Erosion is a result of forces of nature acting on soils and other earth materials. Displacement of soil particles by the force of water followed by their transport away from the initial place can be called water erosion. This possesses a major role in the loss of topsoil, which is the vital top few centimeters of any soil ecosystem and contains the major amounts of soil nutrition & microbial diversity. There are multiple forms of water erosion all of which change the land in various ways. While all the places found on the planet are prone to be eroded by water, regions with sloping topography are normally more extremely affected than those with flatter landscapes. Among the vulnerable areas are the ones with lower organic matter content in the soil, impermeable soil layers, as well as silty soils.
What is water erosion?
Water-erosion is a complicated process with many different forms, but it can be summarized as the removal or displacement of soil away from its original location by the water. This results from rainfall, melted snow, flowing rivers, the movement of glaciers, or the freeze/thaw cycles. Similar to erosion caused by water, it will slowly erode away landscapes over time if left unaddressed and can be very hazardous to those whose livelihoods depend on the land remaining intact like farmlands and ecological landscapes. Different forms of water-erosion commonly found are rills erosion, gullies erosion, splash effect erosion, sheet flow erosion, and tunneling erosion, each of which this article will address in more detail further below.
What causes water erosion?
Like many environmental phenomena, there are both natural and exacerbated forms of erosion, the latter of which are the direct or indirect result of our activities. For example, the erosion of banks from coursing rivers over hundreds of years is a natural form of water erosion that would occur with or without human interference. Conversely, water-erosion that has been intensified or created by our activity may include anything from the flooding and the resulting degradation of farmlands from improper irrigation to the accelerated melting of glaciers from the enhanced greenhouse effect from human-caused pollution. It can be difficult to pinpoint the exact cause behind water erosion when considering the complex interactions present in ecological and physical systems that are affected, but it is clear that our activity has increased the overall levels of water erosion, particularly in the major farming regions of the world.
However, the main reason for water-erosion in farmland can be attributed to several intentional or unintentional activities such as improper irrigation, unsuitable quality, and quantity of fertilizers, extreme or insufficient water availability, harmful cropping practices, etc.
The following factors are responsible to define the magnitude of water erosion:
Rainfall Characteristics:
Rainfall can be considered as the major culprit of water erosion since most of the water is circulated as rainfall in the water cycle. Moreover, the intensity of rainfall, amount of rainfall, and the seasonal distribution of rainfall largely affect water erosion. It is obvious that the higher the intensity and amount of rainfall, the greater the likelihood of severe water erosion. Also, the seasonal distribution of rainfall plays a major role, especially in today’s changing climate since erratic rainfall has increasingly been a major issue for rain-fed agricultural systems around the world.
Catchment Characteristics:
The catchment is the expanse of land that has a common outlet for the flowing water. Water erosion occurring in a catchment is largely dependent upon several catchment characteristics such as the size, shape, and slope of the catchment as well as the presence or absence of a well-defined water channel. Like mentioned earlier, an area with a greater slope has higher chances for water erosion since the effect of water is exacerbated by the action of gravity. A well-drained catchment has lower chances of water erosion than one without water channels.
Soil Characteristics:
Soil differs from one place to the other and each type of soil reacts differently to the effect of eroding water. The properties of soil that affects water erosion are physical properties (texture, structure, porosity, and density), chemical properties (pH, Cation exchange capacity, nutrient content), and biological properties (presence or absence of vegetation, microorganism, and soil fauna).
Atmospheric Characteristics:
The climatic and weather condition of the region also has a significant contribution to erosion through the water. Weather events can affect the hydrological cycle as well as the biological cycle of the vegetation which in turn affects water erosion.
Types of water erosion
There are five main types of water erosion, all of which arise from separate circumstances or weather events, and many of which are interlinked with one another. Some are more severe than others, such as gully and tunnel erosion, and all are natural occurrences that we have seen enhanced by human activity, particularly in farming areas.
Splash erosion
Splash erosion occurs from the impact of raindrops on delicate topsoil, which creates a small crater that may widen over time. Typically single or small soil particles are removed from the main soil structure. The scientific explanation for the splash effect of raindrops is that a moving (falling) body has kinetic energy and upon impact, that energy is transferred to the dislodge soil particles from the ground. While the individual events are less impactful than other types of water erosion on this list, it is the combined effect of countless raindrops that makes the effect significant. Especially, splash erosion is especially a matter of concern in areas that completely lack vegetation.
Sheet erosion
Sheet erosion is the erosion of soil in thin layers by excess water flowing as sheets above the soil. When the amount of rainfall or water on land exceeds the ability of the land to absorb water, water cannot infiltrate into the ground and it carries a thin layer of soil with it when it flows downslope. Sheet erosion can be defined as the overflowing of water above the ground in a uniform layer and eroding the small and light particles found on the top layer. Sheet erosion is related to the formation of gullies, where the pounding rainfall exceeds the ability of the soil to absorb the water. On a hill or sloping landscape, this can result in the accumulation of soil particles in a mound at the bottom of the slope, hence even resulting in the formation of a rill in the surface layer.
Rill erosion
Just as sheet erosion creates rill erosion, rill erosion eventually leads to gully erosion when left unaddressed. A rill can be defined as a shallow channel that has developed from water erosion and typically does not exceed 30-50 cm in width or depth. It is the initiation of the development of water-flowing channels which, with time, increases in depth and width as it erodes more and more of the soil materials and carries it downstream through the rills. They are typically found in hilly or sloping areas, as well as areas where the soil is naked and vulnerable to erosion like deforested areas or commercial farmland.
Gully erosion
Gully erosion is a type of advanced water erosion that originates from the formation of rills that deepen until they gradually become deep and wide trenches that are much more difficult to repair and highly impractical for machinery in the case of farming. The water flowing through gullies is also often of poorer quality as it contains a high concentration of sediment and soil particles as it continues to erode the landscape.
Gullies are a much serious concern in farmland productivity as well as on any other land use since they require more time and effort to repair and if not corrected properly and given favorable conditions, they can keep growing up to a point where the entire land has to be rendered unusable. In areas where advanced gullies are found, sediment loading in the downstream rivers is also a serious problem.
Tunnel erosion
Tunnel erosion is a unique type of subsurface water erosion typically caused by water penetration into a hollow underground zone that then moves soil particles away from their original location below the soil surface, forming tunnels. This type of erosion is very dangerous as it may not be apparent for some time but create unstable landscapes that are prone to collapse. Tunnel erosion is mainly found in specific orders of soil like the sodosol which has an unstable subsoil. The advancement of tunnel erosion occurs when water enters the interior of soil through holes or cracks on the land formed by tree roots or other causes. Tunnel erosion seriously affects the water holding capacity of the soil.
Dealing with water erosion
Water erosion comes in various forms and degrees of severity, all of which lead to land degradation and the loss of productive capacity of the land. While the most accurate and effective solutions to deal with water erosion depends on factors such as topography, vegetation, atmospheric condition, etc., there are some established ways or practices that alone, or indifferent combinations that can be used to prevent, mitigate or reduce the action and impacts of water erosion:
Select Appropriate Land Use:
The land use selected for a given land should match the type and vulnerability of the soil for erosion. The location, physical and chemical characteristics of the soil should govern the choice of land use. For example, steep slopes are best suited for the production of forage crops while forests are appropriate land use for marginal lands with degraded and low productive soils.
Maintain Organic Matter:
Organic matter is what binds the soil together. Soils with higher organic matter content are more stable with good infiltration and high water holding capacity, making the soil less prone to be eroded. Not only that, organic matter is vital for microorganism activities and better vegetation production. This further helps the soil to be resistant to the eroding action of water. It is mainly effective in checking rill erosion, sheet erosion, tunnel erosion, etc.
Maintenance of organic matter content usually requires the checking of balancing between the rate of decomposition and the rate of organic matter buildup. Planned practices such as reducing disturbance to the soil structure and adding manure or leaving crop residue serve to establish and maintain that balance.
Establish Crop Residue cover:
Providing cover to the soil by leaving residues of crops on the ground is a proven method to mainly reduce splash erosion. By doing so, the rainfall does not come in direct contact with the soil and hence isn’t eroded easily. Furthermore, it also controls sheet erosion by reducing the rate of water flow on the surface.
Establishing crop residue is obtained by reducing tillage operations or by using the conservation tillage method.
Reduced tillage:
Intense tillage operation is used in hopes of making the soil more arable but it makes the soil easily erodible. It disrupts the structural integrity of the soil, reduces the moisture content, and makes it vulnerable to the splashing effect of raindrops. So the reduction in the tillage operation or by using alternative methods is essential to deal with all types of erosion.
Use zero tillage or direct seeding:
Zero tillage and direct seeding are two effective methods of dealing with soil erosion and are often used in combination to increase the agricultural productivity of the land. Zero tillage leaves most of the crop residue uniformly distributed across the area and the stubbles are also left intact. Direct seeding involves seeding crops directly on the earlier crop residue with the use of fertilizers and herbicides to replace tillage for weed control and nutrient cycling. It is an economical and beneficial method to reduce the negative impacts of water erosion along with increasing production.
Use conservation fallow:
The use of fallows has been an effective method of revitalizing the productivity of the land by leaving it barren for one or more vegetative periods. However, cases of moderate to severe erosion have been a common occurrence in fallows left without any crop residue cover because the organic matter content in the soil declines with increased decomposition. To solve this problem, conservational fallow leaves the crop residue in the fallows which keeps on feeding organic matter to the soil while reducing tillage operations.
Grow forages and use crop rotations:
The use of crop rotations and the integration of forages in crop rotation substantially reduces water erosion in farms. The perennial forages have a fibrous root system that protects the soil from both above and below the ground. The selection of appropriate time for rotation as well as the best crop varieties allows the farm to maintain a healthy nutrient cycle. Legumes and cereals when rotated alternatively with forages, give the best results against water erosion control.
Use direct seeding for pasture conversion:
The method of directly seeding crops into the sod helps to eliminate the heavy plowing and harrowing requirements in the pasture lands while maintaining the same amount of yield. Direct seeding can be done with the use of a disc as well as an air drill.
Controlling severe erosion
While adopting good practices to deal with normal water erosion is effective, controlling severe cases of water erosion like gullies and extreme sedimentation requires more specialized measures such as:
Grassed Waterways
Just as the name suggests, grassed waterways are the channels in farmlands that have grass on them to carry large amounts of water from the land to a safe outlet. Waterways serve to dispose of the farmlands of the excess water without allowing it to erode the farmland soil. However, newly constructed waterways are prone to failure due to the eroding action of water. So, maintaining a good cover of grass on the waterways ensures sustainability.
Grassed waterways should be large enough to be able to drain peak volumes of water from storms and melting snow and should thus consider the historical records as well the size of the land to be drained using the waterway. While establishing a grass waterway, a gentle slope should be given and the normal drainage pattern should be adhered to as far as possible.
Lined Channels
Artificial channels, as well as natural gullies, help to carry excess water away from the farmlands. Lining the channels with matting that helps to control the erosion within the channel helps to make the channels more stable against channel bank and channel bed erosion. The lined channels can be constructed by using biological means such as turfs, rocks, and grasses as well as fabricated geotextiles and concrete, both of which are highly effective in restricting the expansion of gullies and conveying periodic water over slopes with a gradient of 10 percent. Lined channels also help to maintain the water quality downstream.
Drop Structures
The channel through which water flows is hardly uniform and flat. Sometimes, the elevation difference across a small distance is high enough to accelerate the kinetic and gravitational forces of water causing erosion. In such locations, drop structures are constructed to safely drop down the water to lower grounds. To do so, different types of drop structures such as vertical drop structure, grouted drop structure, and pipe drop structure are used. Among them, pipe drop structures are easier and less costly to construct.
Cross Section of a Pipe Drop Structure:
Pipe drop structures can also vary based on the needs of the site and the method of construction. The simplest types of pipe drop structure include laying out a pipe along with the flow of the channel connecting the upward and downward elevations. The flowing water is concentrated into the inlet of the pipe and then the water flows inside the pipe which is released back into the channel through the outlet.
A more complex pipe drop structure includes constructing a berm or a storage area upstream where water is retained temporarily. One or more plastic pipes with small openings as inlets in the berm are laid into the ground with their outlets opening up on the channels below. In doing so, the water is removed at a slowed pace, and holding water upstream also helps to increase moisture levels.
Terracing
In rugged topography with steep uninterrupted slopes, terraces are the best option to deal with water erosion and mass movements such as landslides caused by water erosion. Establishing channels in steep slopes is hard and the natural water channels cannot drain the entire area. As a result, water tends to flow along the slope and through the soil downslope. Terraces are built along the contour to control the flow of water.
Cross Section of Terrace
Establishing terraces in such areas results in the runoff water being intercepted by the terraces and the water thus intercepted is drained through the channels that separate the terraces. Constructing terraces in sloppy land includes cutting or removing materials to form channels and using the material to form flat land called berms where crops are cultivated.
It is rather expensive to build terraces, but considering the lack of alternatives in sloping lands, terraces are one of the best options for controlling severe water erosion which further helps to stabilize the slope and increase the productive potential of the soil.
Examples of water erosion
Examples of water erosion can be seen everywhere around us. The peculiar shape of the hills and the mountains are a result of the constant wearing action of water and wind. Some of the examples of water erosion include:
Caves: Caves are one of the most fascinating natural landforms found on earth and are largely formed by the action of flowing water over many years. This process is further exacerbated by carbonic acid from the rock’s calcium carbide which can result in caves hundreds of kilometers long like the mammoth cave of the United States.
River Bank Erosion: Riverbank erosion is the removal of the river materials present in the banks of the river as flowing water pounds on the river bends. Riverbank erosion is a serious problem in flood plains agricultural systems where thousands of acres of cultivable land is being washed away annually due to the cutting of river banks.
Canyons: One of the grandest examples of water erosion is the Grand Canyon which was formed by the eroding action of the Colorado River millions of years ago. The Grand Canyon and other canyons across the globe are scenic representations of the power of water erosion.
Coastal Erosion: Coastal erosion is the erosion of lands on the coastline which results in the formation of coastal cliffs and regular removal of coastal lands.
Conclusion
Water is a strong force of nature and its eroding effect can cause serious harm to the overall farmland productivity everywhere, especially on marginal lands. While the causes of water erosion can be both natural as well as human-accelerated, there are certain factors like soil properties and rainfall characteristics that define the severity of water erosion. Furthermore, water erosion is best dealt with as early as possible where normal tillage operations and the adoption of good farming practices can control water erosion effectively and economically. However, severe erosion caused by water or its potential erosion risk asks for other measures such as drop structures, terraces, grassed waterways, and lined channels. Finally, the means of controlling water erosion can be summarized down to maintaining a good crown cover and the structural integrity of the soil. Prevention and mitigation of water erosion are vital to sustaining agricultural and farming operations in farmlands.
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