Wind Erosion

Extent of Wind Erosion:

Let's talk about wind erosion. As you can imagine, looking at this picture of a huge dust storm, wind erosion has negative effects within and outside the area where the soil is blown away.

This makes wind erosion a serious problem in many parts of the world. In fact it occurs over more than one third of the land surface of the Earth. The image shows the risk of wind erosion on agricultural land around the world.

As this map shows, it is worse in arid and semiarid regions. Here wind erosion often takes place at a large scale with effects that can be seen at a global scale like in this satellite image of a dust storm. In less arid regions wind erosion is more of a local problem.

So what makes an area at risk for wind erosion?

Wind erosion occurs mostly in flat, bare areas with dry, sandy soils or anywhere the soil is loose, dry, and finely granulated and where the wind is strong enough. At this point I want to make a distinction between potential and actual wind erosion risk. Potential wind erosion risk is determined by soil type and climate only. In some regions the wind climate is one of the main reasons of the high occurrence of wind erosion. For instance If you drive, or better cycle towards the coast you will notice that the wind becomes stronger. So areas near the coast have a higher potential wind erosion risk than other areas with the same soil type. However if the soil is a type that is not susceptible to wind erosion, the potential wind erosion risk is still zero.

Actual wind erosion risk in a region depends on the potential erosion risk and land use and land management. In other words it depends on the role of us humans. As I already said the soil needs to be bare or nearly bare before wind erosion can take place. Here is an example. This photo which was taken shortly after a wind erosion event shows fields where nothing happened, fields that were clearly blown away and parts of other fields covered by sediment.

Here you can say the potential risk was the same for the whole area but only the fields with little or no crop cover were actually eroded.

Wind Erosion Processes:

There are four sub processes of wind erosion.

Dislodging of soil particles.

Entrainment, which is the lifting of soil particles from the surface.

Transport of the soil particles by wind.

Deposition of the soil particles.

When wind reaches a critical velocity, also called ‘threshold velocity’, soil particles breakup and become dislodged and available to be moved and/or picked up by the wind. 

The entrainment, or lifting and starting the movement, of the loose particles happens by different forces. Particles are lifted off the surface by ‘upward suction’. ‘Drag forces’ produced by the difference in pressure between the side of the particle facing the wind (windward) and the side that’s away from the wind (leeward) induces rolling and sliding. 

And when the wind is already carrying a large number of grains, the near ground velocities decline, and almost all further entrainment happens by ‘bombardment’ caused by the impact of grains falling back to the surface. Once the particles are mobile they can be transported in several different modes. The first mode is ‘creep’. This is when particles roll or slide along the surface. The distance over which a particle is moved by creep is about 1 to several meters. Smaller particles can be lifted and carried by the wind for a short distance before falling back to the surface. This transport mode is called ‘saltation’ and stands for a long-leaping, rebounding motion of high-energy grains. Particles falling back to the surface can cause an impact on the surface that results in dislodging and splashing of other grains. We call this ‘reptation’. Saltating grains can also dislodge very fine particles. When these very fine particles are lifted into the air they can go into ‘suspension’ and be moved over high altitudes and large distances before they fall back to earth.

Deposition takes place when the grains stop moving and settle again on the surface. If you look in the landscape where these wind erosion processes occur you can distinguish a zone where there is a net loss of soil particles – more lost than gained. This is the ‘deflation zone’. The zone where the energy of the wind is fully used but where there is no net loss or gain of soil particles is called the ‘transport zone’. Locations where the capacity of the wind decreases resulting in the deposition of soil particles is called the ‘accumulation or deposition zone’.

Causes and Effects of Wind Erosion:

There are different drivers that can cause an increase in wind erosion activity. There are natural causes like extreme droughts in combination with high winds. The impact of these extreme weather events however, depends on the state of the land and its vulnerability to wind erosion (actual erosion risk). The most common causes of an increase in wind erosion activity are changes in land use and changes in farm management practices. Important drivers that have initiated these kinds of changes are new technical developments like industrialization and the introduction of chemical fertilizers. 

With industrialization of farming, a shift occurred from small scale farming to large scale high intensity farming, along with a shift from multi cropping systems towards mono cropping.  The main effect of this change was that during a significant part of the year (winter and spring) soils were no longer protected by vegetation resulting in a significant increase in wind erosion risk.

With the introduction of fertilizers, it also became possible for farmers to crop larger areas. Larger fields with the same crop have a much higher erosion risk, compared to many small fields with a variety of different crops. Another effect of the introduction of fertilizers is that the input of fresh organic matter into the soil decreased, resulting in a loss of soil structure and higher erodibility of the soil. 

Societal and economic drivers have also resulted in an increase in erosion risk, e.g., increased demand for wool led to higher sheep stock rates per ha and overgrazing which makes land more prone to wind erosion.

But also bad land management practices can lead to an increase in wind erosion, e.g. burning of heath lands to enhance their rejuvenation has led in many cases to an increase in wind erosion. Large scale increases in wind erosion activity is often caused by a combination of these different causes. 

Effects of wind erosion can be divided into on-site and off-site effects. As shown in the diagrams below, moving sand by wind on-site can cause blistering of seedlings and plants, removal of seed or seedlings and burying of seedlings and plants. Blistered and buried plants can either die, due to plant injury or drying out, or become more vulnerable to diseases and plagues, or be setback in growth leading to lower production. Seed and seedlings blown away will also reduce production. 

The storage capacity of plant-available water in the soil can also be reduced. Furthermore, loss of plant nutrients and organic matter, as well as degradation of soil structure will reduce the soil productivity. Pathogens (e.g. nematodes) and agro-chemicals can be moved by the sand and deposited elsewhere. Off-site, the deposition of the transported material can clog up drainage channels, and contaminate areas with agri-chemicals. Drifting sand can bury cultivated land and decrease soil fertility. In greenhouse areas dust on the glass surfaces may limit light incidence reducing plant growth. Finally, dust can hinder traffic obstruct respiration and cause other health problems. It can also affect houses and other buildings and increase the need for cleaning. 

Prevention of Wind Erosion:

Measures to minimize the actual erosion risk often have only a short-term effect and are management practices that usually have to be repeated every year. Most of these measures aim at minimizing the period the land is without a proper protective cover. This can be achieved by sowing autumn-sown varieties like winter Barley instead of summer Barley, by applying an artificial protection layer with organic or synthetic soil stabilizers, or by postponing the time of cultivation and leaving the stubble on the field till just before the sowing date of the next crop.

Another way to protect the main crop against wind erosion is by sowing a nursing crop, a short period before sowing the main crop. A nursing crop is a rapid emerging and fast growing crop like Barley. The Barley provides a good soil coverage during the period the main crop, such as sugar beet, is not providing good soil cover. Still another temporal measure is to increase the surface roughness which will reduce the wind velocity near the surface. This can be achieved by ploughing perpendicular to the dominant wind direction or by plough and press.

More permanent measures aim at reducing the vulnerability of the broader landscape to wind erosion. This can be achieved by reducing the area that has no crop cover by changing arable land into permanent grassland or forest. Reducing field size can also help to reduce the number of fields without crop cover in the same period. It is also possible to reduce the erodibility of the soil by increasing the clay content (Marling). Reducing the erosivity of the wind can be achieved by increasing the landscape roughness by planting wind barriers.