Soil erosion- types of soil erosion and factors
affecting soil erosion
6.1 Definition
Soil erosion is the process of detachment of soil particles from the top soil and
transportation of the detached soil particles by wind and / or water.
The agents causing erosion are wind and water. The detaching agents are
falling raindrop, channel flow and wind. The transporting agents are flowing water,
rain splash and wind.
6.2 Nature and extent of erosion
The problem of soil erosion exists all over the country. Out of the 329 m.ha of
India’s geographical area about 175 m.ha (53.3%) is subjected to soil erosion and
some kind of land degradation (Druvanarayana, 1993). About 150 m.ha is subjected
to wind and water erosion. It is estimated that about 5333 Mt of soil is detached
annually by soil annually. Out of this 29 % is carried away by rivers to seas and about
10% is deposited in reservoirs resulting in 1-2 % of loss of storage capacity annually.
The estimated annual soil loss is 16.35 tones/ha/year.
Physiographically India is divided into three regions as follows:
a) Himalayan region: Geologic immaturity made this region more vulnerable to
erosion. High degree of seismicity of the area, very steep slopes, weak geological
formation and improper land use practices accelerate erosion losses. Gullying, land
slides and slips are most common.
b)Gangetic plains: Major problems in the region are riverine erosion, drainage,
saline and alkali soil conditions.
c) Peninsular region: Main problems of this region are rill and gully erosion.
Arid regions have severe wind erosion. Semi arid regions are subjected to
sheet and gully erosion and ravines are serious problem in Yamuna and Chambal
region. Floods and stream bank cutting and sand deposition have degraded lands of
north east region with heavy rainfall. South and south east are characterized by
undulating terrain with severe erosion in black and red laterite soils.
6.3 Losses due to erosion:
i) Loss of fertile top soil
ii) Loss of rain water
iii) Loss of nutrients
iv) Silting up of reservoirs
v) Damage to forests
vi) Reduction in soil depth
vii) Floods
viii) Adverse effect on public health
ix) Loss of fertile land
x) Economic losses
6.4 Types of erosion:
There are two major types of soil erosion
a) Geological erosion (Natural or normal erosion): is said to be in equilibrium with
soil forming process. It takes place under natural vegetative cover completely
undisturbed by biotic factors. This is very slow process.
b) Accelerated erosion: is due to disturbance in natural equilibrium by the activities
of man and animals through land mismanagement, destructing of forests over
grazing etc., Soil loss through erosion is more than the soil formed due to soil forming
process.
Based on the agents causing erosion, erosion is divided into
a. Water erosion b. Wind erosion c. Wave erosion
6.4.1 Water erosion
Loss of soil from land surface by water including run off from melted snow
and ice is usually referred to as water erosion.
Major erosive agents in water erosion are impacting/ falling raindrops and
runoff water flowing over soil surface.
6.4.1.1 Process of water erosion
Detachment of soil particles is by either raindrop impact or flowing water.
Individual raindrops strike the soil surface at velocities up to 9 m/s creating very
intensive hydrodynamic force at the point of impact leading to soil particle
detachment. Over land flow detaches soil particles when their erosive hydrodynamic
force exceeds the resistance of soil to erosion. Detached soil particles are transported
by raindrop splash and runoff. The amount of soil transported by runoff is more than
due to raindrop splash. Thus the falling raindrops break the soil aggregates and
detach soil particles from each other. The finer particles (silt and clay) block the soil
pores and increase the rate of runoff and hence loss of water and soil.
6.4.1.2 Forms of water erosion
Water erosion occurs in stages identified as sheet erosion, rills, gullies,
ravines, landslides and stream bank erosion.
a) Sheet erosion: It is the uniform removal of surface soil in thin layers by rainfall
and runoff water. The breaking action of raindrop combined with surface flow is the
major cause of sheet erosion. It is the first stage of erosion and is least conspicuous,
but the most extensive.
b) Rill erosion: When runoff starts, channelisation begins and erosion is no longer
uniform. Raindrop impact does not directly detach any particles below flow line in
rills but increases the detachment and transportation capacity of the flow. Rill
erosion starts when the runoff exceeds 0.3 to 0.7 mm/s. Incisions are formed on the
ground due to runoff and erosion is more apparent than sheet erosion. This is the
second stage of erosion. Rills are small channels, which can be removed by timely
normal tillage operations.
c) Gully erosion: It is the advanced stage of water erosion. Size of the unchecked rills
increase due to runoff. Gullies are formed when channelised runoff form vast sloping
land is sufficient in volume and velocity to cut deep and wide channels. Gullies are
the spectacular symptoms of erosion. If unchecked in time no scope for arable crop
production.
d) Ravines: They are the manifestations of a prolonged process of gully erosion.
They are typically found in deep alluvial soils. They are deep and wide gullies
indicating advanced stage of gully erosion.
e) Landslides: Landslides occur in mountain slopes, when the slope exceeds 20%
and width is 6m. Generally land slides cause blockage of traffic in ghat roads.
f) Stream bank erosion: Small streams, rivulets, torrents (hill streams) are
subjected to stream bank erosion due to obstruction of their flow. Vegetation sprouts
when streams dry up and obstructs the flow causing cutting of bank or changing of
flow course.
6.4.1.3 Factors affecting water erosion
a) Climate: Water erosion is directly a function of rainfall and runoff. Amount,
duration and distribution of rainfall influences runoff and erosion. High intensity
rains of longer duration causes severe erosion. Greater the intensity, larger the size
of the raindrop. Rainfall intensity more than 5 cm/hr is considered as severe. Total
energy of raindrops falling over a hectare land with rainfall intensity of 5 cm /hr is
equal to 625 H.P. This energy can lift 89 times the surface 17.5 cm of soil from one ha
to a height of 3 ft. Two- thirds of the above energy is used for sealing soil pores.
Runoff may occur without erosion but there is no water erosion without runoff. The
raindrop thus breaks down soil aggregates, detaches soil particles and leads the
rainwater with the fine particles. These fine particles seal the pores of the surface soil
and increases runoff causing erosion.
b) Topography: The degree, length and curvature of slope determine the amount of
runoff and extent of erosion. Water flows slowly over a gentle slope where as at a
faster rate over a steeper one. As water flows down the slope, it accelerates under the
forces of gravity. When runoff attains a velocity of about 1 m/s it is capable of
eroding the soil. If the percent of slope is increased by 4 times the velocity of water
flowing down is doubled. Doubling the velocity quadruples the erosive power and
increases the quantity of soil that can be transported by about 32 times and size of
the particles that can be transported by about 64 times.
c) Vegetation: Vegetation intercepts the rainfall and reduces the impact of
raindrops. It also decreases the velocity of runoff by obstructing the flow of water.
The fibrous roots are also effective in forming stable soil aggregates, which increases
infiltration and reduces erosion.
d) Soil Properties: Soil properties that influence soil erodability by water may be
grouped into two types.
i. Those properties that influence the infiltration rate and permeability
ii. Those properties that resist the dispersion, splashing, abrasion and
transporting forces of rainfall and runoff.
The structure, texture, organic matter and moisture content of upper layers
determine the extent of erosion. Sandy soils are readily detachable but not readily
transportable. Soils of medium to high clay content have low infiltration capacities
and they are readily transported by water after they are dispersed, but their
detachability is generally low.
e) Man and beast
Man and beast accelerates erosion by extensive farming and excessive
grazing. Faulty practices like cultivation on steep slopes, cultivation up and down the
slope, felling and burning of forests etc., leads to heavy erosion. Excessive grazing
destroys all vegetation and increases the erosion.
6.4.1.4 Estimation of soil loss by water erosion
Based on the mechanism and factors influencing soil erosion, a universal soil
loss equation (USLE) developed by Wischmeier (1959) is most useful for predicting
soil loss due to water erosion. It is an empirical equation and estimates average
annual soil loss per unit area as a function of major factors affecting sheet and rill
erosion. It enables determination of land management erosion rate relationships for
a wide range of rainfall, soil slope and crop and management conditions and to select
alternative cropping and management combinations that limit erosion rates to
acceptable limits.
A= R x K x L x S x C x P
where, A= predicted soil loss in t/ha/year
R= rainfall erosivity factor or index
K= soil erodibility factor
L= length of slope factor
S= slope steepness factor
C= soil cover and management factor and
P= erosion control factor
6.5 Wind erosion
Erosion of soil by the action of wind is known as wind erosion. It is a serious
problem on lands devoid of vegetation. It is more common in arid and semi arid
regions. It is essentially a dry weather phenomenon stimulated by the soil moisture
deficiency. The process of wind erosion consists of three phases: initiation of
movement, transportation and deposition. About 33 m.ha in India is affected by wind
erosion. This includes 23.49 m.ha of desert and about 6.5 m.ha of coastal sands. The
Thar Desert is formed mainly by blow in sand.
6.5 Mechanism of wind erosion
Lifting and abrasive action of wind results in detachment of tiny soil particles
from the granules or clods. The impact of these rapidly moving particles dislodge
other particles from clods and aggregates. These dislodged particles are ready for
movement. Movement of soil particles in wind erosion is initiated when the pressure
by the wind against the surface soil grains overcomes the force of gravity on the
grains. Minimum wind velocity necessary for initiating the movement of most
erodable soil particles (about 0.1 mm diameter) is about 16 km /hr at a height of 30.5
cm. Most practical limit under field conditions, where a mixture of sizes of single
grained material present is about 21 km/hr at a height of 30.5 cm.
In general movement of soil particles by wind takes place in three stages:
saltation, surface creep and suspension.
a. Saltation: It is the first stage of movement of soil particles in a short series of
bounces or jumps along the ground surface. After being rolled by the wind, soil
particles suddenly leap almost vertically to form the initial stage of movement in
saltation. The size of soil particles moved by saltation is between 0.1 to 0.5 mm in
diameter. This process may account for 50 to 70% of the total movement by wind
erosion.
b. Surface creep: Rolling and sliding of soil particles along the ground surface due to
impact of particles descending and hitting during saltation is called surface creep.
Movement of particles by surface creep causes an abrasive action of soil surface
leading to break down of non-erodable soil aggregates. Coarse particles longer than
0.5 to 2.0 mm diameter are moved by surface creep. This process may account for 5
to 25% of the total movement.
c. Suspension: Movement of fine dust particles smaller than 0.1 mm diameter by
floating in the air is known as suspension. Soil particles carried in suspension are
deposited when the sedimentation force is greater than the force holding the
particles in suspension. This occurs with decrease in wind velocity. Suspension
usually may not account for more than 15% of total movement.