Soil erosion is a process of detachment and transportation of soil by natural agencies of water and wind.
Factors Influencing Soil Erosion:
There are many factors which influence the process of soil erosion; these are discussed below:
Precipitation is the most forceful factor causing erosion.
Erosion is dependent on the amount, duration, intensity and frequency of rainfall. By the action of dashing rain drops on soil, soil granules are loosened, detached and separated into fine particles. Erosion is greater where the rainfall is not only heavy, but concentrated over short periods.
2. Slope of Topography:
The slope accelerates erosion as it increases the velocity of the flowing water.
The vegetative cover protects the soil from the beating and dispersing action of the raindrops by forming a canopy over the soil surface. Vegetation also acts as a mechanical obstruction to flowing water, thus reducing its erosive potential. The plant roots help in building a better structure. They said in opening the soil and thereby accelerating water absorption and reducing surface run-off.
The infiltration and permeability of the soil is improved by the practice of proper tillage and thereby reducing the chances of erosion. But excess tilling exposes the soil to erosion, especially by wind.
5. Nature of the Soil:
Erodability of the soil is influenced by the nature of the soil, particularly its texture, structure, organic matter, amounts and kinds of salts present, presence of hard pan in the soil and presence of high water table.
6. Soil Moisture:
The presence of high water table checks the infiltration and permeability, thus allowing more flow of water on the surface, and greater erosion. At the same time, long continuous rainless periods cause loosening of soil and thus expose the soil to erosion by wind.
7. Wind Velocity:
Stronger winds have greater erosive potential, thus wind velocity is directly proportional to intensity of erosion.
Types of Erosion by Water:
Following are the types of soil erosion caused by water (Fig. 22.3).
1. Splash Erosion:
This type of erosion occurs when the falling raindrops splash on the soil, and beat the bare soil into flowing mud.
2. Sheet Erosion:
This occurs when soil is removed uniformly in a thin layer from the entire surface area. Movement of soil by splash erosion is the primary cause of sheet erosion.
3. Rill Erosion:
This type of erosion takes place when the run-off water, laden with soil flowing along the slopes, forms fingerlike channels. Rill erosion is an intermediate stage between sheet erosion and gulley erosion.
4. Gully Erosion:
As the volume of concentrated run-off increases and attains more velocity on slopes, it enlarges the rill into gullies. At an advanced stage, gullies result in ravines, which are sometimes 50 to 100 feet deep. In India ravines cover about 10 million hectares.
5. Slip Erosion:
Landslides cause slip erosion— big masses of soil and rock slip down, thus damaging the fields in the foothills and causing obstructions in communication. The effect of slip erosion is localised.
6. Stream Bank Erosion:
Streams and rivers change their courses by cutting one bank and depositing the silt loads on the others.
During flash floods, the damage is much accelerated. The Kosi river in Bihar is reported to have changed its course westwards by 100 km within the last 100 years.
7. Sea Shore Erosion:
This type of erosion is caused by the striking action of strong waves.
Causes of Soil Erosion:
Soil erosion is also referred to as the creeping death. The causes of soil erosion are directly related to improper landuse, and are, therefore, entirely man-made.
These include the following:
Removal of vegetation cover has caused widespread erosion in Western Ghats, Uttar Pradesh and in Himachal Pradesh.
2. Faulty Cultivation Methods:
For instance, in the Nilgiris, land has been opened for cultivation of tuber crops like potatoes and ginger without undertaking anti-erosive measures like terracing of slopes. Also forests on slopes have been cleared at places to make way for plantation crops. Such faulty cultivation methods have caused soil erosion—landslides are a common feature in these areas.
3. Shifting Cultivation:
An ecologically destructive and uneconomic cultivation method is slash and burn or shifting cultivation which is practised in hill areas of North-East, Chhotanagpur, Orissa, Madhya Pradesh and Andhra Pradesh. Vast areas have suffered erosion of soil in hill areas of North-East because of shifting cultivation.
A surplus of livestock population in our country is a big strain on grasses and fodder. The tread of cattle hardens the soil and prevents new shoots from emerging. Overgrazing by goats is a serious problem in certain stretches of the Aravalis and in Punjab and Himachal hills. The goats not only pull off leaves and branches, but they also uproot grass, as opposed to sheep, which only nibble the top shoots.
5. Diversion in Natural Drainage Channels by Railway Embankments and Roads:
Railway tracks and roads have had to be constructed in such a manner that they are at a higher level than the surrounding area. But sometimes, road and rail embankments come in the way of natural drainage channels. This causes waterlogging on one side and water loss on the other side of embankments. All these factors contribute to erosion in one way or the other.
6. Lack of Proper Surface Drainage:
Because of proper drainage, waterlogging occurs in low lying areas which loosens the top-soil and makes it prone to erosion.
7. Denuding Forest Fires:
These fires, sometimes natural but often man-made, are very destructive. As a result of these, the forest cover is lost forever and soil is exposed to erosion.
Effects of Soil Erosion:
The main effects of soil erosion are as follows.
1. Loss of Soil:
The top-soil is lost by erosion which is the most fertile section, having evolved over centuries of soil-forming processes. Due to formation of gullies and ravines, valuable agricultural lands are lost.
2. Harmful Effects of Erosion on Organic Matter and Soil Structure:
Erosion of upper layer of soil decreases the content of organic matter and as other nutrients. As a result, the soil structure gets impoverished.
3. Decline in Soil Capacity:
When the soil is removed bodily from field, both potential and available plant food along with mineral material is carried away. As erosion progresses, the compact soil of low infiltration capacity is approached. The ability of the land to supply moisture for plant growth is reduced and the beneficial activity of micro-organisms lessened. Due to these bad effects, the yields are lowered.
4. Deposition of Sand and Gravel on Agricultural Lands:
The wind-borne sand encroaches the arable lands and makes them unfertile. Crops are damaged due to sand storms.
5. Flooding of Streams:
Soil erosion in catchment areas of streams due to deforestation and other destructive activities leads to silting of streams and reservoirs. This reduces the capacity of these water bodies to carry large volumes of water, as they occur during the rainy season. This way the streams are more prone to flooding. One such example is Brahmaputra river which has been exposed to siltation because of large-scale deforestation in the hills, and the floods in Brahmaputra valley have become an annual phenomenon now.
Magnitude of the Problem in India:
In India, nearly 80 mHa area is exposed to the threat of soil erosion, and 43 mHa area is actually affected. In states like Madhya Pradesh. Rajasthan, Maharashtra and Punjab, upto 15 per cent of the total land suffers from soil erosion. It is reported that the annual loss of fertility by erosion is 20 times faster than what is lost by growing crops. Each year, 10,000 hectares area is exposed to erosion. Nearly 145 mHa area in India is in need of conservation measures.
Some Glaring Examples of Soil Erosion:
(Fig. 22.5) Different areas in India are affected by different kinds of erosion.
Potato cultivation on steep slopes without proper terracing has caused widespread erosion here.
2. Gullies or Ravines:
Because of gully erosion, nearly 10 mHa area is affected. These gulleys are known by different names—khars in Gujarat, Maharashtra and Karnataka; ravines in Madhya Pradesh, Uttar Pradesh and Rajasthan along Yamuna and Chambal; kotar lands in Gujarat and chos (the rainy season torrents) and landslides in Hoshiarpur in the Shivaliks.
3. Regions of Low and Uncertain Rainfall:
Extensive regions of low and uncertain rainfall, forming parts of Punjab, Madhya Pradesh, Maharashtra, Andhra Pradesh and Karnataka, are characterised by scanty, ill-distributed and highly erosive rains, undulating topography, high wind velocity and generally shallow soils. The period of heavy downpours from August to October is the period of the heaviest erosion in these regions.
4. Desertification of Rajasthan:
Wind erosion, coupled with loss of surface cover in the Aravallis has made the challenge of halting the march of desert in Rajasthan, one of the most vital ecological problems facing the country today.
5. Coastal Erosion:
An extreme example of soil erosion is to be seen in sand movement from the coast in the Saurashtra region of Gujarat, where the once flourishing ports are now covered with advancing sand-dunes.
Methods to Check Soil Erosion:
These measures are a part of the overall strategy to conserve soil and water. These measures can be biological or mechanical (Fig. 22.6). These are discussed below.
The following are biological methods to control soil erosion:
Improving the Existing Surface Cover:
This can be done by resorting to cover cropping by growing groundnut or berseem (a fodder crop) or through grasslands development by growing grasses like dub, kudzu, pans and dinanath.
This practice consists of growing erosion-permitting crops (jowar, bajra, maize) in alternate strips with erosion checking close- growing crops (grasses, pulses). The erosion checking strips check and hold the flowing water and soil.
This refers to growing of two or more different crops in sequence in a field for maintaining the soil fertility. Continuous growing of clean-cultivated crops (e.g., tobacco) causes more erosion. A good rotation should include densely planted small grains, spreading legume which may check soil erosion.
This means leaving crop residue or vegetative litter on the land as a surface protection against erosion and for conserving moisture by favouring infiltration and reducing evaporation.
Using Organic Manures:
Organic manures like cowdung, green manure, farmyard manure etc., improve the soil structure. Granular and crumby structures increase infiltration and permeability in the soil and conserve soil moisture.
Other measures include checking overgrazing, reducing surplus cattle, stopping shifting cultivation and taking preventive measures against forest fires.
Mechanical Mea sures:
The mechanical measures that can be used to control erosion are as follows.
On sloping lands, all tillage operations should be done at right angles to the slope of the land. This way, each furrow intercepts the flowing water and allows it to soak into the soil.
The idea is to break the slope of the land into smaller, more level compartments by constructing mechanical structures of suitable size along contours. Each bund, thus, holds the rainwater within each compartment.
On steeper slopes, terraces or flat platforms are constructed in steps in a series along the slope. This way water is retained on each terrace which can be used to raise crops.
Constructing Proper Drainage Channels and Plugging the Gullies:
This refers to scooping out small basins at regular intervals on slopes which help in checking the run-off and in conservation of water.
This refers to trapping or channeling of water into low-lying areas. This helps in checking the run-off and also acts as a flood control measure.
Scientific Slope Management:
The cropping activity on slopes should be taken up as per the nature of slope. If the slope is between 1:4 and 1:7, proper farming can be done; if more, pastures should be developed; if still more, forestry operations can be undertaken; if it is still greater, then terracing is required before any cropping activity can be done.