Influencing factors of wind erosion

Wind erosion generally refers to the first three stages of dynamic process of wind action.

Wind erosion is a complex physical process of wind-blown sand, and its occurrence and intensity are determined by airflow and underlying surface conditions. Air flow provides power source for particle erosion and transportation, also known as power factor; The condition of the underlying surface determines the ability of the surface to resist wind erosion, which is called the anti-erosion factor. In addition, human activities such as land use, agricultural structure, farming system, grazing intensity and firewood harvesting mode also affect the wind erosion process by changing the underlying surface and airflow, which is the so-called human factor.

It should be noted that there is not necessarily a simple linear relationship between the influencing factors of wind erosion and the wind erosion intensity, and a small change in the influencing factors may lead to a sharp increase in the wind erosion intensity. Experimental research shows that under the action of arid and windy climate, after the local surface vegetation and topsoil are destroyed, the intensity of soil wind erosion will increase sharply, and the speed of Gobi and desertification is several times, ten times or even hundreds times higher than that of areas with good vegetation development. The reason for this phenomenon is that there is a positive feedback relationship between the underlying surface characteristics and wind erosion. Under the condition of constant wind speed, the deterioration of the underlying surface properties leads to the aggravation of wind erosion, which further worsens the underlying surface properties and creates conditions for the further aggravation of wind erosion, thus forming a self-organization process that can gradually strengthen wind erosion without further strengthening of external driving force.

Dynamic factors

The dynamic source of wind erosion is atmospheric circulation, which is produced by weather processes of different spatial scales and mainly controlled by large-scale weather convection system. The erosive force of atmospheric circulation is determined by wind speed, airflow structure and its spatio-temporal dynamics. At present, wind speed is usually used as the main parameter to describe the dynamic factors of wind erosion.

Generally speaking, the greater the wind speed, the more aggressive it is. Theoretically, under the same other conditions, the wind erosion amount and wind erosion modulus increase with the increase of wind speed. However, the experimental observation shows that when the wind speed is too small, it is not erosive. Only when the wind speed exceeds a certain critical value can soil particles rise from the surface and form wind erosion. This critical value is called starting wind speed. The amount of wind erosion depends on the part where the actual wind speed exceeds the initial wind speed. Corresponding to the starting mode of surface particles, there are also two kinds of starting wind speeds: ① fluid starting wind speed (or static threshold), that is, the minimum wind speed that makes soil particles start to move completely under the direct action of wind; (2) Impact threshold wind speed (or dynamic threshold), that is, the minimum wind speed that makes soil particles start to move when there is gravel impact from the upwind direction. Generally speaking, the starting wind speed of fluid is about 20% higher than that of impact.

Wind speed observation in 2 12 area of the United States shows that only the wind with an average wind speed exceeding 5.4m/s per hour is erosive. When the wind speed exceeds the lower limit of erosion, the wind erosion is directly proportional to the cube of the average wind speed. According to the field observation and laboratory measurement in China, when the wind speed is 5 ~ 6 m/s, wind erosion begins to appear; When the wind speed reaches 6 ~ 7 m/s, sandstorm airflow appears; When the wind speed is ≥ 10m/s, with the increase of wind speed, the wind erosion is on the rise, and the change of sediment transport rate follows a power function relationship.

(2) Corrosion resistance coefficient

1. Texture of surface material

Surface material is the object of wind erosion. Different surface materials have different water-stable structure, calcium carbonate content, organic matter, mechanical composition and aggregate structure, and their wind erosion resistance is quite different.

Under the condition of similar wind speed, the wind erosion resistance of loose sand is much lower than that of sandy loam with a certain aggregate structure (table 1 1-3). In addition, the starting wind speed and wind erosion resistance of different particle sizes are different. Silty soil (0.05~0.002mm) and clay (:0.05mm) are difficult to form aggregate structure and have low wind erosion resistance. Soil particles with a diameter of 0.08 ~ 0.25mm have the lowest corrosion resistance. The more gravel content in the soil, the higher the anti-corrosion ability. Physical clay (particle size

In silty loam and sandy loam, calcium carbonate containing 1% ~ 5% will break clods, reduce their mechanical stability and increase wind erosion. In loam sandy soil, with the increase of calcium carbonate content, wind erosion decreases. Soil organic matter is also an important factor affecting wind erosion. Soil with high organic matter content has strong wind erosion resistance, and decomposed organic matter will reduce soil erosion resistance.

Table 1 1-3 Effects of Soil Structure and Material Composition on Wind Erosion Intensity

(According to Wu Benben, 1962)

2. Water content of surface materials

Unsaturated water in loose sediments on the surface is often called soil water, which can form a hydration film on the surface of soil particles. The electrostatic action of hydration film makes soil particles produce adhesive force and bond them together, thus improving the wind erosion resistance of soil. Therefore, with the increase of soil water content, the initial wind speed of particles increases and the wind erosion rate decreases (table 1 1-4). The results show that the critical wind speed of quicksand samples increases linearly with the increase of water content, while that of farmland soil samples increases exponentially with the increase of water content. However, mobile dune soil is more sensitive to water than farmland soil. When the water content of quicksand drops to 2%, the wind erosion is very small, and when the water content increases, the wind erosion will not change obviously, and the critical point of farmland soil water content is about 4%. Another study shows that the existing form of soil water also has an important influence on the starting wind speed: when the soil water changes from hydrated film to capillary water, the anti-corrosion performance of soil will be greatly improved.

3. Vegetation coverage

When there is vegetation on the ground surface, it is equivalent to covering a protective layer, which can significantly weaken the destruction and separation of surface materials by airflow, improve the starting wind speed of surface particles and reduce wind erosion. Wind tunnel experiments on loess show that with the increase of vegetation coverage, wind erosion amount and wind erosion rate decrease sharply at the same wind speed (table 1 1-5). Liu et al. (1992) conducted wind tunnel experiments with sandy soil, and found that the starting wind speed increased rapidly when the vegetation coverage was 60% (table 1 1-6).

Table 1 1-4 Relationship between different soil water contents and wind erosion (experimental wind speed:15.5m/s)

(According to Liu, 1992)

Table 1 1-5 Wind erosion under different vegetation cover (experimental wind speed:12.7m/s)

(According to Dong Zhibao et al. 1996)

Table 1 1-6 Relationship between different vegetation coverage and soil wind erosion

(According to Liu, 1992)

In addition to affecting the erosion resistance of the surface in the erosion area, vegetation also plays an important role in the deposition process of particulate matter. Vegetation can directly "intercept" the particles carried in the near-surface wind-blown sand flow, and make them produce attached deposits. According to the theory of aeolian physics, the settlement of suspended particles by vegetation is not only related to the density (permeability) of vegetation, but also related to the height of vegetation. The higher the vegetation density and height, the stronger the role of settling particles. In addition, the existence of vegetation can increase the roughness, affect the near-surface wind field, hinder the running airflow and cause vortex deceleration, weaken its load capacity, thus producing particle deposition. Specifically, the roughness of grassland, shrub and arbor forest is also very different because of their different heights. Grassland plants are low and have small roughness, generally only a few tens of centimeters; Shrub plants are high, which not only has great roughness, but also produces a certain low wind speed layer above the roughness height. Trees are tall and have the largest roughness, especially the forest belt has a greater influence on wind speed, and the low wind speed layer is thicker.

(3) Human factors

The influence of human factors on wind erosion and desertification is mainly indirect, that is, the process of wind erosion and desertification can be accelerated or slowed down by changing the air flow or underlying surface conditions, the dynamic factors affecting wind erosion and the anti-erosion factors. At present, it is difficult for human beings to influence large-scale weather system processes and mesoscale weather processes, but they can influence near-small scale airflow or near-surface airflow by changing vegetation coverage, land use patterns, small-scale topography and building artificial structures.

Generally speaking, the influence of human factors on wind erosion is mainly realized by changing the underlying surface conditions and surface anti-erosion factors. Common human factors affecting wind erosion desertification include land use types, water resources development, mineral resources exploitation, farming and various engineering activities. Among them, land use type, engineering activities and farming methods directly affect the anti-erosion factors such as surface coverage type, vegetation coverage, plant community structure and soil structure; The development and utilization of water resources can affect the regional groundwater level, and then affect soil water content, vegetation coverage, plant community structure and other anti-erosion factors; The development of water resources may also cause the shrinkage of surface water such as rivers and lakes in arid areas, exposing a large number of loose sediments to the wind; The exploitation of mineral resources will not only destroy the surface vegetation and the original rock structure, but also reduce the wind erosion resistance of the surface. The tailings piles produced by mining activities will also provide a new material source for wind erosion.