The removal of soil by running water and wind is known as soil erosion. The soil-forming process and the erosional process of running water and wind are continuous. Generally, there is a balance between these two processes.
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The rate of removal of fine particles from the surface is the same as the rate of addition of particles to the soil layer. Sometimes such a balance is disturbed by natural or human factors, leading to a greater rate of removal of soil. When this happens, the entire soil layer may be removed in a few years.
Types of Soil Erosion:
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Wind and water are powerful agents of soil erosion because of their ability to remove soil and transport it.
(a) Erosion by Water:
Erosion by water can be of several types, for example, sheet erosion, gully erosion, stream bank erosion, shore erosion and slip erosion.
1. Sheet Erosion:
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When a layer of the soil on the surface is removed over a large area by running water, it is called sheet erosion. Sheet erosion is harmful since it removes the finer and more fertile top soil.
2. Rill Erosion:
This is the second stage of sheet erosion. Small finger-like rills begin to appear on the landscape. Over a period of time, the fine rills increase in number and also become deeper and wider. This reduces the actual area under cultivation and the yield of crops decline.
3. Gully Erosion:
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When soil is removed by water flowing along definite paths downs the slope or in channels, it is called gully erosion. Gullies cut up agricultural land and make it unfit for cultivation. Badland is a region with a large number of deep gullies or ravines, e.g., Chambal Valley in Madhya Pradesh.
4. Stream Bank Erosion:
The continuously flowing water erodes the banks of streams and rivers. Gradually the bed of the river widens.
5. Shore Erosion:
The tidal waves dash against coastal rocks, causing them to erode bit by bit.
6. Slip Erosion:
During heavy rains, water percolates into the soil until it is unable to penetrate further by the underlying impervious rocks. On steep land, the heavy moisture-laden soil often comes down bodily, resulting in a landslide.
(b) Wind Erosion is significant in desert and semi-desert regions. In regions with heavy rainfall and steep slopes, erosion by running water is more significant.
In some regions, the top soil is blown away by wind in the dry season, and washed away by running water in the wet season.
Causes of Soil Erosion in India:
Soil erosion takes place when soil is removed faster than it is formed. There are many different processes by which soil is removed but the cause is usually the removal of trees and other vegetation that hold the soil together and in place.
1. Topography and Slope of the Land:
Hilly Region:
Rainfall occurs in torrents and washes away the top layer of the soil. Also, the steep slopes stimulate the eroding power of the rainwater.
Plains:
Here the erosion is comparatively less than on slopes. But in regions where the rivers overflow onto the plains, the erosion is severe. For example, the entire basin of the Kosi River is threatened by the over-flowing of river.
2. Nature of Rainfall:
Floods and torrential rains cause more damage than light or moderate showers spread over many days.
i. The action of heavy rain is stronger when there are no trees and the plains are bare.
ii. When a prolonged dry spell is followed by sudden heavy rain, sheet erosion takes place. This is because the ground gets baked hard and the soil is unable to absorb the water easily.
3. Nature of Soil:
Porous soils with good water-absorbing capacity are least subject to erosion, while the impervious soils are gradually eroded by the action of water.
4. The Human Factor:
Man and his activities are responsible for soil erosion to a great extent. As the human population increases, the demand on the land also increases. Forest and other natural vegetation are removed for human settlement, for cultivation, for grazing animals and for various other needs. Improper use of land leads to soil erosion.
(a) Deforestation: is the removal of trees and other vegetation which hold the soil together. When vegetation is removed, the soil surface is loose and more easily removed by running water and wind. Rain water that could have been absorbed by the soil, rapidly runs off the surface carrying soil with it.
(b) Overgrazing of Land: by animals results in removal of grass over a large area making it easy for wind and running water to remove the soil. In many parts of India, hill sides have become barren because of overgrazing by goats.
(c) Improper Farming Techniques: Ploughing fields in the traditional up and down manner along the slopes makes it easier for running water and wind to cause erosion.
Pattern of Soil Erosion in India:
In India, soil erosion is the wearing away of the topsoil cover by natural agencies such as water and wind and also as a result of human and animal interference. Sheet erosion is common on relatively steeper slopes of the heavy rainfall areas in the Himalayan Foothills, over the north-eastern parts of the Peninsula, in Assam and in the Sahyadris and the Eastern Ghats.
Rill erosion is active over a wide area in Bihar, Uttar Pradesh, Madhya Pradesh, and in the semi-arid parts of the Peninsula in Maharashtra, Karnataka, Andhra Pradesh and Tamil Nadu. The chhos of northern Haryana and Punjab and the badlands of Madhya Pradesh, Rajasthan and Uttar Pradesh, have resulted due to gully erosion on an extensive scale.
Gully erosion is the most spectacular type of erosion. They have already degraded about 40 lakh hectares of land in the country. This problem affects mainly the states of Uttar Pradesh, Madhya Pradesh, Bihar, Rajasthan, and Gujarat. Wind erosion is active in dry areas devoid of vegetation cover. This type of soil erosion is common all over Rajasthan and Gujarat.
Human and animal interference in a variety of ways leads to soil erosion. Deforestation, overgrazing and shifting cultivation are responsible for soil erosion in large areas. The chhos of Punjab and Haryana and the ravines of Madhya Pradesh, Rajasthan and Uttar Pradesh have resulted, to a certain extent, due to reckless cutting of forests in these areas.
Erosion due to overgrazing by sheep and goats is very common over the hilly areas of Madhya Pradesh, Rajasthan and the low rainfall areas of Maharashtra, Karnataka and Andhra Pradesh.
It is also common in Jammu and Himachal Pradesh. Shifting cultivation is responsible for soil erosion in many tropical forest areas in the country. This mode of cultivation is a serious menace in Assam, Meghalaya, Tripura, Nagaland, Mizoram, Kerala, Andhra Pradesh, Orissa and parts of Madhya Pradesh. It is estimated that over 80,000 hectares of cultivated land of India have already been lost.
Ravines are wide spread in the Chambal basin. They cover over 6 lakh hectares of land in Gwalior, Morena and Bhind districts of Madhya Pradesh, and Agra, Etawah and Jalaun districts of Uttar Pradesh. In Tamil Nadu, ravines are common in South Arcot, North Arcot, Kanyakumari, Tiruchirapalli, Chingleput, Salem and Coimbatore districts. In West Bengal numerous gullies and ravines exist in the upper catchment areas of the Kangsabati River in Purulia district. The country is losing about 8,000 hectarers to ravines every year.
It is estimated that over 80,000 hectares of cultivated land of India have already been lost and a much larger area is rendered less productive by soil erosion every year. Soil erosion is a national menace for the Indian agriculture and its bad effects are seen in other spheres also. Eroded materials are carried down to rivers and they lower down their carrying capacity, and cause frequent flood and damage to agricultural lands.
The bed of the Kaveri River in Tiruchirapalli and Tanjavur districts of Tamil Nadu, for example, has gradually risen and many of the old irrigation sluices and drainage inlets have got blocked. The shallowing of the Brahmaputra causes flood every year. Silting of tanks is another serious consequence of soil erosion. A large number of tanks in different parts of the country get silted every year.
The two most active agents of soil erosion in India are wind and running water. Wind erosion is common in arid and semi-arid areas of Gujarat; Rajasthan and Haryana Light soils are more susceptible to wind erosion than heavier soil. Sands removed by wind spread over adjoining cultivated lands and destroy their fertility.
Water erosion which is more serious and occurs extensively in different parts of India takes place mainly in the form of sheet and gully erosion. Sheet erosion takes place on lower lands after heavy shower and the soil removal is not easily noticeable. Gully erosion is common on steep slopes. Gullies deepen with rainfall, cut the agricultural lands into small fragments and make them unfit for cultivation.
Several factors contribute to soil deterioration. For example, when the forests are cut, the supply of humus to the soil stops, and the capacity of running water to remove its top layer grow. If drainage system is disturbed, water logging or loss of soil moisture takes place; and if it is overused, it loses its fertility.
The removal of the soil by running water in wet areas and wind in dry areas is referred to as soil erosion and the removal of its organic and mineral content is referred to as soil exhaustion. Soil degradation results from its misuse.
The agencies involved in soil erosion, exhaustion and degradation are running water, wind, snow, animals and human beings. Human beings disturb the soil ecology by deforestation, overgrazing and unscientific methods of cultivation. Areas with poor vegetation and steep slope particularly in undulating terrain and along the river courses often exhibit badland topography.
Erosion caused by the Kosi River has become proverbial. The dry lands of Rajasthan are subjected to wind erosion. Intensive farming and overgrazing accelerate erosion and desertification.
Deforestation is one of the major causes of soil erosion; plants keep soil bound in locks of roots and thus prevent erosion. They also add humus to the soil by shedding leaves. Forests have been denuded practically in whole of India but their implication on soil erosion is more in hilly areas of the country especially in Himachal Pradesh and the Western Ghats.