Soil Erosion: Meaning, Types And Factors

Soil erosion is the wearing away, detachment and transportation of soil from one place and its deposition at another location by moving water and blowing wind or any other cause. Erosion, by definition, is an exogenic process of removal of soil from the crust of the Earth and then transporting it to another location either inches centimetres or kilometres away.

Studies reveal that due to the exogenic erosional processes, on average, the earth’s surface is lowering at the rate of 3 cm per thousand years (Stoddart,1969), having variations from one area to another as depicted by the distribution map of erosion rates in the world.

Asia carries some 80% of the world’s erosion to the oceans each year. In low-land areas, the rate of erosion varies between 2.2 cm to 7.2 cm per thousand years, while in mountains, it varies between 20.6 cm to 91.5 cm per thousand years (Fig.2). The maximum rate of erosion, i.e., 91.5 cm per thousand year stands in the youngest mountain chain of the world, viz., the Himalaya (Schume, 1963; Corbell, 1964).

Many factors influence the erosion rate in a particular area, contributing to worldwide variations in erosion. Map (Fig.) showing the pattern of world erosion suggests that the highest annual yield of above 240 tonnes/km2 occurs in southeast Asia, where the influence of man is especially notable.

The next highest is in the mountains and southeastern parts of the United States of America, followed by the rates in the tropics, which are higher than those of temperate latitudes, in turn, higher than those of the Artic ( Gregory and Walling, 1073).

Spatial variations in distribution of erosion in the world — Spatial variations in the distribution of erosion in the world (after Strakhov, 1967).

Types of Soil Erosion

Genetically, the soil erosion processes are divisible into two major groups. These are water-born and wind-born erosion. The water-born erosion is further divisible into two groups, i.e., pluvial (rainwater-born) processes and fluvial (stream water-born) processes.

Water Erosion

The erosion caused by rainwater, i.e., rain splash and sheet wash erosion, is the outcome of fluvial processes, while erosion caused by channel water, i.e., rill erosion and gully erosion, suspended, dissolved, and bed load flow is the outcome of fluvial processes.

The progressive removal of sediment and mineral material from valley sides and channel floor is known as channel erosion. A brief account of these different types of erosions caused by pluvial and fluvial processes is given in the following paragraphs.

Rain Splash Erosion

Direct force of rain drop causes splashing in which soil particles are detached, lifted and then dropped into a new position. The rainwater from the sky hits the soil, causing a disbursement of soil upon impact and creating a crater where the rain has hit (Fig.). This is a microform of erosion in which soil particles are transported at a distance of millimetres to a few centimetres.

For measurements of rain splash erosion, high-speed photography may be used to demonstrate the explosive effect of raindrop impact (Ellision, 1950) but more quantitative measurements of soil splash can be done by using trays of soil or small field plots or troughs to catch the splashed material. A rainfall simulator is also used to measure rain splash erosion (Osborne, 1953, McQueen, 1963).

Rainsplash erosion by raindrops — Rain splash erosion by raindrops (Source:https://www. google.co.in/search?q=Rainsplash+erosion+photo)

Sheetwash Erosion

The rain splash soil particles are a component of sheet wash erosion. During overland flow, water flows on the land surface in sheet form which is coupled with soil particles. The overland flow’s transportation of soil particles in sheet form is known as sheet wash erosion (Fig.). To measure sheet wash erosion, the Gerlach trough is installed at different slopes (Fig.).

During rain, transported sheet wash material from the plot is caught by the Gerlach trough. The rate and volume of sheet wash erosion are estimated by weighing this material. The study of rain splash and sheet-wash erosion helps understand anthropogenic activities’ impact. i.e., particularly deforestation and grazing.

Gerlech trough (Below) installed at different slopes to measure sheetwash erosion — Gerlech trough (Below) was installed at different slopes to measure sheet wash erosion (after Gregory, K. J. and Walling D.E.,1979).

Rill Erosion

Where hillslopes are steep and runoff from rain storms is extremely high, and sheet wash erosion progresses into a more intense activity, that of rill erosion (Fig.) or rilling. All the ephemeral and seasonal channel on steep hills of the watershed starts rilling during high-intensity rainfall and cause severe soil erosion and floods in the lowland areas of the watershed.

The amount of rill erosion can be measured in two ways. Firstly, by measuring material transported by rills at their outlets and secondly, an attempt may be made to document the development of the rill system in both sections and plan by periodic surveying and to calculate the amount of material removed (Tackfield, 1964).

Gully Erosion

Rills on the diluvium (i.e., mass wasting and regolith deposits) and alluvium (i.e., river terraces and fans) soon begin to integrate into still larger channels, termed as gullies (Fig.). Gully erosion is an advanced stage of rill erosion Gullies are mainly confined on the gentle and level surfaces of watersheds in its valley region. Measurement of the development of gullies may be done by doing periodic monitoring of erosion pins or can also be documented in both sections and planned by a periodic survey.

Suspended Load

The suspended load includes all those material which flows in suspension forms (Fig.8) with channel water, such as clay, slit and sometimes fine sand. Watersheds under natural conditions carry suspended sediment only during the rainy period, but channels of anthropogenically and technologically disturbed watersheds may carry suspended loads throughout the year.

River with suspended material (left) and without suspended material (right)

Dissolved Load

The dissolved load includes chemical elements and compounds that are carried in solution with channel water. Usually, the most abundant cations in the total dissolved solids are Ca++, Mg++, Na++, and K+, and dominant anions are usually Hco-3, So4 and No3-. Apart from these heavy metals, phosphates and phenols are not uncommon.

The major sources of dissolved load in a stream are notably precipitation, chemical weathering and erosion, atmospheric fallout, mineral springs and anthropogenic and technogenic activities of man. Measurement of sediment and dissolved loads require an appropriate sampling technique.

For suspended and dissolved load measurements, a series of water samples (Fig. left) are collected from a river or a water gauging station (Fig. right B). Using the gravimetric method, the concentration of suspended and dissolved loads is used determined.

Suppose frequent measurements of suspended and dissolved load discharge are made at a stream gauging station having a weir to define the variations through time. In that case, sediment yield estimates can be computed on individual rainstorms, daily (during the rainy season) and a weekly or fortnightly basis on dry period.

At the gauging site having no weir, the channel cross-section is divided into different sub-sections of homogeneous flow and water samples are collected at 0.6 depth for suspended and dissolved loads by sediment samplers.

Bedload

Bed load includes the channel transportation material which moves along the channel floor by rolling or sliding and on occasional low leaps. Bed load includes gravel, pebbles, cobble, block and boulders. The bed load samples are collected from the stream bed by using a bedload sampler by constructing a bedload trough (Fig. right B) across the channel behind the weir at the gauging station. In large rivers, a basket sampler (Fig.) is used to measure bedload. On larger rivers, a lifting system to basket samplers from the river bed is also required.

Wind Erosion

Wind erosion is a natural process. It is a common cause of land degradation in the arid and semi-arid regions of the world, where the average annual rainfall is between 100 mm to 150 mm only. The lighter texture soil in the low rainfall regions is the most susceptible to wind erosion.

Wind erosion is one of the processes leading to desertification. Significant wind erosion occurs when strong winds blow over light-textured soils that have been heavily grazed during periods of drought. Wind erosion is also a natural process.

In the arid and semi-arid regions, the wind is the main agent of erosion, which develops arid landscapes constituted of different types of erosional (i.e., inselbergs etc.) and depositional landforms (i.e., sand dunes etc.).

The main factor in wind erosion is the velocity of moving air. Because of the roughness imparted by soil, stones, vegetation and other obstacles, windspeeds are lowest near the ground surface. Windspeed increases exponentially from the surface at which the wind velocity is zero. The processes of wind erosion are divisible into three different types. These are- wind erosion, surface creep, saltation and suspension (Fig.).

Suspension

Suspension describes the movement of fine particles, usually less than 0.1mm in diameter, high in the air and over long distances, which can be moved into the air forming dust storms when taken further upwards by turbulence. These particles include very fine grains of sand, clay particles and organic matter. However, not all dust ejected from the surface is carried in the air indefinitely.

Larger dust particles (0.05 to 0.1 mm) may be dropped within a couple of kilometres of the erosion site. Particles of the order of 0.01 mm may travel hundreds of kilometres and 0.001 mm sized particles may travel thousands of kilometres. Australian soil has been carried to New Zealand and beyond through this process. Fine dust may remain in suspension in the air until it is washed out by rainfall.

Saltation

Saltation is the process of grain movement in a series of jumps. It occurs among middle-sized soil particles that range from 0.05 mm to 0.5 mm in diameter. Such particles are light enough to be lifted off the surface but are too large to become suspended. These particles move through a series of low bounces over the surface, causing an abrasion on the soil surface and attrition (the breaking of particles into smaller particles).

Surface Creep

Surface creep is the rolling of coarse grains along the ground surface. Larger particles ranging from 0.5 mm to 2 mm in diameter, are rolled across the soil surface. This causes them to collide with and dislodge other particles. Surface creep wind erosion results in these larger particles moving only a few metres.

Factors Influencing Erosion

The important factors controlling soil erosion systems are rainfall intensity and wind velocity, also known as erosivity; the physical property of soils and rocks, known as erodibility; topography and slope; vegetation; and anthropogenic activities. A brief account of these influencing factors of soil erosion is presented in the following paragraphs.

Rainfall

Soil erosion is closely related to rainfall, partly through detaching power of raindrops striking the soil surface and partly through the contribution of rain to runoff (Morgan, 1979). Studies reveal that the average soil loss per rain event increases with the storm’s intensity. The low-intensity rain enters the soil where it strikes, and some will slowly run off, but the high-intensity rain, there is not enough time for the water to soak and infiltrate through the soil, and it runs off, causing soil erosion. Therefore, the runoff that causes soil erosion depends upon the intensity, duration, amount and frequency of rainfall.

Soils Properties

Properties of soils, both physical and chemical, play a significant role in determining the erodibility of any region because these define the resistance of the soil to both detachment and transport. Thus, the erodibility is influenced by soil texture, structure, organic matter and chemical composition.

Soil detachability increases as the particle size increases, but soil transportability increases with the decrease in particle size. Fine textured soil clay particles are more difficult to detach than sand but are easily transported even in level slopes because of the very small particle size.

During low-intensity rain, there is less erosion in sandy soil because the rainwater is absorbed readily due to its high permeability in sandy soil. Similarly, more organic manure in the soil improves the granular structure and water-holding capacity. Hence, as organic matter decreases, the erodibility of the soil increases. The fine textured and alkaline soils are more erodible.

Topography and Slope

Topography, with special reference to the slope of the land, also plays a very significant role in determining erodibility of any region. Under the same rainfall intensity and even under the same soil properties, the erodibility may vary from place to place due to different topography specifically different slope conditions. Slope variation changes the direction of rain splash particles and increases their transportation length.

On a flat surface, raindrops splash soil particles randomly in all directions, but on a sloping surface, more soil is splashed downslope to a larger distance compared to upslope. Slope accelerates soil erosion as it increases the velocity of running water. Even a small difference in slope can make a large difference in soil erosion.

Hydrologic studies reveal that a four-time slope increase doubles running water’s velocity. This double velocity accelerates erosive power four times and the carrying capacity by 32 times. Thus, erosion would normally be expected to increase with increases in slope steepness and slope length as a result of increases in velocity as well as the volume of surface runoff.

Vegetation Cover

The ground cover, nature, and extent are directly related to soil erosion. The presence of vegetation retards erosion up to some limit. Trees, shrubs and grasses are more effective in providing cover to control soil erosion.

The canopies of vegetation intercept the erosive beating action of falling raindrops retards the amount and velocity of surface runoff permitting more water flow into the soil for infiltration and creating more groundwater storage capacity in soils and rocks. It is the lack of ground cover that creates erosion-permitting conditions.

The effectiveness of vegetation cover in controlling soil erosion depends upon the height, density and continuity of the canopy above the surface and plants’ root density and depth below the surface. The height of the plant canopy is important because water drops falling from 7m may attain over 90 per cent of their terminal velocity.

Further, raindrops intercepted by the plant canopy may coalesce on the leaves to form larger drops which are more erosive for rain splash activity. Ground cover intercepts the rain; dissipates the energy of running water and winds; imparts roughness to the flow, thereby reducing its velocity; consequently, erosion is largely controlled.

Below the earth’s surface, the plant root network’s main effect is opening up the soil, thereby enabling water to penetrate and increasing infiltration capacity (Morgam,1979).

Thus, vegetation cover is most effective in reducing erosion because of its canopy. For adequate erosion protection, it is recommended, based on various studies, that at least 70 per cent of the ground surface must be covered (Fournier,1972; Elwell and Stocking, 1976).

Anthropogenic Activities

For the shake development, now the man himself has become an active agent of erosion through anthropogenic (i.e., deforestation, grazing, agriculture) and technogenic (i.e., various engineering works such as road and building construction) activities.

Studies from the Himalayan region reveal that the rate of soil erosion has been accelerating considerably due to anthropogenic and technogenic activities. A Himalayan study reveals that the removal of forest cover and agricultural activities has greatly accelerated the rate of soil erosion (Rawat and Rawat, 1993).

This study estimates the erosion rate as 0.09 mm/year on deforested land and 0.18mm/year on agricultural land, by contrast, the natural erosion rates vary between 0.02mm/year on oak forest and 0.04mm/year on pine forest.

Two decades ago the erosion rate of the Himalayas was estimated at 0.91mm/year (Corbel, 1964; Schume,1963). More recently in the Central Himalayas, it has been estimated as 1.7mm/year (Valdiya and Bartarya, 1989).

Several factors may contribute to this acceleration which includes deforestation, poorly managed agriculture, forest fire, overgrazing, and substandard constructions of roads and buildings on ill-suited sites.

However, the actual impact of these human activities remains poorly understood and unquantified (Haigh,1989). A geomorphic study (Rawat et el.,2000) conducted in Uttarakhand Himalaya in India suggests that technogenic (urban) and anthropogenic (deforestation and agriculture) activities increase the intensity of erosion by a factor of 2 to 47.

Soil Erosion: Meaning, Types and Factors

Types of Soil Erosion