Lesson 8.01 Soil Erosion and Human Impact

 

Water is the major agent of soil erosion on Earth. When water erodes soil, it carries loosened particles from one area and deposits those particles at a new site. Soils containing a large amount of silt-sized particles are most susceptible to erosion from moving water whereas soils consisting of clay or sand-sized particles are less susceptible to erosion from moving water.

There are four types of water erosion:

1. rain splash
2. rill
3. gulley
4. sheet

Rain splash occurs from the falling rain that moves soil directly. When the rain hits the soil, its kinetic energy causes the soil to move. Rain splash erosion causes the soil particles to move only a few inches. Rainfall may also move/erode soil indirectly in the form of runoff. Rainwater that is not able to absorb or infiltrate into the soil - because the rainfall is too intense or because the soil is saturated - flows downhill as a result of gravity.

Visit Soil Erosion by Rain Splash to see examples of how rain splash erodes soil.

This rain runoff will travel downhill in channels known as rills or gullies. Rills are small trenches and gullies are large trenches that water travels down. The greater the slope of the terrain, the faster the water flows, increasing erosion.

Sheet erosion is uniform removal of soil by raindrops and runoff that occur over a large area. The effects of this type of erosion can be great but go undetected for long periods of time because it occurs over such a large area. With sheet erosion, there are no gullies or rills that the water travels through.

Visit Major Categories of Erosion to view examples of rain splash, a rill tunnel (listed as a channel), sheet erosion, and wind erosion.

 

Wind is another agent of erosion. Wind is the movement of air. Wind detaches soil particles and moves them from one place and deposits them in another location. As with water, soil with silt-sized particles are the most prone to wind erosion and soils with sand and clay-sized particles are least prone.

Wind moves soil particles in three ways.

1. saltation
2. suspension
3. soil creep

In saltation, the wind causes medium-sized particles to bounce across the ground moving them from one place to another. Saltation accounts for 50-90% of total movement of soil by wind.

The bouncing particles from saltation dislodge smaller particles that become suspended in the air and carried to another location. These suspended particles can be carried high into the atmosphere and travel hundreds of miles.

Larger particles are also dislodged and slide or roll across the ground aided by the bouncing particles from saltation through a process called soil creep. Soil creep accounts for 5-25% or soil movement by wind.

Visit Plant & Soil Sciences eLibrary: Wind Erosion to see a satellite image of a sand storm off the coast of Africa.

Watch the animation Soil Erosion to see a visual of saltation, suspension, and soil creep.

 

Glaciers are a third agent of erosion. Glaciers are slow moving masses of ice and snow formed by the accumulation and compaction of ice and snow on mountaintopsvalley or alpine glaciers or near the Earth's polescontinental glaciers = Greenland and Antarctica. They form when the amount of snow that falls exceeds the amount of snow and ice that melts during the summer/warm months.

Continental glaciers are larger than valley glaciers. Greenland and Antarctica are examples of continental glaciers. Valley glaciers (also known as alpine glaciers) flow downhill and continental glaciers flow from the middle out. Glacial movement is caused by gravity. As glaciers move, they pickup any material that is in their path - sand, rock, large boulders. These materials become frozen to the bottom of the glacier and travel with it becoming a "cutting tool" and eroding the bedrock that the glacier travels over. This process cuts U-shaped valleys in the sides of mountains. As glaciers cut through the landmass, any soil that is in its way can also be carried away and deposited in a new location.

View the following video clip on glaciers. Login Instructions Glaciers (2:40)

Andrews Glacier is an valley glacier in Rocky Mountain National Park in Colorado.

 

There are several factors that determine the severity or rate of erosion:

• soil type
• amount of water/intensity of water
• severity of wind
• temperature
• topography of the land
• type of vegetation present/absent

As mentioned earlier, soils with silt-sized particles are more prone to erosion than soils that have more clay-sized or sand-sized particles. Silt-sized soil particles are therefore washed away by water and blown away by wind much easier than clay-sized and sand-sized soil particles.

The greater the amount of water the greater the erosion. The speed of the moving water also affects the severity of erosion. The faster the water is moving, the greater the effects of erosion. This also holds true for the eroding power of wind. The stronger the wind, the greater the erosion to the soil.

Temperature is another factor to consider with soil erosion. Areas with warmer climates tend to experience more erosion because the soil has less organic matter present. The amount of organic matter in the soil decreases the effects of rain splash. The more organic matter in the soil, the better the soil is at soaking up water, which reduces the effects of runoff.

Topography also plays a role in the severity of erosion. Soil erosion tends to be worse on land areas that are sloped. The longer and steeper the slope, the greater soil erosion. Steeper and longer slopes mean that the water running off is moving faster, which increases soil erosion. Soil erosion is usually greater at the bottoms of slopes because this is where the water's velocity is the greatest.

The most influential factor in soil erosion is vegetation or the lack of vegetation. Soils that are covered with plant life tend to experience less erosion than bare soils. This is because vegetation shields the soils from the effects of wind and runoff of water from rain. The roots of the plants help bind the soils particles together helping increase its resistance to runoff.

 

Human activity is one of the main contributing factors to soil erosion. The main ways that humans contribute to soil erosion is through removing vegetaion. When land is stripped of trees and other vegetation, deforestation occurs. Deforestation is the permanent destruction of forests to make land available for other uses.

The removal of vegetation occurs for many reasons. Humans clear areas to build homes, cut roads and highways, and for other infrastructure projects like utility lines. Humans clear-cut forests and use the trees for building materials and paper. Land is stripped of vegetation to mine precious metals and fuel resources, such as coal. Land is also cleared for agricultural purposes.

Agriculture can impact erosion in a number of ways:

• Large areas of land are plowed, which loosens the soil making it more susceptible to erosion.
• Irrigation systems can cause erosion simply by how the water is delivered to the crops. The water from irrigation systems can hit the ground with such a force to cause a rain splash effect. Also, overwatering crops can cause soil erosion.
• Live stock can also strip the land of vegetation if grazing is not monitored or is done improperly.

In all of the cases described above, native vegetation is stripped from the land leaving the soil exposed to the natural elements - wind and rain. These unprotected soils become more susceptible to the effects of erosion. Loss of topsoil through erosion can make soils less fertile and unable to grow vegetation. This can lead to desertification, which is process of desert like conditions developing in areas that do not normally have a desert climate.

One historical example of the ways that human practices can impact erosion is the Dust Bowl of the 1930s. The Dust Bowl was a mass desertification of farmland as result of poor agricultural and grazing practices.

Read the article and view the video clip on the Dust Bowl. Login Instructions Dust Bowl America in the 1930s: The Dust Bowl (1:02)