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Term Paper on Soil Erosion
Term Paper # 1. Introduction to Soil Erosion:
Soil erosion has been our nation’s most devastating environmental disaster. Soil in quantities of incomprehensible magnitude has been lost from the land where its forfeited productivity could have contributed to our national wealth. Soil erosion is the detachment and transportation of soil material from one place to another through the action of wind, water in motion or by the beating action of raindrops. Soil erosion is caused by detachment and removal of soil particles from land surface. With the advent of modem civilization, the pressure on land increased which leads to it’s over exploitation and subsequently, its degradation.
Types of Soil Erosion:
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The process of soil erosion goes through two main stages of activities, namely the detachment of soil particles and their transportation. The principal agents responsible for these two stages of activities are wind, water and gravity. Wind erosion occurs in arid and semi-arid areas where rainfall is scanty and the day temperatures are very high. Water erosion occurs in areas where rainfall is high and gravity erosion in areas that are near to pits, rivers, roads, etc.
The types of soil erosion caused by three natural agents-winds, water and gravity are described below:
Classification of Erosion:
Erosion may be broadly classified into two groups:
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1. Geological Erosion:
Geological erosion, sometimes referred to as natural or normal erosion, represent erosion under the cover of vegetation. It takes place as a result of the action of rainfall, runoff, topography, wind velocity, atmospheric temperature and gravitational forces. It is continuous and slow but constructive process. Soil removal is fairly balanced with soil formation. Erosion caused by biological activities is called as normal erosion.
2. Accelerated Erosion:
Accelerated erosion of soil is caused mainly by managerial errors such as raising of crops without adopting any soil conservation practice, deforestation, etc. It leads to erosion in excess of the threshold value of the new soil formation causing severe deterioration of the top surface of the land. It occurs in faster rate and considered destructive process. It is the erosion in which rate of soil loss exceeds the rate of soil formation.
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Causes of Soil Erosion:
The causes of soil erosion are as follows:
1. Destruction of Natural Vegetation:
(A) Deforestation:
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i. Shifting cultivation.
ii. Forest fire.
(B) Over Grazing:
i. Excess grazing.
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ii. Non-rotational.
(C) Burning of Grass Land.
Mismanagement of Land:
(A) Improper Land Management:
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i. Up and down hill cultivation on sloppy land.
ii. Un-successive growing of grass, crops.
iii. Removal of organic matter and plant nutrients.
iv. Faulty irrigation systems i.e., flood irrigation on sloppy land.
v. Growing of wider row crops on sloppy land.
(B) Pressure on Agriculture:
i. Animal.
ii. Human being.
Term Paper # 2. Water Erosion:
Erosion of soil by water is caused by its two forms- liquid as the flowing water and solid as the glaciers. The impact of rainfall causes splash erosion. Runoff water causes scarping and transport of soil particles leading to sheet, rill and gully erosion. Water waves cause erosion of bank sides, reservoirs, lakes and ocean. The sub-surface runoff causes soil erosion in the form of pipe erosion, which is termed as tunnel erosion.
The glacial erosion causes heavy landslides. In India, glacial erosions are mainly confined to Himalayan regions. Movement of soil, by rain water running rapidly over the exposed land surface, causes severe erosion. This is most commonly found in humid and sub-humid areas where rainfall is excess and no vegetation over the land. In water erosion, greater soil fertility loss occurs.
Forms of Water Erosion:
i. Raindrop Erosion:
The raindrop characterized by rainfall has a significant effect on the magnitude of splash erosion. The other factors which affect splash erosion are type of soil, land topography and vegetative cover. Splash erosion is caused by the impact of falling raindrops on the soil surface. Splash erosion is influenced by rainfall intensity and rainfall size. Splash erosion takes place, when rainfall intensity exceeds infiltration rate.
On steeper slopes splash erosion is higher as soil particles are pulled downward to very large distance by the gravitational forces. The mechanical energy is manifested in two forms: kinetic and potential. Kinetic energy is the energy possessed by a substance by virtue of its motion and is proportional to the product of the moving mass and half of the square of the velocity of the mass, i.e.,
E =1/2 mv2
where,
E = kinetic energy
m = mass of water or drop in question
v = velocity of the mass of water
Potential energy is the energy possessed by a substance by virtue of its position and is expressed as,
Ep = mgh
Where,
Ep = potential energy of the mass of water
m = mass of water in question
g = acceleration due to gravity
h = height of water mass above the reference level
ii. Sheet Erosion:
This is caused by the unconfined flow of water running across the surface. The effects of sheet erosion are often hard to distinguish because such thin layers of soil are being removed.
The characteristics are:
a. Sheet erosion is the removal of a thin and fairly uniform layer of surface soil by the action of rainfall and runoff water.
b. Sheet erosion takes place on smooth and regular filled slope.
c. Shallow soils suffer greater reduction in production than deep soil.
d. Extremely harmful to the land.
e. Splash erosion and sheet erosion occur side by side.
f. In India, about 60 lakh ha lands are affected by sheet erosion.
iii. Rill Erosion:
It is caused by water concentrating into innumerable, closely-spaced small channels. Rill erosion is the removal of soil by water from small channels which develop due to concentration of surface flow along the slope. It generally develops on bare and unprotected land where soil conservation measures are not adopted.
In advanced stage, it may take the shape of gullies. The rill erosion is an intermediary stage between sheet erosion and gully erosion. Removal of surface soil by running water with the formation of narrow shallow channels can be leveled by normal cultivation. Rill erosion is more serious in soils having a loose shallow top soil.
iv. Gully Erosion:
Gullies are steep-sided trenches formed by the coalescence of many rills. Once they start, it is difficult to stop. It is the advanced stage of rill erosion which cannot be obliterated by normal tillage operations. Rills with more than 30 cm depth are generally called gullies. Gullies obstruct the operation of farm machinery and tend to deepen and widen with every heavy rainfall.
They cut up large fields into small fragments and in course of time, make them unfit for cultivation. The rate of gully erosion depends on several factors, viz., runoff producing characteristics of the watershed, drainage area, soil characteristics, channel alignment, slope, vegetative cover on the ground, etc. The gullies are classified based on size, shape and state of gully.
The development of gully occurs due to following four stages:
(a) Formation Stage:
Channel erosion takes place by downward scour of the topsoil. If the topsoil can provide resistance, this stage proceeds slowly.
(b) Development Stage:
Upstream movement of the gully head and simultaneous enlargement of width and depth take place. The weak parent material is rapidly removed by cutting C horizon.
(c) Healing Stage:
Vegetation begins to grow in the channel and further erosion ceases.
(d) Stabilization Stage:
The gully bed and sides reach a stable slope and sufficient vegetation grows to anchor the soil and to trap soil flowing from upstream. New topsoil develops and vegetative cover grows.
Other Forms of Erosion:
Besides the four main water erosions described above, there are some special forms of erosion also occur.
i. Landslides or Slip Erosion:
During monsoon season or heavy down-pour, the hills get saturated with water resulting in instability of the land mass. The complete surface portion of soil may slip down from its actual position, causing landslides. Landslides are very common in hills and huge quantities of soil are washed away in streams and rivers.
Hills devoid of vegetation are more prone to slip erosion, causing damage to agricultural lands, orchards, communication systems and human lives. Minor landslips occur during the drying phase in the months of May and June, especially in the sandy hills of Shiwaliks.
ii. Stream Bank Erosion:
It is caused by streams and torrents, also called chos, cutting their banks and adjoining fields. It is mainly due to runoff flowing over the side of the stream banks or by scouring and undercutting below the water surface, the latter process being more serious. It is influenced by deforestation and overgrazing. Scour erosion is influenced by the velocity and direction of water flow, depth and width of channel and soil texture.
iii. River Bank Erosion:
This is a common phenomenon in the rivers of the eastern and western Himalayan regions. During floods, the rivers undermine their banks and big chunks of land mass are engulfed by flood water. This leads to broadening of river beds and changes in their courses.
iv. Coastal Erosion:
The strong waves of sea strike against the seashore causing large scale soil erosion. In this case, erosion is a combined effect of wind and water.
Harmful Effect of Water Erosion:
i. Loss of top fertile soils.
ii. Accumulation of sand or other unproductive coarse soil materials on other productive lands.
iii. Silting of lakes and reservoirs.
iv. Silting of drainage and irrigation channels.
v. Decrease in water table.
vi. Fragmentation of land especially in gully erosion.
vii. Increase in flood (heavy floods in the river).
viii. Adverse effect on public health.
ix. Loss of nutrients.
x. Destruction of soil structure.
xi. Adverse effect on crop yield.
xii. Hindrance of farming operations.
xiii. Affecting transportation.
xiv. Destruction of vegetation.
Term Paper # 3. Wind Erosion:
Wind erosion is the detachment and transportation of soil particles by the forces of moving wind. In regions, where the precipitation is low and the atmospheric temperature during the day is high, the velocity of wind is invariably very high. Such climatic conditions are generally prevalent in arid and semi-arid regions where the wind velocity is also very high.
Wind erosion is caused by mismanagement of land resources such as intensive farming, over grazing, deforestation, etc. It is characterized by movement of soil by wind rapidly over the exposed land surface. It is most commonly found in arid and semi-arid areas where rainfall is insufficient and no vegetation over the land is available.
Mechanism of Wind Erosion:
Wind erosion is characterized by transport of soil particles through wind by three processes depending on the size of soil particles that are moved by wind:
i. Surface creep.
ii. Saltation.
iii. Suspension.
i. Surface Creep:
The movement of larger size particles (> 0.5 mm) occurs by the process of surface creep, where the movement is by rolling and sliding of soil particles on the land surface caused by direct push of wind and collisions between the saltating particles, i.e., the particles which are already moving by the process of saltation. Surface creep process destroys soil structure.
ii. Saltation:
It is the next step of movement taken by soil particles which are in process of surface creep, after these particles have gained sufficient speed. On gaining speed, these particles start to hop, leap and bounce off the ground and the movement by this process is called saltation.
Soil particles of medium size (0.1-0.5 mm) are carried by wind in series of short jump/bounces/hop/leap/lift caused by direct pressure of wind on soil particles. Saltation process is less in rough surface. Soil particles can jump up to the height of 1.0 m (average 30- 60 cm) and jump particle moves 200-1000 round/minute.
iii. Suspension:
Very small-sized particles of soil (0.1 mm) are carried by suspension in the air. It is the floating of small sized soil particles in air. The soil particles stay in suspension, because the downward movements of soil particles due to their weight are balanced by the lifting forces of eddies in the fluid. When soil particles are very small (<0.1 mm), they are carried over long distance up to 100 km. The particles come down only with rains or when wind velocity is decreased and it makes smooth surface.
Forms of Wind Erosion:
There are five different forms of wind erosion:
(1) Abration/Corrasion:
Abration form is found in cloddy soils. With the effect of strong winds, the clods are contacted and broken up into small pieces.
(2) Efflation:
This form is found in silty and clay particles dominant in soil.
(3) Efflution:
This form of wind erosion is found in calcareous soil where clay and sand particles are dominant. With the effect of strong wind clay particles carry away a long distance by suspension process and big particles or sliding i.e., sand carry away a short distance by rolling or sliding.
(4) Detrusion:
This form is found where bunds are constructed in cultivated soil with the effect of strong wind, soil particles of bund slip down and deposited in lower place.
(5) Extrusion:
This form of wind erosion is found in sandy soils where sand particles are dominant. With the effect of strong wind, sand particles carry away from one place to another by rolling/ sliding.
Harmful Effects of Wind Erosion:
i. Loss of soil and plant nutrients.
ii. Loss of production.
iii. Change of soil textural class.
iv. Reduction in water holding capacity of the soil.
v. Exposure of hard rocks.
vi. Formation of sand dunes.
vii. Destruction of soil structure.
Wind Erosion Factors:
The following factors affecting soil erosion by wind are assessed by the Wind Erosion Equation (WEE). A Wind erosion equation is an erosion model designed to predict long-term average annual soil losses from a field having specific characteristics.
The equation is:
E = f (IKCLV)
Where,
E = Estimated average annual soil loss expressed in tons per acre per year
I = Soil credibility index
K = Soil ridge roughness factor
C = Climatic factor
L = Equivalent unsheltered distance across the field along the prevailing wind erosion direction
V = Equivalent vegetative cover
Other Factors:
Tillage – Soils are loosened by the excess tillage = More erosion
Structure – Single grain structured soil = More erosion
Organic matter – Low organic matter in soil = More erosion
Vegetation – Soil covered with vegetation = Reduce erosion
Weather – Continuous dry weather = Increase erosion
Wet and moist soils = Decrease erosion
Wind velocity – High wind velocity = Increase erosion
Soil – Rough surface = Less wind erosion
Smooth surface = More wind erosion
Cloddy soil = Less wind erosion
Without clods = More wind erosion
Control of Wind Erosion:
Any practice or measure that reduces the wind velocity or improves the soil characteristics is helpful to control wind erosion. Improved soil characteristics should have better structure, improved cohesive property and good moisture holding capacity. Some of the measures may provide both the requirements. Vegetation improves the soil structure and at the same time retards the surface wind velocity.
In general, the following practices may be followed to control the wind erosion:
i. The soil should be covered with vegetation or crop residues as far as possible.
ii. Limited cultivation should be done.
iii. Dry soils should not be tilled.
iv. Permanent vegetation may be established on unproductive soils.
v. After the rains, the soil may be tilled so that clod formation takes place.
vi. Tillage implements should be selected in such a manner that rough surface is formed and crop residue is not buried.
vii. Overgrazing should be avoided.
Principal methods of reducing surface wind velocity are vegetative control, tillage practices and mechanical method. Vegetative control consists of cultivated crops, field and strip cropping, stubble mulching, shrubs and trees to act as mechanical barrier to wind. Windbreaks may also be used as a mechanical barrier.
Some important practices are discussed below:
i. Strip Cropping:
Field and contour strip cropping consists of alternate strips of row (erosion-susceptible) crops and close growing (erosion-resistant) crops in the same field. The strip cropping is laid out generally parallel to the field boundary or perpendicular to the erosive wind direction.
The main benefits of strip cropping are:
i. Vegetation provides physical protection against blowing of soil.
ii. Soil erosion is limited to a distance equal to the width of the erosion susceptible crop.
iii. Better conservation of moisture takes place.
iv. Particles carried in saltation are trapped.
In a mechanized farm, movement of machinery becomes difficult due to narrow strips. In case of attack by insects, there is more number of edges for protection. The width of the strips should be selected in a way such that the farming operation is not hampered and at the same time much erosion does not take place. For example, in a sandy soil the width of the erosion susceptible crop should be limited to 6 m. But for movement of machinery, the width has to be increased. In a sandy loam soil the width can be increased up to 30 m.
ii. Windbreak:
A windbreak is defined as type of barrier for protection from winds and refers to any mechanical or vegetative barriers of buildings, gardens, orchards and feed lots. Windbreaks made up of just mechanical barriers are not very useful for field crops. However, they are frequently used for the protection of farm sheds and small areas. The mechanical barriers include brush fences, board walls, vertical burlap or paper strips.
Brush matting, rock or gravel barriers are also used as windbreaks. Some of these barriers are impermeable and others are semi-impermeable. Generally the semi-impermeable barriers are more useful as they provide better diffusion and eddying effects on the leeward side of the barrier.
When vegetable crops in organic soils are required to be protected, vertical burlap or paper strips are often used. Brush matting, debris, rock, gravel etc. are more useful for stabilizing sand dune areas.
iii. Shelterbelts (Shrubs and Trees):
A shelterbelt usually consisting of shrubs and trees is a longer barrier than windbreak. It is primarily used for protection of field crops, soils and conservation of moisture. The shelterbelt provides larger protection and wind erosion control. It also saves fuel, increases livestock production, reduces evaporation, prevents firing of crops from hot winds. In addition, it may provide better fruiting in orchards, make spraying of trees for insect control more effective manner.
iv. Tillage Practices:
Tillage practices if properly adopted can reduce the soil blowing to a great extent. Similarly, faulty tillage operations increase the soil erosion by wind. If the soil is pulverized and the crop residues are buried due tillage operations, erosion problem increases.
The effective way of prevention of wind erosion is by producing a rough, cloddy surface and exposing the crop residues on the surface. If the land is ploughed at optimum moisture content after the rains, big clod and large aggregate formation takes place, which ultimately decreases the process of erosion.
Factors Affecting Soil Erosion:
Numerous factors affect soil erosion depending upon the local conditions with regard to physical, chemical and biological nature of the soil.
The major factors affecting soil erosion are:
i. Climate, especially rainfall and wind velocity and their intensity in time and space
ii. Topography, slope of land, its nature, degree and length
iii. Physical, chemical and biological characteristics of the soil
iv. Nature of ground cover and land use pattern
v. Biotic interference due to human beings and animals
vi. Conservation practices
vii. Mass erosion, such as earthquakes, landslides, torrents, floods, etc.
Mathematically, soil erosion may be depicted by functional equation:
Erosion= f (cl, t, v, s, h,..…)
i. Climate:
The major climatic factors influencing runoff and erosion are rainfall, temperature and wind. Rainfall is by far the most important factor causing soil erosion by water. The soil erosion depends on the intensity, kinetic energy, amount, duration and frequency of rainfall.
The intensity, duration and frequency of rainfall govern the rate and the volume of runoff. A light rain can be easily absorbed in the soil and causes no runoff and soil loss. When the intensity of rain is more than infiltration rate, it causes runoff and soil loss.
The following are the factors in the soil erosion process:
i. The detaching capacity of the erosive agent.
ii. The detachability of the soil.
iii. The transporting capacity of the erosive agent.
iv. The transportability of the soil.
ii. Topography:
On flat lands, erosion is usually not a problem. It is the sloping lands that experience increasingly greater problems of erosion. The degree of slope and the length of slope are the two main features of topography affecting erosion. Velocity of the runoff water is influenced mainly by the degree of slope. If the land slope is increased four times, the velocity of water flowing over it is approximately doubled.
When the velocity is doubled, the erosive capacity as represented by the kinetic energy of the flowing water, is increased about four times. The land slope mainly its degree and length, are important in determining the extent of soil erosion. Higher the degree of slope and longer is its length, more is the soil erosion due to increased velocity of water running down the slope.
iii. Vegetation:
Vegetative cover retards the surface velocity of runoff water and transportation capacity reduces. Also the water remains on the surface for longer period and more amount infiltrates into the ground. The root system of the plants acts as binding agent for the soils and the detachability reduces. The decaying of roots, leaves, etc. increases the porosity of soil and the capacity to absorb water increases. Addition of humus increases the biological activity inside the soil.
Accelerated biological activity is favourable for growth of bacteria, earthworms, etc. This also creates a favourable condition for absorption and holding of more moisture inside the soil. Thus vegetative cover helps to conserve both soil and moisture in several ways. The nature of vegetative cover on the soil surface is a major factor in influencing soil erosion.
The following are the major effects of vegetation on soil erosion:
(a) Interception of Rainfall:
A part of the rainfall intercepted by the canopy of vegetation never reaches the soil, but is evaporated directly from the leaves and stems. This part of the rainfall does not contribute to the runoff. In addition, the vegetative canopy absorbs the impact of the raindrops and thereby minimizes the dispersion of soil by the raindrops.
(b) Runoff Velocity:
Vegetative cover is a hindrance to runoff. A well-distributed and close-growing vegetation not only slowdown the rate at which water travels down the slope but also tends to prevent a concentration of water. These two effects greatly reduce the erosive capacity of the runoff water.
(c) Root Effects:
The knitting and binding effect of root systems in the surface layer of soils, aggregates the soil into granules and increases its resistance to erosion.
(d) Biological Influence:
The soil fauna are most active in soils having ample vegetative cover. The soil under a thick forest covers increase aeration and good environment for the activities of beneficial bacteria and earthworms.
Soil Characteristics:
Erodibility of soil is influenced by its physical and chemical properties, including soil texture, structure and organic matter, nature of clay, amounts and kinds of salts present. Soil structure, texture, organic matter infiltration and permeability influence the runoff and soil loss. Fine soils are more susceptible to erosion than coarse soils. The organic matter helps in binding the soil particles and improves the water holding capacity of soil.
The three most significant soil characteristics which influence soil erosion are:
(i) Infiltration capacity.
(ii) Structural stability.
(iii) Antecedent soil moisture.
Biological Factors:
Biological factors that influence the soil erosion are the activities of man and animal. Felling of trees and faulty cultivation practices such as up and down cultivation in hilly areas, overgrazing by cattle, etc. contribute much to the soil erosion. Without such activities, the soil, climate, vegetation, etc., will remain in a balanced condition and no erosion will take place.
Predicting Soil Erosion:
Universal Soil Loss Equation (USLE):
By taking all factors into account, a prediction equation was developed for calculating the soil loss, called as Universal Soil Loss Equation (USLE). It accounts all parameters affecting the soils loss and predicts the annual soil loss. It computes sheet erosion. It is not used when slope are steeper than 20 per cent.
A = RKLSCP
Where,
A = Estimated gross erosion (t/ha/year)
R = Rainfall erosivity factor
K = Soil erodibility factor
L = Slope length factor
S = Slope gradient factor
C = Crop cover or vegetation management factor
P = Support conservation practice factor
Demerits of USLE:
This equation does not compute the sediment yield from the watershed directly. Annual sediment yield can be determined which is less important for design of water storage structures. This equation is used in small watershed to prediction of soil loss.
Modified Universal Soil Loss Equation (MUSLE):
This equation computes sediment yield either monthly or seasonal. This equation is used in large watershed for prediction of soil loss.
Y = 95 (Qq)0.56 KLSCP
Where,
Y = Sediment yield for an individual storm (t/ha)
Q = Volume of runoff (ha-m)
q = Peak flow rate (m3)
E = f (IKCLV)
Where,
E = Average annual soil loss (t/ha/year)
I = Soil erodibility
K = Roughness
C = Control of soil conservation measure
L = Slope length
V = Vegetation
[Note: Soil erosion = f (Erosivity of rain) x (Erodibilty of soil)]