In this article we will discuss about erosivity and soil erodibility.
Erosivity:
It is defined as the potential ability of rain to cause the erosion. It is dependent on the physical characteristics of rainfall, which include raindrop size, drop size distribution, kinetic energy, terminal velocity etc. For a given soil condition, the potential of two storms can be compared, quantitatively regarding soil erosion to be caused by them.
Rainfall erosivity is the rainfall energy to detach the soil particles, because energy is required to break the soil aggregates into finer, so that they can be splashed out and subsequently moved off through runoff. A raindrop, when falls from a height to the ground surface, it acquires some kinetic energy, which is used in detaching the soil particles.
In addition, when overland flow/surface runoff takes place, it also gains some kinetic energy, which is responsible for scouring the soil particles from the bottom as well as sides of the channel. The comparative investment of kinetic energy of a falling raindrop and surface runoff, is presented in Table 8.1.
The table values shows that there is large difference in energy gained by the raindrop in splash erosion process and the surface runoff. The raindrop acquires about 256 times more kinetic energy than the surface runoff. Though it is an approximation but certainly there is large difference in energy status of falling raindrop and overland flow/runoff, resulting into difference between raindrop erosion and sheet erosion/scouring through surface runoff.
The raindrop detaches the soil particles, which are consequently transported by the overland flow/runoff. In field, a greater contribution of erosion is mainly due to splash erosion. Soil erosion reaches at maximum level, when splash erosion and sheet flow combine together.
Factors Affecting Rain Storm Erosivity:
The term ‘erosivity’ is solely a property of rainfall, which can be quantitatively evaluated as the potential capacity of rain to cause erosion in the given circumstance. The rainfall intensity affects directly to the storm erosivity.
The tropical storms are more erosive than the temperate storms, because they have high rainfall intensity. The literature reports that the majority of tropical rainstorms fall in the category of erosive storms, whereas temperate storms are found erosive only in the magnitude of 5 to 10% of total storms.
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The factors, which affect the erosiveness of rain storm are given as under:
(1) Rainfall intensity
(2) Drop size distribution
(3) Terminal velocity
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(4) Wind velocity, and
(5) Direction of slope.
Intensity of rainfall is referred as the rate of fall of rainfall over the land surface. It is one of the most important factors or characters of rain storm, which makes the rainfall in erosive nature. The rainfall intensity acts as the force, by which an individual water droplet strikes over the soil surface. The force or kinetic energy of individual drop is related to its size and the intensity.
The kinetic energy and intensity of rainfall are related by the following equation, given by Wischmeir and Smith (1958):
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Ek = 210.3 + 89 log10 I … (8.1)
Where,
Ek = kinetic energy of rainfall (metric-tonnes per ha per cm of rain)
I = rainfall intensity (cm/h)
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The drop size distribution in a particular rainstorm influences the energy, momentum and erosivity of the rain in cumulative way. The increase in median drop size increases the rainfall intensity. The relationship between median drop size (i.e. D50) and rainfall intensity is given as under –
D50 = 2.23 I0.182 … (8.2)
In which, D50 is the median drop size (inch) and I is the rainfall intensity (inch/h). The maximum size of rain drop is closed to 7 mm. Hudson (1971) for tropical rain storms reported a relationship between D50 and I as – D50 ∝ Ib which is valid only for low rainfall intensities. This relationship is not fit for erosive storms occurring in humid tropics.
The effect of terminal velocity of falling rain drops is counted interms of kinetic energy of respective rain drops at the time of their impact over the soil surface. The terminal velocity of rainstorm is the function of drop size.
A rainstorm composed of large proportion of bigger size raindrops has greater terminal velocity and vice-versa. The kinetic energy of rain storm has following relationship with the terminal velocity –
Where,
Ek = rainfall energy (watt/m2)
I = intensity of rainfall (mm/s)
V = terminal velocity of rainfall before impact (m/s)
Ellison (1947) developed following empirical relationship among terminal velocity, drop diameter and rainfall intensity for computing the amount of soil detachment by the rainfall –
E = K. V.4.33 d1.07. I0.65 … (8.4)
Where,
E = relative amount of soil detached
K = constant, depends on the soil characteristics
V = velocity of raindrop (feet/s)
d = drop diameter (mm)
I = rainfall intensity (inch/h)
The wind velocity affects the power of rainfall to cause soil detachment by influencing the kinetic energy of rain storm. In tropical regions the occurrence of windy storm is very common. The wind-driven storms are more effective for breaking the aggregates.
Lyles et. al (1969) reported that about 73% more soil is detached by the rain storms accompanied with 13.4 m/s wind velocity than the rain storm without wind. However, the effect of wind velocity on soil detachment by rain storm is shown in Table 8.2.
The direction of land slope also develops significant effect on rainfall erosivity. Slope direction in the direction of the rain storm effectively alters the actual kinetic energy of rain drop by which it falls on the soil surface because the angle at which it strikes the soil particle, is changed. The maximum rain drop impact is there, when it strikes the soil surface at 90° angle.
Soil Erodibility:
It is the vulnerability or susceptibility of the soil to get erosion. Erodibility is the function of physical characteristics of soil and land management practices, used. The involved physical characteristics of soil are the texture, structure, organic matter, land use pattern etc. On the basis of erodibility, one soil can be compared quantitatively with the other for a given rainfall condition.
Soil erodibility depends on the physical properties of the soil and land management practices, used. The physical properties of the soil play an important and long term role on soil erosion, as compared to the land management practices.
Therefore, the effect of physical properties of the soil should always be evaluated more precisely to determine the erodibility, than the land management practices. Bouyoucos (1935) suggested that the soil erodibility depends on mechanical composition of soil, such as sand, silt and clay.
It is presented in the ratio form, given as –
Erodibility, E = (% Sand + % Silt) / (% Clay) … (8.5)
The range of particle’s diameter of clay, sand and silt is given as under:
Clay – below 0.002 mm
Sand – from 0.06 to 2.0 mm
Silt – from 0.002 to 0.006 mm.
Bouyoucos (1935) intended the erosion parameter E to express the proportion of sandy substances in the colloid-bound proportion of the line earth. The research has shown that the ratio of clay content to the remainder of fine earth is a reliable indicator of soil erodibility, but only in some cases.
Middleton (1930) reported that the soil erodibility depends on several factors, which are important properties of the soil.
They are mainly:
i. Mechanical structure
ii. Colloidal content
iii. Moisture content
iv. Soil density
v. Capillary water balance
vi. Plasticity
vii. Soil swelling capacity, and
viii. Dispersion ratio.
He formulated following equation for computing the soil erodibility:
ER = DR.ME/Col … (8.6)
Where,
ER = erosion ratio (soil erodibility)
DR = dispersion ratio
MR = moisture equivalent
Col = colloidal content.
The ER < 10 indicates to non-erodible soil, while ER ranging from 12 to 115 shows to the erodible soils. The dispersion ratio is expressed as the ratio of clay content (particles smaller than 0.05 mm in diameter) to the particle content measured by dispersing 10 g of soil in 1000 ml. of water.
A soil is resistant to erosion which has large amount of clay content, high colloidal content, greater specific weight with respect to solid phase of the soil, lower plastic limit, lower dust content and smaller dispersion and erosion ratio.
Pasak (1967) developed a relationship between soil erodibility, content of non-erodible particles, moisture content of soil and wind velocity at the soil surface.
The equation is given as under –
E = 22.02 – 0.72 P – 1.69 V + 2.64 R … (8.7)
Where,
E = soil erodibility
P = content of non-erodible particles in the soil
V = relative moisture content = Vm – Vn
Vm = instantaneous moisture content
Vn = metrical or bound moisture content
R = wind velocity at the soil surface.
Slater and Byers (1931) pointed that, for determining soil erodibility the permeability of soil, expressed as percolation ratio, should be taken into consideration. The percolation ratio can be expressed as –
The percolation ratio computed so, approaches very close to the erosion ratio, determined by the use of Middleton’s equation.
Lulz (1934-1935) came on the conclusion that the aggregation of the finest fractions has considerable importance on soil erodibility and durability of aggregates, depending on the coagulation of non-hydrated colloids. Or in other terms, the soil erodibility is related to the ability of colloids to hydrate.
Andre and Anderson (1961) developed an index of soil erodibility for water erosion, called surface aggregation ratio, given as under –
Lugo-Lopez (1969) defined the erosion ratio for evaluating the soil erodibility. It is given below –
Henin, Monnier and Combeau (1958) developed an index for predicting the soil erodibility, called instability index, described as under –
Where,
Is = instability index
Agair = % of aggregates > 0.2 mm size, after wet sieving without pre-treatment
Agbenz, Agalc = % of aggregates > 0.2 mm size, after wet sieving with pre-treatment of soil by benzene and alcohal, respectively.
Chorley (1959) derived following relationship for computing erodibility index, as –
Erodibility index = [1/ (Mean shearing resistance x Permeability)] … (8.11)
Voznesensky and Artsrui (1940) developed following equation for predicting erosion index to evaluate soil erodibility.
E = d.h/a … (8.12)
Where,
E = erosion index
d = index of dispersion, which is the ratio of % particles > 0.05 mm without dispersion to % particles > 0.05 mm after dispersion of soil by NaCl.
h = index of water-retaining capacity, which is described as water retention in soil relative to 1g of colloids.
a = index of aggregation, which is % aggregates > 0.25 mm after subjecting the soil to a water flow of 100 cm/min for one hour duration.
Bryan (1968) concluded that, the aggregate stability is good indicator to show the effect on soil erodibility. He considered the proportion of water-stable non-primary aggregates larger than 0.5 mm in diameter present in the soil, as an indicator of soil erodibility.
The greater is the proportion of above content in the soil, more will be the soil resistant to erosion. A more common index as K, is used to predict the soil loss, defined by Wischmeir (1965) given as under –
K = Soil loss per unit El30 … (8.13)
In which, EI30 is the rainfall erosivity.
Wischmeir, Johnson and Cross (1971) have developed a nomograph for computing the K value for use in USLE, shown in Fig. 8.1. For determining the K using nomograph, the grain size distribution, organic content, structure and permeability class of the soil should be known.
The procedure is described as under:
i. The soil data of % silt + very fine sand, enter to the scale at left side and draw a straight line to cut the curve indicating percent sand of given soil. Let it is the point ‘A’.
ii. From point ‘A’ draw a vertical straight line to the curve showing given organic matter (%). Suppose it intersects the curve at point ‘B’.
iii. Draw a horizontal line from point ‘B’ to cut the curve of given soil structure. Let, it cuts at point ‘C’.
iv. Again from point ‘C’ draw a vertical line to the curve of given permeability.
v. Lastly, draw a horizontal line from the point of intersection of permeability curve to the axis indicating soil-erodibility factor (K). The value at intersecting point is the K value of soil taken into consideration.
On the basis of long term experiments, Voznesenski and Artsruni (1940) have made recommendations to assess the soil erodibility, shown in Table 8.3.
Voris and Bernuth (1990) conducted a laboratory study to determine the erodibility of disturbed soil; and developed following relationship for determining the K value:
K = f (d50, M, G, B, W, p, TOC) … (8.14)
Where,
K = USLE soil-erodibility factor
f = an unknown function
d50 = representative soil particle size
M = particles size variation term
G = soil specific gravity
B = bulk density
W = initial water content
p = saturated hydraulic conductivity
TOC = total organic carbon
The different parameters associated to equation (8.14) are described as under:
i. D50:
It is the diameter of a representative soil particle, at which 50 percent of the soil mass contains the particles of smaller diameters. The d50 is used to denote as representative soil particle size for evaluating the soil erodibility.
ii. M:
It is used to denote the particle size variation, introduced by Wischemeir et.al (1971), as the product of percent sand and silt (particles size ranging from 2 to 2000 μm) and percent very fine sand and silt (particle size ranging from 2 to 100 μm). They reported that the M alone accounted about 85 percent of the variance in observed K values.
iii. Specific Gravity (G):
The soil specific gravity refers to the mass per unit volume of soil material. It also provides some effect on soil erodibility.
iv. Bulk Density (B):
It is defined as the mass per unit volume of soil, air and water, refers to the density of particles and their packing, both.
v. Water Content (W):
The initial water content of the soil has very effective role in detachment of soil particles, especially in case of wind erosion. At greater m.c. the wind erosion is about to zero. It is expressed in terms of mass of water per unit volume of soil, air and water.
vi. Saturated Hydraulic Conductivity (P):
The saturated hydraulic conductivity is used as a measure of permeability. The permeability of soil is affected by the structure and texture of soil. The infiltration is more important lo soil erosion, but its greater variability makes it less desirable for a prediction equation.
vii. Total Organic Carbon (TOC):
It corresponds to the organic matter of the soil. The organic matter is often calculated by measuring TOC. The organic matter is equivalent to 58% of carbon.
The role of soil texture on credibility is very significant. For example, the larger particles are resistant to get transport, because greater force is required to entrain them. On the other hand the finer particles are resistant to detachment, because of their cohesiveness character. The least resistant particles are the silts and line sands.
Thus, the soils composed of high silt contents are more erodible. In this regard, Richter and Negendank (1977) have pointed that the soils with 40 to 60 percent silt content are most credible. Evans (1930) reported that, the soils restricted with clay fraction (9 to 30%) are more susceptible to erosion.
The percent of clay content in the soil is also considered as an indicator of soil erodibility.
Theoretically, it is justified by the following points:
i. The clay particles combined with the organic matters form soil aggregates or clods, which provide greater resistance to soil erosion.
ii. Soils containing high content of base minerals are found more stable, as these materials contribute chemical bonding to the aggregates.
iii. The soil moisture has great effect on soil credibility.
The credibility of different soils based on moisture content is given by Strednansky (1977) shown in Table 8.4.
When soil is wetted the aggregates are weakened, because the cementing substances which make the soil cohesiveness, get dispersed throughout the aggregates, as result the resistance lo erosion is reduced. Apart from this particular effect of moisture content on soil resistance or credibility, the wetting of soil also causes swelling of clay particles, which also affects the soil credibility.
The interaction of moisture content and clay content is very complex, that is why it is very difficult to predict that how clays are susceptible lo swelling Grissinger and Asmussen (1963) reported that the strength of soil is regained, if swelling brings the particles in the alignment parallel to the eroding water flow.
He also mentioned that the strength of clay is largely dependent on the Sodium Adsorption Ratio (SAR). As, its value increases, i.e., the replacement of Ca++ and Mg++ ions by Na++ gets increase, the water uptake increases, which causes the likelihood of swelling and collapsing of the aggregates.
The soil cohesiveness and resistance to erosion are also indicated by the shear strength of the soil.
The shear strength of soil can be expressed by the following equation:
τ = c + ρ.tan ɸ … (8.5)
in which, τ is the shear strength required to detach the soil particles, c is the cohesion, ρ is the stress normal to the shear plane and ɸ is the angle of internal friction. A high soil moisture content decreases the shear strength, and thus change the soil behaviour in respect of erosion. At low moisture content, the soil behaves like solid, but as the moisture increases the soil becomes in the nature of plastic; in this condition the soil particles are not detached by wind action.
The organic and chemical constituents of the soil also have significant effect on soil erodibility. These constituents have their influence on aggregate stability. Evans (1930) reported that the soil containing less than 2% as organic carbon, which is equivalent to about 3.5% of organic content, can be grouped under erodible soil. Similarly, Voroney et.al (1981) concluded that with increase in organic content over 0 to 10% the soil erodibility gets decrease, linearly.
The erodibility is the soil born property affecting the soil detachment by splash during rainfall or by surface flow during sheet erosion or both. The soil erodibility is related to the integrated effect of rainfall, runoff and infiltration, and is commonly called as the soil-erodibility factor (K).
The factor (K) in RUSLE accounts for the influence of soil properties on soil loss during storm occurrence. As for as its definition is concerned, it is the rate of soil loss per unit rainfall erosion index (ton. acre. h) from a unit plot. The dimension of unit plot is 22.1 m long with 9% slope, is continuously under a clean-tilled fallow condition. The recommended minimum plot width is 1.83 m.
Guidelines for preparation and maintenance of natural runoff plots given by D.D. Smith are as follows:
i. Plough the plot to a normal depth and smooth immediately by disking and cultivating two or more times, except for the areas where wind erosion during winter poses a serious problem. In wind erosion condition the disking or cultivating should be delayed as per wind blow pattern.
ii. Cultivation should be for new crop planting or when necessary to remove the serious crust formations.
iii. Chemical weed control may be done if it is not possible with cultivation practices.
iv. Ploughing and cultivation should be up and down slope; and should not be at excessively wet soil condition.
Practically, the soil-erodibility factor is the average long-term soil and soil-profile response to the erosive powers of rainstorms. In other words, the soil-erodibility is a lumped parameter, which refers to the integrated average annual value of total soil and soil profile reactions to a large number of erosion and hydrologic processes.
These processes are the detachment and transport of soil particles either by raindrop impact or surface flow (overland/runoff), deposition of soil particles due to topography, tillage-induced roughness, and rainwater infiltration into the soil profile.