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Fundamental of Soil and Water Conservation Engineering

Author(s): Vikas Sharma, Dr. P. K. Singh,
Abstract:
Fundamental of Soil and Water Conservation Engineering
Authors
Vikas Sharma
Ph.D. Research Scholar, Department of Soil and Water Engineering,
College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur, Rajasthan, India
Dr. P.K. Singh
Professor, Department of Soil and Water Engineering,
College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur, Rajasthan, India
Publication Month and Year: December 2019
Pages: 36
E-BOOK ISBN: 978-81-944644-1-9
Academic Publications
C-11, 169, Sector-3, Rohini, Delhi, India
Website: www.publishbookonline.com
Email: publishbookonline@gmail.com
Phone: +91-9999744933
Preface
Soil and water’ conservation practices play an important role in conservation of water’ and soil on the earth surface. It enhance saving of natural resources in long run agriculture. In now days there is need of to now basic fundamental of different soil conservation practices used in field of agriculture.
This book covers all the technique related to all aspect of soil and water’ conservation structures. It includes all inclusive technical details of various conservation techniques.
This is anticipated as a ebook for students of agricultural engineering. It will providea technical support for students of engineering at undergraduate, postgraduate and useful for PhD students of agricultural engineering. In addition, the book will be a valuable reference to professional and agricultural scientists working in the field of micro irrigation.
It is hope that farmers, undergraduate, postgraduate and Ph.D. students of agricultural engineering will find this ebook useful. In preparation of this ebook, authors have received help, suggestions and encouragement from several individuals. The authors would like to express their gratitude to Dr. Ajay Kumar Sharma, Dean, College of Technology and Engineering and Dr. Mahesh Kothari, HoD, Department of Soil and Water’ Engineering for their encouragement and providing all necessary support in preparation of this task. The authors are also grateful to all colleagues of Department of Soil and Water’ Engineering for their assistance and having useful discussions in preparation of this manual.
We hope that readers would provide the scope for future alteration and improvement of this manual by their valuable comments and suggestions.
Content
S. No.ChapterPage No.
1.Soil Erosion01-02
2.Water’ Erosion03-07
3.Wind Erosion08-09
4.Engineering Measures for Soil and Water’ Conservation10-18
5.Universal Soil Loss Equations19-24
6.Grassed Water Way25-27
7.Land use Capability Classification28-30
8.Flow Measuring Structures31-34
References35
Chapter - 1
Soil Erosion
What is Soil Erosion?
Soil erosion is the detachment, transportation and deposition of soil material from one place to another through the action of rain, wind and water in motion.
In other word we can say symptoms of soil erosion are approachable and visible to everyone, but recognition of the implications is less easy in future condition. So now day what is happening to most of agricultural lands we must look at erosion seriously? We must separate erosion in natural/normal and accelerated erosion and must know the significance of each. Continuity of slopes on the surface of land, streams with normal open channels and well-adjusted to the valleys in which they flow, slow uniform downhill creep of the soil mantle, and streams and rivers flowing clear except when in flood are all signs of natural erosion-indications of a perfectly normal condition of the landscape.
Causes of Soil Erosion: The major causes of soil erosion can be described as follows:
Destruction or Removal of Natural Protective Cover by
Deforestation
Forest fire
Overgrazing
Improper or inefficient use of the Land
Keeping the large area of land barren subjecting it to the action of rain and wind
Growing of more crops that accelerate soil erosion
Cultivation along slope of land
Faulty and inefficient methods of irrigation
Removal of organic matter and plant nutrients by injudicious cropping pattern
Types of Soil Erosion
Geological Erosion: it is mainly caused due to natural phenomena is called geological erosion. In this case rate of formation of soil is equal to rate of loss of soil. Relatively slow, continuous process that often gets unnoticed. Responsible for the formation of topographical features such as valley, channels etc.
Accelerated Erosion: it also refers to as anthropogenic erosion or man induce erosion. In this case rate of formation of soil is not equal to rate of loss of soil generally. Fast process as compared to geological erosion.
It is Two Types: It is Mainly Two Types
water’ erosion
wind erosion
Chapter - 2
Water’ Erosion
Water Erosion
Water erosion may be defined as detachment, transportation and deposition of soil particle from one place to another place under the influence of water’, is called water’ erosion.
Mechanics of Water’ Erosion
Hydraulic Action: It mainly takes place when high velocity water runs over the surface of soil compressing the soil, as a result of which the air mass present in the voids of soil exerts a high pressure on the particles of soil and this enhanced to the soil detachment. The pressure exerted by the air voids is called hydraulic pressure. The soil particles so detached from their places, are scoured by the running water’.
Abrasion: In this condition particles of soil mixed with the running water’ create more an abrasive power
Attrition: Mechanical breakdown of soil particles takes place in this condition.
Solution: This form is associated with the chemical action between running water’ and soil
Transportation: The process of soil transportation by running or flowing water’ is completed under the given forms
Solution: The water’ soluble contents present in the water’ are transported by the water’ in solution form.
Suspension: It refers to the transportation of finer or very small soil particles, which are present in suspension form in the flowing water’.
Saltation and Surface Creep: It refers to transportation of medium size of soil particles that are not suited in suspension form, but are mixed in water’ and flow over the stream bed in the form of mud. The surface creep action is responsible for transporting the coarser soil particles.
Deposition: Deposition of the particles occurs when the gravitational force is greater than the forces holding the particles in water’
Factors Affecting Water’ Erosion
Climatic Factors: it includes characteristics of rainfall, temperature and wind velocity
Characteristics of rainfall includes amount, intensity, frequency and duration of rainfall
Temperature also has a positive influence on soil erosion. Frozen soils are relatively erosion resistant
Wind, contributes to the drying of soil and enhances the need for irrigation for new plantings and for applying wind erosion control practices
Characteristics of Soil: it includes texture of soil, structure, organic matter content and permeability. In addition, in many situations, compaction is significant. These characteristics greatly determine the erodibility of soil
Vegetation Cover: Vegetative cover is an extremely important factor in reducing erosion at a site
Roots of vegetation binds soil particles
Topographic Effect: The major topographic factors which influence the soil erosion are land slope, length of slope and shape of slope
A flat land has no difficulties of erosion, while sloping land predominantly effected by erosion.
As the slope enhances, the runoff coefficient, kinetic energy and carrying capacity of surface runoff also enhance thereby decreasing the soil stability.
Types of Water’ Erosion
Splash Erosion
It is a called first stage of water erosion it is mainly caused by the impact of raindrops on exposed soil surface. The process of raindrop erosion mainly occurs as when raindrop strikes on open soil surface it forms a crater. The disturbed soil may be splashed into air up to a height of 50 to 70 cm depending upon rain drops size. Instant soil particles also move horizontally as much as 1.45 m on level land surface. On sloping land, more than half of the splashed particles move down with the runoff.
Sheet Erosion
It is basically a second stage of water erosion. It may be defined as less or more uniform removal or disturbance of soil in the form of a thin layer or in “sheet” form by the flowing water’ from sloping land. It is an inconspicuous type of soil erosion because the total amount of soil removed during any storm is usually small. In the sheet erosion 2 basic erosion processes are involved. In first process soil particles are detached from the soil surface by falling of raindrop. Second process is one in which detached soil particles are transported away by surface runoff from the original place.
Rill Erosion
It is regarded as a transition stage between sheet erosion and gully
It is also called known as micro channel erosion
It is the removal of soil by running water’ with the formation of a areas of small branching channels
Rills are small in size and can be leveled by tillage operations
Gully Erosion: It is advanced form of rill erosion in which the size of rills is enlarged which cannot be smoothened by tillage operations.
Gully Development Stages: The gully development is recognized in 4 stages
Formation Stage: in this stage, the channel erosion and deepening of the gully takes place. It normally proceeds slowly where the top soil is fairly resistant to erosion.
Development Stage: it mainly causes upstream movement of the gully head and further enlargement of the gully in depth and width. The gully cuts to the C-horizon of soil, and the parent materials are removed rapidly as water’ flows.
Healing Stage: In this stage vegetation starts growing in the gully. No appreciable erosion takes place.
Stabilization Stage: In this stage gully reaches a stable gradient, and attain a stable slope.
Classification of Gullies
Gullies can be classified based on three factors viz. their size, shape (cross section) and state of gully.
Based on Size
Gully classification based on size.
ClassificationSymbolSpecifications of gully
Very Small gulliesY13m deep, bed width<18m, side slope varies
Small gulliesY23m deep, bed width>18m, side slope varies
Medium gulliesY33m-9m deep, bed width>18m, side are uniformly sloping
Deep and narrow gulliesY4>9m deep, bed width varies, side slope varies, mostly steep and vertical
Based on Shape of Gully
U-Shaped: These are formed where the topsoil and subsoil have the uniform resistance against erosion. Because the subsoil is eroded as easily as the topsoil, nearly vertical walls are developed on each side of the gully
V-Shaped: it forms where the subsoil has more resistance than tops oil against erosion.
Based on the Stage
Active Gullies: Dimensions enlarge with time. Found in plain areas generally
Inactive Gullies: Dimensions constant with time found in rocky areas
Stream Bank Erosion
In this kind of erosion, running water take material from the side and bottom of a stream or water channel and the cutting of river bank. It is mainly due to removal of vegetation, over grazing or cultivation on the area near to the streams banks.
Agronomical Measures of Water’ Erosion Control
Soil conservation is a preservation technique, in which disturbance of soil and its losses are eliminated or minimized by using it within its capabilities and applying conservation techniques for protection as well as improvement of soil. In soil and water’ conservation, the agronomical measure is a more economical, long lasting and effective technique. Agronomic conservation measures function by reducing the impact of raindrops through interception and thus reducing soil erosion. They also enhance infiltration rates and thereby reduce surface runoff. Widely used agronomic measures for water’ erosion control are listed below.
Contour Cropping
Contour cropping is mainly conservation farming method that is used on sloppy condition to control soil losses due to water’ erosion. Contour cropping involves planting crops across the slope instead of up and down the slope. Use of contour cropping save the top of soil surface by reducing the velocity of runoff water’ and inducing more infiltration.
Strip Cropping
Strip cropping is the agronomical practice in which growing crop in having poor potential for erosion control, such as root crop (intertilled crops), etc., alternated with strips of crops having best potentials for control of erosion, such as fodder crops, grasses, A crop rotation with a combination of inter tilled and close growing crops, farmed on contours, provides food, fodder and conserves soil moisture.
Contour Strip Cropping: In contour strip cropping, alternate strips of crop are sown more or less following the contours, similar to contouring. Suitable rotation of crops and tillage operations are followed during the farming operations
Mulching
Mulches are used to minimizing rain splash, decrease evaporation, control weeds, reduce temperature of soil in very hot climates, and moderate the temperature to a level conducive to microbial activity. Mulches reduced impact of raindrops, prevent splash and dissipation of soil structure, obstruct the flow of runoff to reduce their velocity and prevent sheet and rill erosion. They also help in improving the infiltration capacity by maintaining a conductive soil structure at the top surface of land.
Chapter - 3
Wind Erosion
Wind erosion is a natural process in which soil particle moves from one place to another place under the influence of wind. It can cause significant economic and environmental damage.
The Effects of Wind Erosion
Wind erosion may have the following impacts:
Soil fertility is reduced because of the destruction of the plant nutrients that are concentrated on small soil particles and organic matter in the topsoil. This reduces the soils capacity to support productive pastures and sustain biodiversity
Sand grains transported by winds can impact vegetation in their path by sandblasting
Air pollution caused by small particles in suspension can affect people's health and cause other difficulties
Wind Erosion Processes
The three processes of wind erosion are surface creep, saltation and suspension. The three processes of soil erosion
Suspension: It implies of soil having less than 0.1 mm in diameter can be moved into the air by saltation, forming dust storms when taken further upwards by turbulent action of wind.
Saltation: Occurs among middle-sized particle of soil that ranges between 0.05 mm to 0.5 mm in diameter. The particle of soil move through a series of low bounces over the surface, causing abrasion on the soil surface and attrition (the breaking of particles into smaller particles).
Surface Creep: It refer the movement of large particles ranging from 0.5 mm to 2 mm in diameter, are rolled across the soil surface. This causes them to collide with, and dislodge, other particles. Surface creep wind erosion results in these larger particles moving only a few metres.
Control Measures
Trees and shrubs minimize wind velocities and so provide protection from wind erosion.
Shelter belts or tree corridors contribute to biodiversity, provide shelter for livestock and reduce the drying effects of winds. However, it is not practical to plant belts of trees in the vast, arid inland areas where wind erosion is most prevalent. Hence well-managed pastures are the key to providing erosion protection in these areas.
Tillage Practices
The tillage practices, such as ploughing are importantly adopted for controlling wind erosion.
The Common Tillage Practices used for Wind Erosion Control are as under
Primary and Secondary Tillage
Strip Cropping
Use of Crop Residues
Mechanical Measures
This method consists of some mechanical obstacles, constructed across the direction of prevailing wind, to minimize the impact of blowing wind on the soil surface.
Sand Dunes Stabilization
A ‘Dune’ is derived from English word ‘Dun’ means hilly topographical feature. Therefore a sand dune is a mount, hill or ridge of sand that lies behind the part of the beach affected by tides.
Vegetative Measures
This method is a most effective, least expensive, aesthetically pleasing method which formed a natural system with self‐repairing provision. However, it has some disadvantages as the plant establishment phase is critical, it needs irrigation and maintenance until self-sustaining system is developed.
Chapter - 4
Engineering Measures for Soil and Water’ Conservation
Terracing
A terracing is a practice of constructing of multiple continuous lines of earth embankment constructed across the slope of field in order to check erosion of soil.
As a means of controlling erosion and conserving rainfall on sloping land, farmers long ago introduced the use of hillside ditches. The ditches themselves proved inadequate, but the principle of controlling erosion by systematically intercepting surface runoff on sloping lands has led to the use of farm terracing. The development of terracing as recommended today has required years of use, extensive held observations and experimentation, and many modifications from time to time in construction procedure. When terraces are properly used and constructed and adequately supported by approved cropping and tillage practices they provide one of the most effective erosion-control measures applicable to cultivated lands. When improperly constructed or not coordinated with good land use and soil conserving practices, they often accelerate rather than retard soil losses. This section gives up-to-date information on terrace construction and maintenance in coordination with other recommended soil conservation practices. The information it contains is based on the terracing work of the Soil Conservation Service in every important agricultural region of the United States. The basic factor that must be recognized in the application of erosion control measures is the proper utilization of the land. The use of land in accordance with its capabilities is a guide in considering what areas or fields are to be terraced and what areas need a combination of terracing and other supporting measures. Land used for cultivated crops should be terraced where runoff and erosion cannot be controlled by use of vegetation or tillage practices in the proposed cropping rotation or with contour strip cropping. Land unsuited for cultivated crops should not be terraced except in special cases. For instance, where land has been severely impoverished, terraces may be desirable to assist in the establishment of a permanent stand of grass. In such places terraces are used as a temporary measure and may be constructed according to standards somewhat below those required for a permanent system. Experience indicates that terracing on land not suited to cultivation usually results in expensive failures. The original cost and subsequent maintenance are high, owing to steep slopes, erosion, or unsatisfactory working conditions of the soil. The high cost of construction and maintenance coupled with the unusually low yields in cultivated crops would indicate that a less intensive use, such as for pasture, meadow, or woods, would result in greater net income. The success of terracing depends primarily on maintenance and management. Neglect of the terrace system will destroy the terraces and cause serious erosion in the field. In keeping terraces in repair, the most important operation is proper plowing.
Types of Terraces
The ultimate objective of all terraces is soil conservation. This objective is achieved by terraces that provide for the interception and diversion of runoff or the impounding of surface runoff for enhanced absorption. From a functional aspect, terraces are classified as
Broad-Channel Terraces
The broad-channel terrace acts primarily to conduct excess rainfall from the Helds at nonerosive velocities. Since low-velocity surface removal of excess rainfall is required, the channel and not the rifte is of numbers importance. A wide, relatively shallow channel of low? Radient that has gentle side slopes and ample water’ capacity will give the most desirable results (fig. 80). The excavated earth is used to bring the lower side of the channel to a height sufficient to provide necessary capacity. A high ridge is not desirable since it seriously interferes with tillage operations, enhances construction costs, and frequently requires for its formation a large part of the topsoil scraped from the held. In the broad-channel terrace the ridge should be considered as supplemental to the channel and should blend gradually into the surface slopes to afford a minimum of interference wuh machinery operations. Moreover, the low gradient of the downslope side of the rider which IS the critical part of ridge terraces, so nearly conforms with the natural slope of the land that there is much less danger of erosion at this point.
Ridge Terraces
Erosion control by the ridge terrace is accomplished indirectly by water’ conservation. In order to enhance absorption the terrace is constructed so as to flood collected runoff over as wide an area as possible. If this is to be done most effectively the surface slopes on which the terraces are built should be fairly flat, the ridge should be of sufficient height to pond water’ over a relatively large surface, and the earth required for the ridge so excavated as to avoid concentration of runoff on a small area.
Bench Terraces
Bench terracing is one of the oldest mechanical methods of erosion control, having been used for many centuries in thickly populated countries where economic conditions necessitated the cultivation of steep slopes. Bench terracing was highly developed by the Inca civilization centuries ago in the Andes Mountains of Peru, in parts of China, the Mediterranean countries, and in other places. It consists principally in transforming relatively steep land-20-to 50-percent slopes-into a series of level or nearly level strips, or steps, running across the slope.
Plate 1: A view of bench terraces
Table Top Terraces
These are normally constructed exactly on contour bund in order to trap all amount of rainfall. These types of terraces used to enhance infiltration of water’.
Sloppy Outward Terraces
In this type of terracing a small slope is provided in outer direction and effectively used in low rainfall with a permeable soil.
Sloppy Inward Terrace
In this type of terracing a small slope is provided in inner direction and effectively used in heavy rainfall with a permeable soil. In this type of terrace a suitable drain is provided in inner side for safe disposal of runoff as well as eliminating the ponding of water’.
Plate 2: Bench terraces and its component
In other Word we can define as
Level Bench Terrace
It consists of level top surface. Level bench terraces are generally used in the areas which receive medium rainfall and have highly permeable soils. Since the soils are highly permeable, it is expected that most of the flowing surface runoff passing through these terraces are absorbed by the soil and the remaining portion is drained in to a drain. This type of terraces is also called irrigated bench terraces provided that they must be under irrigation. Sometimes level bench terraces are also called as table top, because such terraces have top surface in level condition that can be easily impounded with water’ and plantation of paddy crop can be performed.
Bench Terrace Sloping Outward
Such bench terraces are adopted in low rainfall areas with permeable soil. For these terraces a shoulder bund is essential even though this bund is to provide the stability to the outer edge of the terrace. In addition, this bund also helps in retaining the surface runoff on the benches that is either absorbed by the soil or drained. Bench terraces sloping outward are also known as orchard type bench terraces. For outwardly slopping bench terraces constructed on soils having poor permeability, the provision of graded channel at lower end is most essential for disposing surplus surface water’ to the grass water’ way. Whereas, in very less permeable soil case, a strong bund along with spillway arrangement should be essentially equipped, for making the terrace safe against heavy storm and allowing the water’ very safely, downward to the next terrace.
Bench Terraces Slopping Inward
Bench terraces slopping inward are adopted to construct in the areas having high rainfall and less permeable soils, from where large portion of rain water’ is drained as surface runoff. The drain ultimately leads to a suitable outlet (grassed water’ ways). This type of bench terrace is also known as hill type bench terrace. The inwardly sloping bench terraces are usually preferred for growing those crops, which are extremely susceptible to water’ logging.
Design of Bench Terraces
Terrace spacing is generally expressed as the vertical interval between two terraces. The vertical interval (D) is dependent upon the depth of the cut and since the cut and fill are to be balanced, it is equal to double the depth of cut. In higher slopes greater depth of cuts result in greater heights of embankments which may become unstable.
The width of the bench terraces (W) should be as per the requirement (purpose) for which the terraces are to be put after construction. Once the width of the terrace is decided, the depth of cut required can be calculated using the following formulae.
Case 1: When the terrace cuts are vertical.
S is the land slope in percent; D/2 is the depth of cut and W is the width of terrace.
Case 2: When the batter slope is 1:1.
Case 3: When the batter slope is ½: 1.
Source: Singh, P.K Water’ shed Management Book.
After deciding the required width, the depth of cut can be calculated from one of the above formulae.
Fig 1: Cross section of bench terraces
The design of the terrace cross section consists of deciding
The batter slope
dimensions of the shoulder bund
Inward slope of the terrace and the dimensions of the drainage channel in case of terraces sloping inward
Outward slope in case of terraces sloping outward (Fig. 1)
The batter slope is mainly for the stability of the fill or the embankment. The flatter the batter slope, the larger the area lost due to bench terracing. VI are to be used in very stable soils and when the depth of the cut is small (up to 1 m). In case of terraces with flat top and sloping outwards, larger sections of shoulder bunds are required as water’ stands against these bunds. The bund cross section mainly based upon the terrace width and soil conditions. The inward slope of the terrace may be from 1 in 50 to 1 in 10 depending upon the soil conditions. For these terraces a drainage channel is to be provided at the inner edge of the terrace to dispose of the runoff.
Area Lost for Cultivation Due to Bench Terracing
The area lost for cultivation due to bench terracing of a slope can be calculated as follows.
Consider a batter slope of 1:1. Let D be the vertical interval of the benches to be laid out on a land with a slope of S %, along AB in and the batter of the risers is 1:1. L is the HI between the benches Actual distance of AB is given by:
If W is the width available for cultivation after terracing:
Width not available for cultivation after terracing (from equations 5.5 and 5.6).
Width loss in percentage of original inclined width AB.
By dividing the numerator and the denominator by 100 width lost in percentage of the original width.
The percentage width lost can be taken as the percentage area lost. When the batter is vertical, the length of bench terrace per hectare in metres will be 10000/W where W is in metres.
Contour Bund
Bund is simply embankment like structure, constructed across the land slope. When bunds are constructed on contour, they are called contour bunds. It is used for impounding runoff water’ behind them so that it could gradually infiltrate in to the soil. It is a type of soil and conservation structure.
Plate 3: View of Contour Bund
Design Specifications
Spacing of Contour Bund
The main base of selecting spacing of bund is to intercept the water’ before it attains erosive velocity. It will depends on soil type, slope, rainfall and cropping pattern.
By C.E Ramser
VI = 0.3 (( S)/( 3)+ 2)
Where,
VI = Vertical interval between two consecutive bund (m)
S = Land slope (%)
Horizontal Spacing of Bund
It measured on the land slope by following formula:
HI =( VI)/( S)× 100
Where,
HI = Horizontal interval (m)
VI = Vertical interval
S = Land Slope
Cross-Section of Bund
The bund are mainly constructed in trapezoidal shape, so area of bund is calculated by following formula-
Cross section area of bund = ( Top width+Bottom width)/( 2)× Height
Length and Area Lost During Contour Bunding per Hectare
It can be calculated by formula-
Length of contour bund (m) = 10000/( HI)
Area Lost Due to Contour Bunding (m2/hac) = L ×B
Where,
B = Bottom width of contour bund (m)
Chapter - 5
Universal Soil Loss Equations
Wischmeier and Smith (1965) developed the universal soil loss equation given below
A = R.K.L.S.C.P.
Where,
A = average annual soil loss soil (t ha-1yr-1)
R = rainfall erosivity factor
K = soil erodibility factor
L = slope length factor
S = slope steepness factor
C = cover management factor
P = conservation practice factor
Rainfall Erosivity Factor (R)
It refers to the rainfall erosivity index, which expresses the ability of rainfall to erode the soil particles from an unprotected field. It is a numerical value. From the long field experiments it has been obtained that the extent of soil loss from a barren field is directly proportional to the product of two rainfall characteristics: kinetic energy of the storm and its 30-minute maximum intensity. The product of these two characteristics is termed as EI or EI30 or rainfall erosivity.
Soil Erodibility Factor (K)
The soil erodibility factor (K) in the USLE relates to the rate at which different soils erode. The formula used for estimating K is as follows:
Where,
K = Soil erodibility factor
A0 = Observed soil loss
S = Slope factor
ΣEI = Total rainfall erosivity index
Topographic Factor (LS)
The two factors L and S are usually combined into one factor LS called topographic factor. This factor is defined as the ratio of soil loss from a field having specific steepness and length of slope (i.e., 9% slope and 22.13 m length) to the soil loss from a continuous fallow land. The value of LS can be calculated by using the formula given by Wischmeier and Smith (1962):
Where,
L = field slope length in feet
S = percent land slope
Crop Management Factor (C)
The crop management factor C may be defined as the expected ratio of soil loss from a cropped land under specific crop to the soil loss from a continuous fallow land, provided that the type of soil, slope and rainfall conditions are identical.
The Five Stages are
Period F (Rough Fallow): It includes the summer ploughing or seed bed preparation.
Period 1 (Seed Bed): It refers to the period from seeding to 1 one month thereafter.
Period 2 (Establishment): The duration ranges from 1 to 2 months after seeding.
Period 3 (Growing Period): It ranges from period 2 to the period of crop harvesting.
Period 4 (Residue or Stubble): The period ranges from the harvesting of crop to the summer ploughing or new seed bed preparation.
Support Practice Factor (P)
This factor is the ratio of soil loss with a support practice to that with straight row farming up and down the slope. The conservation practice consists of mainly contouring, terracing and strip cropping. The soil loss varies due to different practices followed.
Use of USLE
There are three important applications of the universal soil loss equation. They are as follows:
It predicts the soil loss
It helps in identification and selection of agricultural practices
Limitations of Universal Soil Loss Equation
Empirical
The USLE is totally empirical equation. Mathematically, it does not illustrate the actual soil erosion process. The possibility to introduce predictive errors in the calculation is overcome by using empirical coefficients.
Prediction of Average Annual Soil Loss
This equation was developed mainly on the basis of average annual soil loss data; hence its applicability is limited for estimation of only average annual soil loss of the given area.
Non-Computation of Gully Erosion
This equation is employed for assessing the sheet and rill erosions only but cannot be used for the prediction of gully erosion.
Modified Universal Soil Loss Equation (MUSLE)
The USLE was modified by Williams in 1975 to MUSLE by replacing the rainfall energy factor (R) with another factor called as ‘runoff factor’. The MUSLE is expressed as
Where,
Y = sediment yield from an individual storm (in metric tones),
Q = storm runoff volume in m3and qp = the peak rate of runoff in m3/s.
All other factors K, (LS), C and P have the same meaning as in USLE
Or
Y=95(Q×qp).56 K(LS)CP
Y = sediment yield for an individual storm (tones)
Q = volume of runoff (acre-feet)
qp = peak flow rate (cfs)
All other factors K, (LS), C and P have the same meaning as in USLE.
Erosivity
It may be defined as the potential ability of rain to cause the erosion
It is a property of rainfall
It is the input force to detach the soil particles
Erosivity is a fun of characteristics of rainfall
Factors Affecting Rainfall Erosivity
The various factors, which affect the erosivity of rain storm, are given as under:
Rainfall Intensity
The kinetic energy is related to the intensity of rainfall by the equation proposed by Wischmeier and Smith (1958) as follows:
Where,
KE = kinetic energy of rainfall, (tons per ha per cm of rainfall)
I = rainfall intensity (cm/h)
Drop Size Distribution
The relationship between the median drop size (D50) and rainfall intensity, is given as under (Laws and Parsons, 1943):
In which, D50is the median drop size (inch) and I is the intensity (inch/h).
Terminal Velocity
The kinetic energy of rain storm has following relationship with terminal velocity, as:
Where,
Ek = rainfall energy (watts/m2)3
I = Intensity of rainfall (mm/s)
V = Terminal velocity of rainfall before impact (m/s)
Wind Velocity
Wind velocity affects the power of rainfall to cause soil detachment, by influencing the kinetic energy of rain storm.
Direction of Slope
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 the rain drop. It enhances the impact force of the raindrop as the velocity component in the direction of slope becomes more.
Estimation of Erosivity from Rainfall Data
The rainfall erosivity is related to the kinetic energy of rainfall. The following two methods are widely used for computing the erosivity of rainfall.
EI30 Index Method
KE > 25 Index Method
EI30 Index Method
This method was introduced by Wischmeier (1965).
It is based on the fact that the product of kinetic energy of the storm and the 30-minute maximum rainfall intensity gives the best estimation of soil loss.
The greatest average intensity experienced in any 30 minute period during the storm is computed from recording rain gauge charts by locating the maximum amount of rain which falls in 30 minute period and later converting the same to intensity in mm/hour. This measure of erosivity is referred to as the EI30 index and can be computed for individual storms, and the storm values can be added over periods of time to give weekly, monthly or yearly values of erosivity.
The rainfall erosivity factor EI30value is computed as follows:
Where,
KE is rainfall kinetic energy.
I30is the maximum rainfall intensity for a 30-minute period. Kinetic energy for the storm is compute das.
KE=916+331 log10I.
Limitation
The EI30 index method was developed under American condition and is not found suitable for tropical and sub-tropical zones for estimating the erosivity.
KE > 25 Index Method
This is an alternate method introduced by Hudson for computing the rainfall erosivity of tropical storms.
This method is based on the concept that erosion takes place only at threshold value of rainfall intensity.
From experiments, it was obtained that the rainfall intensities less than 25 mm/h are not able to yield the soil erosion in significant amount. Thus, this method takes care of only those rainfall intensities, which are greater than 25mm/h.
That is why the name is K.E. > 25 Index method. It is used in the same manner as the EI30 index and the calculation procedure is also similar.
Erodibility
It is the vulnerability or susceptibility of the soil to get erosion. It is the property of soil.
It act as a resistance force to make resistance against erosivity.
Erodibility is a function of physical characteristics of soil (texture, structure, organic matter, land use pattern).
Bouyoucos (1935) suggested that the soil erodibility depends on mechanical composition of soil, such as sand, silt, and clay, presented by the ratio as:
The range of particle diameter of clay, sand and silt is:
Clay = < 0.002 mm
Silt = 0.002-0.006 mm
Sand = 0.06-2.0 mm
Chapter - 6
Grassed Water Way
Grass water’ way are basically natural or constructed water’ courses covered with erosion resistant grasses and are used to safe disposal of surface water’ for all erosion control practices excluding level terraces. It is used for following purposes:
Grassed water’ ways are used as outlets to prevent rill and gully formation
These water’ ways can also be used as outlets for water’ released from contoured and terraced systems and from diverted channels
This best management practice can reduce sedimentation of nearby water’ bodies and pollutants in runoff
The vegetative cover slows the water’ flow, minimizing channel surface erosion. When properly constructed, grassed water’ way scan safely transport large water’ flows to the downslope
Plate 1: View of grass water’ way
Design of Grass Water’ Way
In order to design grass water’ way, the shape, size and location of construction play an important role. The temporary soil and water’ conservation structure designed to carry the maximum runoff from a storm of 10 years recurrence interval.
Size of Water’ Way
The size of water’ way decided on the basis of maximum expected peak runoff rate for 10 year recurrence interval. The peak runoff rate mainly calculated by rational formula and area-velocity relationship which are given below:
Rational Formula
Qp = CIA/36
Where,
Qp = Peak runoff rate (m3/sec), C = Coefficient
I = Intensity of rainfall (mm/hr)
A = Area of water’ shed (hac)
Area-Velocity Method
Q=AV
A = Q/V
Where,
Q = Expected maximum runoff (m3/sec)
A = Area of cross section (m2)
V = Velocity of flow (m/sec)
Table 1: Calculation for parabolic and trapezoidal cross section of grass water’ way
Cross-Sectional Area, aWetted Perimeter, PHydraulic Radius,
Top Width
Cross-Sectional Area, aWetted Perimeter, PHydraulic Radius,
Top Width
The velocity of flow in a grassed water’ way is mainly dependent on the condition of the vegetation and the soil erodibility. It is recommended to have a uniform cover of vegetation over the channel surface to ensure channel stability and smooth flow. The velocity of flow through the grassed water’ way depends upon the ability of the vegetation in the channel to resist erosion. Even though different types of grasses have different capabilities to resist erosion; an average of 1.0 m/sec to 2.5 m/sec are the average velocities used for design purposes. It may be noted that the average velocity of flow is higher than the actual velocity in contact with the bed of the channel.
Chapter - 7
Land use Capability Classification
Land use capability classification may be defined asit is a systematic arrangement of various types of land according their properties that determine the ability of land to produce on virtually permanent basis.
In this classification land is classified in 8 classes under two land suitability groups. Groups are mainly based onsuitability of land for cultivation:
Group 1: Land suitable for cultivation (Classes I to IV)
Group 2: Land not suitable for cultivation (Classes V to VIII)
Fig 1: Suitability of Land use capability class for different use
Group 1: Land Suitable for Cultivation(Classes I to IV)
Class-I (Green Colour): In this class, land is best suitable for cultivation and having no limitation and hazards.It does not require any type of soil conservation practices except ordinary crop management practices.It is sutable to grow all kind of crop.
Class-II (Yellow Colour): In this class, land is sutable for cultivation but having moderate limitations like less deep soil and moderate slope. This kind of land require simple and moderate soil and water’ conservation practices like strip cropping, bunding and terracing.
Class-III (Red Colour): In this class, land is moderately good but has severe limitation for use like stepper slope, sallow soil and les fertility. It require intensive soil and water’ conservation practices and requires careful management of soil.
Class-IV (Blue Colour):In this class, land has very severe limitation for use and SWC pratices are more difficult to maintain.In some irrigated areas, part of the soils in class III have limited use because of high water’ table, slow permeability, and the hazard of salt or sodic accumulation.
Group 2: Land Not Suitable for Cultivation (ClasseS V to VIII)
Class-V (Dark Green Colour): It is basically level land but not suitable for cultivation because of wetness, stoniness and adverse climatic condition. This type of land suitable for grazing and perennial vegetation.
Class-VI (Orange Colour): This type of land has modearte limitation and not suited for cultivation. The land is more severly eroded having steep slope. The type of land used for pasture and forestry.
Class-VII (Brown Colour)
This type of land are stony and having steep slope. They can be used safely for grazing or woodland or wildlife food and cover or for some combination of these under proper management. Depending upon the soil characteristics and local climate, soils in this class may be well or poorly suited to woodland. They are not suited to any of the common cultivated crops; in unusual instances, some soils in this class may be used for special crops under unusual management practices. Some areas of class VII may need seeding or planting to protect the soil and to prevent damage to adjoining areas.
Class-VII (Purple Colour)
It include rough, completely barren, bad land and high mountain. It is only suited for wildlife recreation or water’shed protection.
Land Capability Sub-Classes: (e,w,s,c)
The land capabilty classes further devided in to four sub-classeson the basis of their limitation and hazards.
Sub-Class ‘e’ (Erosion): Where the susceptibility of erosion is the dominant hazard in their use (e.g., mass movement, gully/rill, wind).
Sub-Class ‘w’ (Wetness): Where excess water’ is the dominant hazard in their use (e.g., high water’ table, slow drainage, flooding).
Sub-Class ‘s’ (Soil): Where the limitation like shallowness of root zone, moisture holding capacity are the major limitation in their use (e.g., shallow soil, stoniness, water’ holding capacity).
Sub-Class ‘c’ (Climate): Where the climate is the only major hazard in their use. (e.g., dry, wet, frost/snow).
Chapter - 8
Flow Measuring Structures
In safe disposal of runoff or high velocity surface water, the most commonly used devices for measuring water are different type of weirs and orifices.
Weirs
Weirs are normally used to measure the flow in channel or small stream. The basic formula for estimating discharge through a weir is:
Q = CLHm
In which,
Q = Discharge capacity, m3/sec
C = A coefficient dependent on the nature of the crest and approach conditions
L = Length of crest
H = Head of crest
m = An exponent, depending upon the weir opening
Rectangular Weirs
The discharge through rectangular weirs may be estimated by the Franci's formula which is given below:
Suspended Rectangular Weir
Q = 0.0184 LH3/2
In which,
Q = Discharge capacity (m3/sec)
L = Length of crest, cm
H = Head over the weir, cm
Contracted Rectangular Weir (with end Contractions at Both Sides)
Q = 0.0184 (L-0.2H) H3/2
Cipoletti/Trapezoidal Weir
The Cipoletti weir is normally trapezoidal weir, in which each side of the H: V as 1:4. The discharge through a Cipoletti weir is computed by the following formula:
Q = 0.0186 LH3/2
In which,
Q = Discharge capacity, cubic meters per second
L = Length of crest, cm
H = Head over the crest, cm
V-Notch/Triangular Weir
The 90o V-notch weir is commonly used to estimate discharge through channel.
The discharge can be calculated by following formula:
Q = 0.0138 H5/2
In which,
Q = Discharge capacity, cubic meters per second
H = Head, cm
For heads lower than 5 cm, the weir should preferably be calibrated to obtain the discharge.
Concept of Design of Soil and Water Conservation structures:
The design of permanent soil and water conservation structure mainly based on three types of design concept:
Hydrologic Design
It consists of estimation of runoff or peak runoff rate, which the structure is expected to handle.
Hydraulic Design
It consists of determining the dimensions of the structure for handling the designing runoff so that flow water passes through the structure safely without disturbance/overtopping.
Structural Design
It consists of determining the different dimensions of the different of structure from safety point of view.
Drop Spillway
It is a weir structure in which flow passes through opening of weir, drop on approximately level apron and then flows through downstream channel. It is mostly used for drop in head less than 3m.
Component of Drop Spillway
It consist by following components-
Head wall
Cut off wall
Apron
Toe wall
Side wall
Wing wall
Plate 1: A view of Drop Spillway
Chute Spillway
It is a soil water conservation structure which is used to convey safe discharge of high velocity runoff from upstream channel to lower stream channel, when head drop between upstream and downstream, section is more than 3m.
Plate 2: A view of Chute Spillway
References
Anonymous, 2016 A field guideline on Bench Terraces. NRMD.
Singh, P.K. 2000. Watershed Management. E Media Publications
Suresh, R. Soil and Water Engineering, Standard Publication House.
Sharma, V., Singh, Y.P., Gunjan P., Singh, P.K..2017. Effect of Different Level of Irrigation on Biometric Parameters and Estimation of Crop Water Requirement for Summer Rice Crop under Drip Irrigation in Tarai Region of Uttarakhand, Int. J Pure App. Biosci.5(6):730-739. doi: http://dx.doi.org/10.18782/2320-7051.6050
Sharma, V., Gunjan, P., Singh, Y.P. and Singh, P.K. 2019. Growth, Yield and Yield Contributing Factors of Rice Crop as Influenced by Different Level and Methods of Irrigation in Tarai Region of Uttarakhand, India. Int. J Curr.Microbiol.App.Sci. 8(04): 1088-1098. doi: https://doi.org/10.20546/ijcmas.2019.804.126
Sharma, V, Singh, P.K, Bhakar, S.R, Yadav, K.K. &Lakhawat, S.S. (2019). Integration of Soil Moisture Sensor Based Automated Drip Irrigation System for Okra Crop. Indian Journal of Pure and Applied Biosciences. 7(4), 277-282.
About the Authors
Vikas Sharma is Ph.D. Scholar of the Department of Soil and Water’ Engineering, College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur. He did his B. Tech. (Agriculture Engineering) from Uttar Pradesh Technical University (UP) and M.Tech. (Irrigation and Drainage Engineering) from College of Technology, G.B Pant University of Agriculture and Technology, Pantnagar. Vikas Sharma was awarded with Bronze Medal for standing third in order of merit in B.Tech (Agricultural Engineering) in year 2014. He has published 11 papers, 10 abstracts and 4 articles in various journals and proceedings. He is a Member of Indian Society of Agrometeorologist, Anand.
Dr. P.K. Singh is currently working as Professor in Department of Soil and Water’ Engineering at College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur. He has published more than 50 research article in national and international journal. He has received University Best Teacher Award in year 2018-19. He has published several popular articles in regional magazine. He is a Member of Indian Society of Agrometeorologist, Anand, Indian society of soil and water’ conservation and Indian Society of Agricultural Engineers.
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