Soil Erosion and Its Control

Introduction

Detachment and removal of soil particle from land surface is the basic cause of soil erosion. It is a natural physical phenomenon which has helped to shape the present form of the earth’s surface. Advent of modern civilization has increased the pressure on land, leading to its overexploitation and, subsequently, its degradation. This trigged a very fast pace of soil erosion from land surface because of action of two fluids, w'ater and w'ind. Soil erosion triggered due to overexploitation of land surface is called accelerated erosion and that caused due to natural phenomena is termed geologic erosion. Geologic erosion of soil is caused mainly by effect of rainfall, atmospheric temperature runoff, wind velocity, topography, and gravitational force. It is a continuous, slow, but constructive process resulting in wearing a way of mountains, building up of coastal plains and flood plains, and development of some of the most fertile valleys of the world such as Indo-Gangetic valley in the Indian subcontinent, Nile Valley in Africa, etc. Accelerated erosion of soil is mainly caused by error in management, such as raising of crops without adopting any soil conservation practices, deforestations, etc. It leads to erosion in excess of threshold value of new soil information, causing severe deterioration of the top surface of the land. Deterioration is sometimes so rapid that it disrupts the equilibrium between relationships of soil- plant environment.

Types of Soil Erosion

The process of soil erosion goes through two central phases of activities: detachment of soil particle and their transportation. The major agents responsible for these two phases of activities are wind, water, and gravity. Wind erosion occurs in semi-arid and arid areas where precipitation is scanty and day temperature is very high. Water erosion occurs in regions where precipitation is high and gravity erosion occurs in areas that are near to river, pits, roads, etc. Gravity erosion leads to mass soil movement due to pull of protruding land mass by gravitational forces. As geologic erosion is a part of natural recurring process, it is called as permitted soil erosion. Significant aspects which activate soil erosion process include climatic, hydrologic, geologic, topographical, soil type, and vegetation cover. In addition to these aspects, the socioeconomic condition and the level of technical know'-how have a substantial impact on rate of exploitation of land surface. Invariably, these factors have a combined effect on soil erosion. Three natural agents - wind, w'ater, and gravity - are primary causes of soil erosion.

a. Wind

b. Water

1. Water forms

I. Runoff

  • • Subsurface runoff
  • • Surface runoff
  • - Rill erosion
  • - Gully erosion
  • - Stream-bank erosion
  • - Inter-rill erosion

II. Reservoir

III. Rainfall

2. Glaciers

c. Mass-soil movement

  • 1. Landslide
  • 2. Debris
  • 3. Creep
  • 4. Landfall

Wind Erosion

It is detachment and transportation of soil particles by the force of moving wind. In areas where precipitation is low, atmospheric temperature during the day is high and velocity of wind is invariably very high. Such climatic conditions generally prevail in semi-arid and arid regions, where velocity of wind is very high. Wind erosion is caused by mismanagement of land resources, such as overgrazing, intensive farming, disforestation, etc. These practices destroy cohesive properties of soil particle making them susceptible to wind erosion. Such erosion slowly leads to formation of sandy tracts, which in turn become more vulnerable to wind erosion.

Wind erosion passes through three phases:

  • 1. Destruction of soil surface by wind forces
  • 2. Transportation of eroded soil to other destinations
  • 3. Deposition of sediment particle when the wind velocity reduces

The soil eroding force is called erosivity of wind. The process of soil erosion caused by wind is very complex in nature because of two opposite forces acting on soil particles: one is the erosivity of wind which tries to blow off soil particles and the other is the erodibility of soil which is the property by which soil particles resist separation from their bond with each other (Chepil 1953). Sandy soils have less cohesion and are eroded easily, whereas clay particles are able to resist erosion to greater extent.

Pasak (1974) developed the following equation to determine the erodibility of soil by wind:

where

(gm/

SE is the erodibility of soil by wind /m2)

P is the content of non-erodible particle in soil (70.8 mm)

Ms is the relative soil moisture, Ms = M%„

Mq is the instantaneous moisture and M„ =

M is the content of clay particle (0.01 mm) in the soil R is the wind speed near the soil surface

He found that equations based on clay content are easier for using in the field, and gave the following equation for determining erodibility of soil by wind for different kinds of soil:

Lai (1990) and Siddoway in 1965 proposed the following parametric model for prediction of erosion by wind:

where

WE = potential average annual erosion I' = soil erodibility index = ^г

X, = quantity eroded from soil comprising 60% clods (>0.84 mm)

X2 = quantity eroded under same condition from soil comprising any other proportion of clods more than 0.84 mm K' = soil ridge roughness factor

C = wind erosivity, also called the wind erosion climatic factor (Chepil et al., 1963)

v = mean annual wind velocity

where

P = precipitation >12.7 mm

L' = field length, unsheltered distance across field along prevailing wind erosion direction = Df - Dh Df = distance across field Db = sheltered distance

V = vegetative cover equivalent quantity = RvxSx K„

R, = quantity of vegetative cover and mulch = 1.2 x washed oven dry residue

  • 5 = kind of vegetative cover = 1.0 for small-grain stubble and solver
  • 0.25 for sorghum stubble and solver
  • 0.20 for maize stubble and solver
  • 2.5 for small grain in seeding and stooling stages)

Gillette et al. (1972) proposed a formula for estimating soil erosion by wind:

where

  • 8s = soil erosion (g/cm-s)
  • 8m = maximum soil flux L = length of field exposed to wind erosion / = erodibility index r = wind drag corrected soil moisture Yr = reference wind drag of 3300 kg/ha R = surface residue kg/ha К = soil surface roughness, cm

Erosive forces of wind are also called entrainment forces. These forces cause lift, drag, and particle movement through collisions among each other. Chepil 1960 recommended different widths of wind strips for erosion susceptible crops. Soil resisting forces are friction, gravity, and cohesive properties of soil particle. Wind erosion takes place when erosive forces are higher than soil-resisting forces (erodibility properties of soil).

Energy of moving wind causes lift and drag action of soil particles by following Bernaulli’s effect. Soil particle gets lifted upward from surface of land due to pressure difference between air mass and land surface, created by reduction in the air pressure due to high velocity of wind and unevenness in pressure created by soil clods and capillaries. Drag is created by the wind velocity, which gives energy for detachment and transportation of soil particles over the land surface. Thus, the velocity of moving wind is proportional to the drag force per unit area, near the ground surface. The drag force is expressed as

where

x = drag force

К = coefficient based on the height of measurement of wind velocity from the ground surface and surface roughness V = velocity of wind

The profile of wind velocity varies with changes in soil cover and surface roughness. The variation of wind velocity with height is called velocity gradient and in general, an average wind velocity is considered for estimating wind erosion. Chepil and Woodruff (1963) determined average wind velocity as given below:

where

v, = velocity of wind at height z v* = friction velocity

К = height above mean aerodynamic surface Zo, where wind velocity is zero

The value of к is zero for small crops and smooth surface.

The wind velocity over solid surface, such as ground surface, is zero at height Zo, but velocity is not zero over vegetation since vegetation is porous and wind is able to flow through it.

Skidmore and Woodruff (1968) determined the magnitude of wind erosion force vector using the following formula:

where

>-j = wind erosion force vector

v, = mean wind speed with the ith speed

f = duration factor in percentage of total observation in the /th direction within the /'th speed group

J = subscript indicating the 16 directions of a compass from 0 to 15 The total wind erosion is then given by

FT is a vector quantity. (It is the relative capacity of wind to cause the blowing off of the soil in any direction.)

Bagnold (1941) developed the formula to determine threshold velocity (v„,) of wind:

v,h is the critical shear velocity of wind above the threshold level, a and p are the particle and air densities, respectively G is the acceleration due to gravity D is the mean particle diameter

A" is a constant=0.1 (when Reynolds number exceeds 3.5)

The amount of transported soil particle is found by the following formula (Finkel and Noveh, 1986):

where

<2s amount of soil moved per unit area C, is the constant of proportionality

Hsu (1973) proposed a formula to determine the weight of soil moved per unit area: where

Qs is the tonne per meter width per year:

Z is the height of measurement of wind speed D is the mean diameter of soil particle v- is the hourly average mean velocity

Water Erosion

Soil erosion due to water is caused by its two forms: liquid as the flowing water, solid as the glaciers. The impact of rainfall causes splash erosion. Runoff water causes scrapping and transport of soil particle leading to rill, sheet, and gully erosion. Water waves cause erosion of bank sides of reservoir, lakes, and ocean. The subsurface runoff causes soil erosion in the form of pipe erosion, which is also known as tunnel erosion. Glacier erosion causes heavy landslides. In India, glacier erosions are mostly confined to the Himalayan region.

 
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