the drag coefficient of an isolated roughness element to that of the bare surface) and a constant m (accounting for the difference between the average and peak surface shear stresses) from the present experiments (β = 184–210 and m = 0.68–0.79). The peak mean stress ratio of the present flexible plants is not a constant (1.07–1.54) because it is affected by the lateral cover, which is different from previous studies that consider the ratio to be constant without regard for the lateral cover.

Influence of dust storms on atmospheric particulate pollution and acid rain in northern China

Northern China is the area with the highest incidence of dust storms in the world, which are the main sources of its soil dust emissions. In addition, the region consumes huge amounts of fossil fuels and has serious atmospheric particulate pollution. Existing observation results show that a single dust storm has significant influence on atmospheric particulate pollutant concentrations and precipitation acidity. Proving the influence of dust storms on atmospheric particulate pollution, acid rain, and the acid rain ratio and determining whether there is a causal relationship among them on a longer time scale will help us recognize the impact of dust storms on the atmospheric environment. This paper proves that dust storms are the direct cause of the variations in the number of acid rain days and acid rain ratio, as well as the changes in atmospheric particulate pollution, in spring by using the Granger Causality Test and correlation analysis methods based on 1993 to 2007 data, including the number of days of dust storms, atmospheric particulate pollution, and acid rain. Atmospheric particulate pollution is the direct cause of variations in the number of acid rain days and the acid rain ratio in spring; for the other seasons, additional data combined with atmospheric particulate pollution are needed to explain the causes of the acid rain day and ratio changes.

The Characteristics and Distribution of the Source Area of Aeolian Sand in the Valley Lhasa Rivers Lower Reaches, Tibet, China

Based on IKONOS and Quickbird interpretation and field survey, the effects of human activities and characteristic of sand distribution, grain diameter of sand sources and vegetation in the valley of lower reaches of Lhasa River were discussed. The results indicate that valley sand sources distribute along river valley and its two sides, in patch and particle form from slope to bottom of valley, and have a close interrelation with large, medium and small scale of wind field in space; There are three categories of accumulation substances which are sources of sand in the lower reaches of Lhasa River valley: river alluvium, flood deposit and weathering substances of mountain rock. Among them, river alluvium is the most important sand source, grain diameter of sand source is mainly made of silver sand, extreme silver sand and sticky sand, over 90% of whose grain size is less than 0.15mm, and tends to produce wind erosion. Aeolian sand activity is the primary factor affecting sand-nurtured vegetation, especially on movable and semi-movable land is affected most seriously, the species and coverage of vegetation can well reflect the stability of sand sources. Vegetation succession is developing in the tendency in favor of aeolian sand activities, which is one of a main driving factor and outcome of worsening aeolian sand activities. Natural factors are the main cause of the aeolian sand activities in this region, and human activities will further strengthen aeolian sand activities.