Soil Erosion: Meaning, Factors, Effects, Types, Causes and Prevention
Introduction :
Soil erosion is a two ways word. It hurts the land and people where it takes place, so also the land and people where it gets deposited.
· Total geographical area of India – 328 million hectares
· Total area subjected to erosion by wind and water – 150 million hectares
· Area at critical stage of deterioration due to erosion – 69 million hectares
· Area subjected to wind erosion – 32 million hectares
· Area affected by gully erosion & ravines – 4 million hectares
· Area affected by shifting cultivation – 3 million hectares
· Area under rainfed farming (non-paddy) – 70 million hectares
The rapid erosion of soil by wind and water has been a problem since man began cultivating the land. Although it is a less emotive topic today than in the past The prevention of soil erosion, which means reducing the rate of soil loss to approximately that which would occur under natural conditions, relies on selecting appropriate strategies for soil conservation and this, in turn, requires a thorough understanding of the processes of erosion.
The factors which influence the rate of erosion are rainfall, runoff, wind, soil, slope, plant cover and the presence or absence of conservation measures. These and other related factors are grouped under three headings; energy, resistance and protection (Fig. 7.1)
1. Energy Group:
This includes the potential ability of rainfall, runoff and wind to cause erosion. This ability is termed erosivity. In this are also included those factors which directly affect the power of the erosive agents such as the reduction in the length of runoff or wind blow through the construction of terraces and wind breaks respectively.
2. Resistance Group:
The resistance group includes credibility of the soil which depends upon its mechanical and chemical properties. Factors which encourage the infiltration of water into the soil and thereby reduce runoff and decrease erodibility whilst any activity that pulverizes the soil increases it Thus, cultivation may decrease the erodibility of clay soils but increase that of sandy soils.
3. Protection Group:
It focuses on factors relating to the plant cover. By intercepting rainfall and reducing the velocity of runoff and wind, a plant cover protects the soil from erosion. Different plant covers afford different degrees of protection so that, by determining the land use, man, to a considerable degree, can control the rate of erosion.
Theoretically, the rate of soil loss is judged relative to the rate of soil formation. If soil properties such as nutrient status, texture, and thickness remain unchanged through time it is assumed that their rate of erosion balances the rate of soil formation.
Effects:
The effects of erosion are:
1. Surface Runoff:
Top soil is removed, bedrock exposed and the land entrenched by gullies.
2. Deposition and Clogging:
Ground is covered with sand and silt deposits; ditches and canals are clogged with sediment and reservoirs silt up.
3. Reduction in Productivity:
As a result of erosion there is deterioration in the quality of cropping and grazing land which brings about reduced productivity and increased expenditure on fertilizers to maintain fertility.
4. Barren Land:
In extreme cases yield become so poor that land has to be taken out of cultivation.
5. Pollutant:
Siltation of rivers reduces their capacity, creating flood hazard, and the sediment is a major pollutant, lowering water quality.
Process of Erosion:
Soil erosion is a two-phase process consisting of the detachment of individual particles from the soil mass and their transport by erosive agents such as running water and wind. When sufficient energy is no longer available to transport the particles a third phase, deposition, occurs.
The phases in the process of erosion are:
1. Loosen the Soil:
Rain splash is the most important detaching agent. As a result of raindrops striking a bare soil surface, soil particles may be thrown through the air over distances of several centimeters. Continuous exposure to intense rainstorms considerably weakens the soil. The soil is also broken up by weathering processes, both mechanical, by alternate wetting and drying, freezing and thawing and frost action and biochemical. Soil is disturbed by tillage operations and by the trampling of people and livestock. Running water and wind are further contributors to the detachment of soil particles.
2. Agent of Transport:
The transporting agents comprise:
A. Agents which act and contribute to the removal of a relatively uniform thickness of soil. This group consists of rain splash, surface runoff in the form of shallow flows of infinite width, sometimes termed sheet flow but more correctly called overland flow and wind.
B. Agents which concentrate their action in channels. This group covers water flow in small channels, known as rills, which can be obliterated by weathering and ploughing, or in the larger, more permanent features of gullies and rivers; picking up material from and carrying it over the ground surface; soil flows, slides and creep, in which water affects the soil internally.
3. Extent of Erosion:
The severity of erosion depends upon the quantity of material supplied by detachment and the capacity of the eroding agents to transport it.
The extents of erosion are:
(i) Detachment – limited:
When the agents have the capacity to transport more material than is supplied by detachment, than the erosion is described as detachment-limited.
(ii) Transport – limited:
When more material is supplied than can be transported, the erosion is transport-limited.
Energy for Erosion:
The energy available for erosion takes two forms:
Potential and kinetic. Potential energy (PE) results from the difference in height of one body with respect to another. It is the product of mass (m), height difference (h) and acceleration due to gravity (g), so that PE = mhg, which in units of kg. m and ms-2 respectively, yield a value in joules. The potential energy for erosion is converted into kinetic energy (KE), the energy of motion. This is related to the mass and the velocity (v) of the eroding agent in the expression
KE = 1/2mv2
Which in units of kg and (ms-1)2, also gives a value in joules. Most of this energy is dissipated in friction with the surface over which the agent moves so that only 3 to 4 per cent of the energy of running water and 0.2 per cent of that of falling raindrops is used in erosion.
Types of Soil Erosion:
1. Rain Splash Erosion:
The action of raindrops on soil particles is most easily understood by considering the momentum of a single raindrop falling on a sloping surface. The down-slops component of this momentum is transferred in full to the soil surface but only a small proportion of the component normal to the surface is transferred and the remainder being reflected.
The transference of momentum to the soil particles has two effects:
(i) It provides a consolidating force, compacting the soil,
(ii) It imparts a velocity to some of the soil particles, launching them into the air.
Thus, raindrops are agents of both consolidation (e.g. formation of surface crust) and dispersion.
Rain splash erosion acts uniformly over the land surface; its effects are seen only where stones or tree roots selectively protect the underlying soil. On sandy soils 2 cm high splash pedestals can form in one year.
2. Gully Erosion:
The erosion through channel worn by running water is called gully (or gulley) erosion. Gullies are relatively permanent steep-sided water courses which experience ephemeral flows during rainstorms. They are almost always associated with accelerated erosion.
Formation of Gully:
Their initiation is a more complex process. In the first stage small depressions or Knicks form on a hillside as a result of localized weakening of depressions where near-vertical scarps develop over which supercritical flow occurs.
Types of gullies net-work:
Three types of gullies network can be recognized. The types are related to differences in soils and the effects that these have on the processes of gully formation.
These are as follows:
1. Axial Gullying:
This consists of individual gullies with single head cuts that retreat upslope by surface erosion, occurs in gravelly deposits.
2. Digitate Gullying:
It occurs in several head cuts extending in the direction of tributary depressions. It is characteristic of clay loams.
3. Frontal Gullying:
This is associated with piping and is found particularly on loamy sands with columnar structure.
Example of Gully Erosion:
There is regular gully erosion in the Assam Uplands where monthly rainfall may total 2000 to 5000 mm and in the Darjeeling Hills where over 50 mm of rain falls on an average of twelve days each year and rainfall intensities are often highest at the end of a rain event.
3. Rill Erosion:
The removal of soil by narrow finger-like furrows at an outlet of the river is called rill erosion. Both rill and overland flow processes generally affect the same part of a hill slope and their hydraulic characteristics are similar. Rills are ephemeral. The rills formed from one storm are often obliterated before the next storm of sufficient intensity to cause rilling. Most rill systems are discontinuous, that is they have no connection with the main river system.
Only occasionally does a master rill develop a permanent course with an outlet to the river. The rills are initiated at a critical distance down-slope where overland flow becomes channeled. By rill erosion bulk of the sediment is removed from a hillside.
4. Overland Flow:
It occurs on hillside during a rainstorm or prolonged rain or with intense rain by which there are surface depression storages or small soil moisture storages when the infiltration capacity of the soil exceeded. The flow is rarely in the form a sheet of water of uniform depth and more commonly is a mass of anatomizing water courses with no pronounced channels.
The flow is broken up by large stones and cobbles and by the vegetation cover, often swirling around tufts of grass and small shrubs. The amount of soil loss resulting from erosion by overland flow varies with the velocity and the turbulence of flow.
Distribution of Overland Flow:
The flow results from the intensity of the rainfall being greater than the infiltration capacity of the soil and is distributed over the hill slopes in the following pattern. At the top of a slope is a zone without flow which forms a belt of no erosion.
At a critical distance from the crest sufficient water has accumulated on the surface for flow to begin. Further down-slope the depth of flow increases with distance from the crest until, at a further critical distance, the flow becomes channeled and breaks up into rills. In well-vegetated areas, the overland flow occurs rarely. Removal of die plant cover can enhance erosion by overland flow. It frequently occurs in bare soil.
5. Subsurface Flow:
The lateral movement of water down-slope through the upper layers of the soil is called subsurface flow. Less is known about the eroding ability of water moving through the pore spaces in the soil, although it has been suggested that fine particles may be washed out by this process.
The subsurface flow is more important because the concentrations of base minerals in the water are twice than those found in surface flow. Essential plant nutrients, particularly those added in fertilizers can be removed by this process, thereby impoverishing the soil and reducing its resistance to erosion.
6. Wind Erosion: Cutting Power of Wind:
The erosion of soil particles by wind is effected by the application of a sufficiently large force and by the bombardment of the soil by grains already in motion. These two forces identify two threshold velocities required to initiate grain movement. The static or fluid threshold applies to the direct action of the wind and the dynamic or impact threshold allows for the bombarding effect of moving particles.
The critical velocities vary with the grain size of the material, being least for particles of 0.10 to 0.15 mm in diameter and increasing with both increasing and decreasing grain size. The resistance of the larger particles results from their size and weighty. That of the finer particles is due to their cohesiveness and the protection afforded by surrounding coarser grains.
Transport of particles by wind:
The transport of soil and sand particles by wind takes place in the following three ways viz:
(1) Suspension:
In this there is a movement of fine particles, usually less than 0.2 mm diameter, high in the air and over long distances.
(2) Surface Creep:
It is the rolling of coarse grains along the ground surface.
(3) Saltation:
It is the process of grain movement in a series of jumps.
Types of wind erosion:
There are two types of wind erosion:
(i) Selective Erosion:
The sorting of sand or relatively fine soil particles by wind from an eroding surface is called selective wind erosion. Selective erosion results in the loss of the finer and more fertile portion of the soil from the eroding area giving rise to the local accumulation of sand into hummocks or dunes.
(ii) Mass Removal:
The sorting of silt and clay or finer soils from basic soil material by wind action is known as mass removal. In desert areas, progressive removal of the surface material results, eventually in exposure to the surface of zones of high lime accumulation; lowered crop yields and increase erosion hazard.
Case histories of Erosion:
In Thar at Bikaner (Rajasthan, India) during most vulnerable period of wind erosion of 75 days from April to June as much as 4 cm or 615 t/ha soil loss was observed when wind velocity ranged from 26 to 39 km h”1.
Ahman (1975) reported a loss of 10-15 cm of surface soil per year through wind erosion in Vomb Valley, Southern Scania. Borsy (1975) observed a loss of 55 kgnr2 soil from a wind-blown area in Hungary during a storm which lasted for 10 hours.
Wind erosion has ruined vast tracts of land in Asia, in the Mediterranean basin, in the savannas of the Sahel, and in North America, particularly in the Mid-West of the USA. In the USA, some 20 million hectares of land and at least one third of the top soil have been completely destroyed over the last 200 years by erosion arising from cultivation.
No Soil Erosion:
Soil erosion does not occur in forests and is only very slight on grassland, even if it slopes quite steep. The continuous carpet of litter and moss in afforested areas acts like a sponage: 1 kilogramme of dry moss can absorb 5 litres of water so that 1 hectare of Mediterranean forest, for instance, retains some 400 m3 of water after a violent storm. Part of this is lost through evapotranspiration and the rest seeps slowly downwards and gradually replenishes the ground waters below. There is no run-off.
Causes of Soil Erosion:
Use of unsuitable, unrestricted cultivation of fragile soil and techniques are main causes of erosion.
The causes of soil erosion are following:
1. Natural:
A. Water:
The erosion by water is most prevalent in regions with high relief, although it can occur even in land with quite moderate slopes. Consult rill, gully, and overland, subsurface flow.
B. Wind:
A substantial amount of wind erosion occurs in steppe like regions where the soil has a sandy texture or consists of fine periglacial alluvium.
2. Human Activities:
A. Agriculture:
The development of modern industrial agriculture based on a very restricted number of cultivated crops or even on monoculture (groundnuts in the tropics; wheat or maize in temperate regions) has become a sizeable contributor to soil erosion. Pimentel et al. (1976) showed that the cultivation of maize in the USA has been accompanied by an annual loss of soil amounting to between 6.6 and 200 tones per hectare. It should also be noted that rotation of crops gives the soil better protection.
The movements of large agricultural machines like heavy tractors, gang-ploughs and use of disc harrow etc., in agricultural operations accelerate the soil erosion. The bad agronomic practices can also lead to catastrophic erosion of the surface layer.
B. Deforestation:
Deforestation or overgrazing, on the other hand, increases erosion by allowing much more violent impact of the rain on the bare surface and a greater run-off. The elimination of hedges, the leveling of embankments and filling in the ditches also aggravate the soil erosion.
C. Mining:
It is a localize specific activity. In this extensive quarrying, denudation of hill slopes and large scale loosening of rock faces-all leading to condition that help the process of soil erosion.
Factors of Soil Erosion:
Factors effecting Soil Erosion:
The factors controlling the working of soil erosion are the erosivity of the eroding agent, the erodibility of the soil, the slope of the land and the nature of the plant cover. Only those variables which are commonly accepted as important are discussed here.
1. Rainfall:
Soil loss is closely related to rainfall partly through the detaching power of raindrops striking the soil surface and partly through the contribution of rain to runoff. This applies particularly to erosion by overland flow and rills for which intensity is generally considered to be the most important rainfall characteristic.
2. Erodibility:
Erodibility defines the resistance of the soil to both detachment and transport. Although soil resistance to erosion depends in part on topographic position, slope steepness and the amount of disturbance created by man, for example during tillage, the properties of the soil are the most important determinants. Erodibility varies with soil texture, aggregate stability, shear strength, infiltration capacity and organic and chemical content.
The large particles are resistant to transport because of the greater force required to entrain them and that fine particles are resistant to detachment because of their cohesiveness. Soils with less than 2 per cent organic matter can be considered erodible.
3. Effect of Slope:
Erosion would normally be expected to increase with increase in slope steepness and slope length as a result of respective increase in velocity and volume of surface runoff.
4. Effect of Plant Cover:
The plant cover is important in reducing erosion. The effectiveness of a plant cover in reducing erosion depends upon the height and continuity of the canopy, the density of the ground cover and the root density. A ground cover not only intercepts the rain but also dissipates the energy of running water and wind, imparts roughness to the flow and thereby reduces its velocity.
The main effect of the root network is in opening up the soil, thereby enabling water to penetrate and increasing infiltration capacity. Generally, forests are the most effective in reducing erosion because of their canopy but a dense growth of grass may be almost as efficient.
Strategies for Erosion Control or The Fight against Soil Erosion:
Soil erosion is a constant threat to the stability of agro ecosystems-Saxena N.B.
The protection of cultivated land against erosion involves a combination of civil engineering and biological techniques. On land of moderate slope, contour ploughing and on more steeply ground, terrace ploughing are essential. Biological methods for increasing resistance to erosion either involve reinforcement of soils by the addition of litter, manure or other organic matter, or else they involve the crops themselves through the adoption of different techniques of cultivation. Crop rotation with successive plantings of row crops and cover crops considerably reduces erosion.
The strategies for soil conservation must be based on covering the soil to protect it from raindrop impact; increasing the infiltration capacity of the soil to reduce runoff; improving the aggregate stability of the soil; and increasing surface roughness to reduce the velocity of runoff and wind. The various conservation methods are described under biological and engineering techniques.
I. Biological Techniques:
(1) Crop Rotation:
The simplest way to combine different crops is to grow them consecutively in rotation. The suitable crops for use in rotations are legumes and grasses. These provide good ground cover, help to maintain or even improve the organic status of the soil, thereby contributing to soil fertility.
(2) Cover Crops:
Cover crops are grown as a conservation measure either during the offseason or as ground protection under trees. Ground covers are grown under tree crops to protect the soil from the impact of water-drops falling from the canopy.
(3) Strip Cropping:
In strip cropping, row crops and protection-effective crops are grown in alternating strips aligned on the contour or perpendicular to the wind. Erosion is largely limited to the row-crop strips and soil removed from these is trapped in the next strip down-slope or downwind.
(4) Mulching:
The mulching is the covering of the soil with crop residues such as straw, maize stalks, palm fronds or standing stubble or synthetic mulch. The cover protects the soil from raindrop impact and reduces the velocity of runoff and wind.
(5) Wind Breaks and Shelter-Belts:
The shelter belts and wind breaks are barriers of trees and shrubs planted to reduce wind velocity, evaporation and wind erosion. Shelterbelts are placed at right angles to erosive winds at regular intervals, break up the length of open wind blow. Shelterbelts are strictly living wind breaks.
2. Engineering Techniques:
The engineering techniques are used to control the movement of water and wind over the soil surface. A range of techniques is available and the decision on which to adopt depends on whether the objective is to reduce the velocity of runoff and wind, increase surface water storage capacity or safely dispose of excess water. These are normally employed in conjunction with biological techniques.
(1) Contour Bunds:
The contour bunds are earth banks, 1.5 to 2 m wide, thrown across the slope to act as a barrier to runoff, to form a water storage area on their upslope side and to break up a slope into segments shorter in length.
(2) Terraces:
These are earth embankments constructed across the slope to intercept surface runoff and convey it to a stable outlet at a non-erosive velocity, and to shorten slope length. The terraces may be of three types: diversion, retention and bench.
(3) Stabilization Structures:
The stabilization structures play an important role in gully reclamation and gully erosion control. Small dams, usually 0.4 to 2.0 m in height, made from locally available materials such as earth, wooden planks, brushwood or loose rock, are built across gullies to trap sediment and thereby reduce channel depth and slope. These structures have a high risk of failure but provide temporary stability and are therefore used in association with agronomic treatment of the surrounding land where grasses, trees and shrubs are planted.
Erosion is a natural continuing process and will go on into the future regardless of anything man may do. It is abnormal and undesirable process started by man’s activities and subject to his control. Unchecked erosion produces poverty and undermines the strength of nations, because soil is the strength of Nation. So soil must be saved by holding it in the fields.