AGE 505: Soil and Water AGE 505: Soil and Water ConservationConservation

References:References:1.1.Soil and Water Conservation Engineering, by Glenn Soil and Water Conservation Engineering, by Glenn O. Schwab et. alO. Schwab et. al2.2. Michael, A.M.: Irrigation: Theory and PracticeMichael, A.M.: Irrigation: Theory and Practice3.3. Design of Small Dams: USDA: Bureau of Land Design of Small Dams: USDA: Bureau of Land ReclamationReclamation4.4. Soil Conservation: N.W. HudsonSoil Conservation: N.W. Hudson5.5. Field Engineering for Agricultural Development: Field Engineering for Agricultural Development: N.W. Hudson.N.W. Hudson.

IntroductionIntroduction

Measures that provide for the management of water and soilMeasures that provide for the management of water and soil Conservation practices involves the soil, the plant and the climate, each of Conservation practices involves the soil, the plant and the climate, each of

which is of utmost importance.which is of utmost importance. The engineering approach to soil and water conservation problems involves the The engineering approach to soil and water conservation problems involves the

physical integration of soil, water and plants in the design of a co-ordinated physical integration of soil, water and plants in the design of a co-ordinated water managementwater management

The engineering problems involved in soil and water conservation may be The engineering problems involved in soil and water conservation may be divided into the six following phases:divided into the six following phases: Erosion controlErosion control DrainageDrainage IrrigationIrrigation Flood controlFlood control Moisture conservation andMoisture conservation and Water resource developmentWater resource developmentThe conservation of these vital resources implies utilization without waste so as to make possible a The conservation of these vital resources implies utilization without waste so as to make possible a

high level of production which can be continued indefinately.high level of production which can be continued indefinately.

Types of ErosionTypes of Erosion

Two major types of erosionTwo major types of erosion Geological erosionGeological erosion Accelerated erosionAccelerated erosionGeological erosionGeological erosion: includes soil-forming as well as soil eroding : includes soil-forming as well as soil eroding

processes which maintain the soil in a favorable balance.processes which maintain the soil in a favorable balance.Accelerated erosionAccelerated erosion: includes the deterioration and loss of soil as a : includes the deterioration and loss of soil as a

result of man’s activities. Although, soil removal are recognized in result of man’s activities. Although, soil removal are recognized in both cases, only accelerated erosion is considered in conservation both cases, only accelerated erosion is considered in conservation activities. activities.

The forces involved in accelerated erosion are: The forces involved in accelerated erosion are: 1.1. Attacking forces which remove and transport the soil particles andAttacking forces which remove and transport the soil particles and2.2. Resisting forces which retard erosion. Resisting forces which retard erosion.

Soil erosion by waterSoil erosion by water

Water erosion is the removal of soil from the lands surface by running Water erosion is the removal of soil from the lands surface by running water including runoff from melted snow and ice. Water erosion is sub-water including runoff from melted snow and ice. Water erosion is sub-divided into raindrop, sheet, rill, gully and stream channel erosion.divided into raindrop, sheet, rill, gully and stream channel erosion.

Major Factors Affecting Erosion by Water Major Factors Affecting Erosion by Water 1.1. Climate, 2. Soil, 3. Vegetation and 4. TopographyClimate, 2. Soil, 3. Vegetation and 4. Topography

Climate: - Precipitation, temperature, wind, humidity and solar radiationClimate: - Precipitation, temperature, wind, humidity and solar radiationTemperature and wind: - evident through their effect on evaporation and Temperature and wind: - evident through their effect on evaporation and

transpiration. However, wind also changes raindrop velocities and transpiration. However, wind also changes raindrop velocities and angle of impact. Humidity and solar radiation are less directly involved angle of impact. Humidity and solar radiation are less directly involved since they are associated with temperature.since they are associated with temperature.

Soil:Soil: Physical properties of soil affects the infiltration capacity of the Physical properties of soil affects the infiltration capacity of the soil. The extend to which it can be dispersed and transported. These soil. The extend to which it can be dispersed and transported. These properties which influence soil include:properties which influence soil include:

- Soil structureSoil structure- TextureTexture- Organic matterOrganic matter- Moisture contentMoisture content- Density or compactnessDensity or compactness- Chemical and biological characteristicsChemical and biological characteristics

VegetationVegetationMajor effect of vegetation in reducing erosion are:Major effect of vegetation in reducing erosion are: Interception of rainfallInterception of rainfall Retardation of erosion by decrease of surface velocityRetardation of erosion by decrease of surface velocity Physical restraint of soil movementPhysical restraint of soil movement Improvement of aggregation and porosity of the soil by Improvement of aggregation and porosity of the soil by

roots and plants residueroots and plants residue Increase biological activitiesIncrease biological activities Transpiration – decrease soil moisture resulting in Transpiration – decrease soil moisture resulting in

increased storage capacity.increased storage capacity.These vegetative influences vary with the season, crops, These vegetative influences vary with the season, crops,

degree of maturity, soil & climate as well as with kind of degree of maturity, soil & climate as well as with kind of vegetative materials namely: roots, plant tops, plant residuevegetative materials namely: roots, plant tops, plant residue

TopographyTopography

Features that influence erosion are:Features that influence erosion are: degree of slopedegree of slopeLength of slopeLength of slopeSize and shape of the watershedSize and shape of the watershed

StraightStraightComplexComplexConcaveConcaveConvex. Convex.

RaindropRaindrop characteristicscharacteristics

TheThe relationshiprelationship betweenbetween erosion and rain fall erosion and rain fall momentum and energy id determined by raindrop momentum and energy id determined by raindrop mass, size distribution, shap, velocity and direction. mass, size distribution, shap, velocity and direction. The characterization and measurement of these The characterization and measurement of these individual factors demand the utmost ingenuity and individual factors demand the utmost ingenuity and precision.precision. The The energy equation that has been developed by energy equation that has been developed by wischmeier and smith (1958)wischmeier and smith (1958)

E=916 +331logiE=916 +331logiw and s E=kinetic energy in w and s E=kinetic energy in

The resistance of a soil to erosion depends on many The resistance of a soil to erosion depends on many factors and so to measure erodibility numerically an factors and so to measure erodibility numerically an assessment has to be made of each factor. assessment has to be made of each factor.

-nature of soil-nature of soil -slope of land-slope of land -kind of crop.-kind of crop. Erosivity-is the aggressiveness or potential ability of rain Erosivity-is the aggressiveness or potential ability of rain to cause erosion. to cause erosion. Erodibility-is the vulnerability or susceptibility of the soil Erodibility-is the vulnerability or susceptibility of the soil to erosion.to erosion.

Factors influencing erodibilityFactors influencing erodibility Two groups of factors.Two groups of factors. 1.physical features of the soil. i.e what kind of soil1.physical features of the soil. i.e what kind of soil 2.treatment of the soil . i.e what is done with it. the part 2.treatment of the soil . i.e what is done with it. the part

concerned with treatment has much the greater effect. concerned with treatment has much the greater effect. And is also most difficult to access. i.e average increase And is also most difficult to access. i.e average increase in soil loss per unit increase in E I.in soil loss per unit increase in E I.

These is a great deal of experimental These is a great deal of experimental evidence to suggest a link between erosive power and evidence to suggest a link between erosive power and the mass and velocity of falling drops.the mass and velocity of falling drops.

Ellison(1944).Ellison(1944).

Bisal(1960) suggests in a similar lab.Bisal(1960) suggests in a similar lab.G =k D V1.4G =k D V1.4S=weight of soil splashed in gramsS=weight of soil splashed in grams

k=constant for the soil typek=constant for the soil typeD=drop diameter in mmD=drop diameter in mmv=impact velocity m/s.v=impact velocity m/s.

In both these studies the combination of power of drop In both these studies the combination of power of drop velocity and drop mass are not very different from velocity and drop mass are not very different from combining mass and velocity into the parameter combining mass and velocity into the parameter kinetic energy.kinetic energy.

Rose (1960) challenges the above assumption that Rose (1960) challenges the above assumption that results such as these proves that splash erosion is results such as these proves that splash erosion is dependent only on the k.E. of natural or artificial rain, dependent only on the k.E. of natural or artificial rain, and shows that if such a relationship exist it is equally and shows that if such a relationship exist it is equally valid , though different relationship will exist between valid , though different relationship will exist between erosion and momentum or any other function of mass erosion and momentum or any other function of mass and velocity .since mass occurs in the same form in the and velocity .since mass occurs in the same form in the formula for both momentum and energy, it is formula for both momentum and energy, it is necessary to vary velocity in order to resolve the necessary to vary velocity in order to resolve the problem of whether energy or momentum is the better problem of whether energy or momentum is the better index of erosivity. Conclusively-it has been shown that index of erosivity. Conclusively-it has been shown that for natural rain the relationships between intensity and for natural rain the relationships between intensity and either momentum or kinetic energy are equally close either momentum or kinetic energy are equally close and of the same pattern.and of the same pattern.

Estimating erosivity from rainfall Estimating erosivity from rainfall datadata

The EIzo index. R=EIzo The EIzo index. R=EIzo This is the product of kinetic energy of the stem and This is the product of kinetic energy of the stem and the 30-mints intensity. This latter term requires some the 30-mints intensity. This latter term requires some explanation .explanation . -It is the greatest average intensity -It is the greatest average intensity experienced in any 30-min period during the stem.experienced in any 30-min period during the stem.

-This amount could be doubled to set the same dimension as -This amount could be doubled to set the same dimension as intensity, i.e. inches/hour, mm/hr. The measure of erosivity intensity, i.e. inches/hour, mm/hr. The measure of erosivity is described as the EIzo index. it can be computed for is described as the EIzo index. it can be computed for individuals storms, and the storm values can be summed individuals storms, and the storm values can be summed over periods of time to give weekly, monthly, or annual over periods of time to give weekly, monthly, or annual values of erosivity . values of erosivity .

Application of an index of erosivityApplication of an index of erosivity

The ability to access numerically the erosive power of The ability to access numerically the erosive power of rainfall has two main applications. In practical soil rainfall has two main applications. In practical soil conservation it helps :conservation it helps :

1.to improve the design of 1.to improve the design of conservation works.conservation works. 2.in research, it helps to 2.in research, it helps to increase our knowledge and understanding of erosion.increase our knowledge and understanding of erosion.

Soil detachment and transportationSoil detachment and transportation

The process of soil erosion involves soil detachment and The process of soil erosion involves soil detachment and soil transportation .soil transportation . Generally, soil detachability Generally, soil detachability increases as the size of the soil particles increase and increases as the size of the soil particles increase and soil transportability increases with a decrease in particle soil transportability increases with a decrease in particle size.size.

detachment causes damage because:detachment causes damage because:1.1. The soil particles are removed from the soil mass and The soil particles are removed from the soil mass and

thus easily transported.thus easily transported.2.2. The five materials and plant nutrients are removed.The five materials and plant nutrients are removed.3.3. Seeds may be separated and washed out of the soil.Seeds may be separated and washed out of the soil.

Sheet erosionSheet erosion

Uniform removal of soil in thin layers from sloping land-Uniform removal of soil in thin layers from sloping land-resulting from sheet or overland flow occurring in thin resulting from sheet or overland flow occurring in thin layers. minute rilling takes place almost simultaneously layers. minute rilling takes place almost simultaneously with the first detachment and movement of soil with the first detachment and movement of soil particles. the constant meander and change of position particles. the constant meander and change of position of these microscopic rills.of these microscopic rills.

Rill erosionRill erosion

Removal of soil by water from small but well defined Removal of soil by water from small but well defined channels or streamlets where there is a concentration channels or streamlets where there is a concentration of overland flow. Obviously,rill erosion occurs when of overland flow. Obviously,rill erosion occurs when these channels have become sufficiently large and these channels have become sufficiently large and stable to be readily seen.stable to be readily seen.

Gully erosionGully erosion

Gully erosion produces channels larger then rills. These Gully erosion produces channels larger then rills. These Channels carry water during and immediately after Channels carry water during and immediately after rain.rain.

Principles of gully erosionPrinciples of gully erosion

The rate of gully erosion depends primarily The rate of gully erosion depends primarily on the runoff producing characteristics of the on the runoff producing characteristics of the

watershedwatershed the drainage areathe drainage area soil characteristicssoil characteristics the alignmentthe alignment size and shape of gully size and shape of gully the slope in the channel.the slope in the channel.

Gully development processesGully development processes

1.1. Water fall erosion at the gully head.Water fall erosion at the gully head.2.2. Channel erosion caused by water flowing through the Channel erosion caused by water flowing through the

gully or by raindrop splash on unprotected soil.gully or by raindrop splash on unprotected soil.3.3. Alternate freezing and thawing of exposed soil banks.Alternate freezing and thawing of exposed soil banks.4.4. Slides or mass movement of soil in the gully.Slides or mass movement of soil in the gully.

Four stages of gully developmentFour stages of gully developmentStage1Stage1 Channel erosion by down ward scour of the topsoil. This Channel erosion by down ward scour of the topsoil. This

stage normally proceeds slowly where the topsoil is fairly stage normally proceeds slowly where the topsoil is fairly resistant to erosionresistant to erosion

Stage2:Stage2: upstream movement of the gully head and enlargement of the upstream movement of the gully head and enlargement of the gully in width and depth. The gully cuts to the horizon and the weak gully in width and depth. The gully cuts to the horizon and the weak parent material is rapidly removed.parent material is rapidly removed.

stage3: stage3: Healing stage with vegetation to grow in the channel. Healing stage with vegetation to grow in the channel.

Stage 4: Stage 4: Stabilization of the gully. The channel reaches a stable Stabilization of the gully. The channel reaches a stable gradient, gully walls reach and stable slope and vegetation begins gradient, gully walls reach and stable slope and vegetation begins to grow in sufficient abundance to anchor the soil and permit to grow in sufficient abundance to anchor the soil and permit development of new topsoildevelopment of new topsoil

Sediment movement in channelsSediment movement in channelsSediments in streams is transported by : Sediments in streams is transported by :

1.1. SuspensionSuspension2.2. SiltationSiltation3.3. Bad load movement.Bad load movement. Suspension:Suspension: suspended sediment is that which remains in suspended sediment is that which remains in

suspension in flowing water for a considerable period of time suspension in flowing water for a considerable period of time without contact with the stream bed.without contact with the stream bed.

saltation:saltation: sediment movement by saltation occurs where the sediment movement by saltation occurs where the particle skip or bounce along the stream bed. In comparison to particle skip or bounce along the stream bed. In comparison to total sediment transported ,saltation is considered relatively total sediment transported ,saltation is considered relatively unimportant.unimportant.

Sediment movement in channelsSediment movement in channels

Bed load:Bed load: Bed load is sediment that moves in almost Bed load is sediment that moves in almost continous contact with the stream bed being rolled or continous contact with the stream bed being rolled or pushed along the bottom by the force of the water.pushed along the bottom by the force of the water.

Mavis (1935),developed an Mavis (1935),developed an equation for unigranular materials ranging in diameter equation for unigranular materials ranging in diameter from 0.35 to 0.57 millimeters and specifically from 1.83 from 0.35 to 0.57 millimeters and specifically from 1.83 to 2.64.to 2.64.

Universal soil loss equationUniversal soil loss equationSmith and wisehmeier (1957,1962) developed an equation Smith and wisehmeier (1957,1962) developed an equation

for estimating the average annual soil loss.for estimating the average annual soil loss.A=RKLSCPA=RKLSCP

Am=2.24RKLSCP metric unit.Am=2.24RKLSCP metric unit. A=average soil loss +/a tons/acre.A=average soil loss +/a tons/acre. R=rainfall erosivity index.R=rainfall erosivity index. k=soil erodibity index.k=soil erodibity index. L=slope length factorL=slope length factor S=slope gradient factor.S=slope gradient factor. C=cropping management factor.C=cropping management factor. P=conservation particle factor.P=conservation particle factor. Ls= topographic factor evaluated.Ls= topographic factor evaluated.

Personal managementPersonal management

soil loss under standard management .This varies soil loss under standard management .This varies normally from 0-1.normally from 0-1.

P:values depends on law and slope it also depends on P:values depends on law and slope it also depends on the type of farming system we have.the type of farming system we have.

Example A:Determine soil loss from the following Example A:Determine soil loss from the following condition.condition.

K=0.1 ton/acreK=0.1 ton/acre S=10%S=10% L=400L=400 C=0.15C=0.15Field is to be confouredField is to be confoured p=0.6p=0.6

A= RKLSCP.A= RKLSCP. =0.1 =0.1 400 x 0.1 x 0.18 x 0.6 x R 400 x 0.1 x 0.18 x 0.6 x R B=Also determine soil loss from the following condition.B=Also determine soil loss from the following condition. K=0.1 ton/acreK=0.1 ton/acre L=400L=400 S=80%S=80% C=0.18C=0.18 P=0.6P=0.6 if the soil loss is 6.7 ton/acre what is the max slope length if the soil loss is 6.7 ton/acre what is the max slope length

and corresponding tar ale spacing to reduce soil loss to 3 and corresponding tar ale spacing to reduce soil loss to 3 tons/acre . tons/acre .

we want max Ls value to reduce soil lose to 3 tons/acre.we want max Ls value to reduce soil lose to 3 tons/acre. Ls=2x 3/67 =0.9Ls=2x 3/67 =0.9

Therefore =70’Therefore =70’ max slope length max slope length V.IV.I /70 = 8/100 /70 = 8/100 V.I = 5.6’ V.I = 5.6’ practical application of using universal soil loss practical application of using universal soil loss

equation.equation.1.1. To predict erosion.To predict erosion.2.2. To select crop management practices.To select crop management practices.3.3. To predict erosion from catch crops.To predict erosion from catch crops.

Land useLand use

Very suitable Very suitable land classification. land classification. Fairly suitable Fairly suitable according to suitability factor. according to suitability factor. Not suitable Not suitable a particular crop. a particular crop. Land capability classification.Land capability classification. The type of soil.The type of soil. Depth of soil.Depth of soil. Texture.Texture. Land slope.Land slope. Past erosion on the land.Past erosion on the land.

Soil erosion by windSoil erosion by wind

Wind erosion is more frequent when the mean annual rainfall is Wind erosion is more frequent when the mean annual rainfall is low.low.

Major factor that affects soil erosion by wind are: Major factor that affects soil erosion by wind are: 1.1. climateclimate2.2. Soil characteristicsSoil characteristics3.3. VegetationVegetation

climateclimate:: Rainfall affects soil moisture.Rainfall affects soil moisture. Temperature humidity.Temperature humidity. Wind.Wind.

Wind characteristics that affects soil erosion.Wind characteristics that affects soil erosion.

Duration.Duration. Turbulent of the wind (velocity).Turbulent of the wind (velocity). For any given soil condition the amount the amount of soil which will be blown For any given soil condition the amount the amount of soil which will be blown

depends on two factors:depends on two factors: The wind velocity.The wind velocity. The roughness of the soil surface.The roughness of the soil surface. Soil:Soil: factors factors soil texture. soil texture. density of soil particle and density of soil mass.density of soil particle and density of soil mass. organic matter content.organic matter content. soil moisture.soil moisture.

Vegetation:Vegetation: height of vegetation.height of vegetation. density of cover.density of cover. types of vegetation.types of vegetation. seasonal distribution of vegetation.seasonal distribution of vegetation. Types of soil movement by wind Types of soil movement by wind 1.1. Suspension.Suspension.2.2. Saltation.Saltation.3.3. Surface. creepSurface. creep

These three distinct types of movement usually These three distinct types of movement usually simultaneously.simultaneously.

suspension suspension particle were carried about 1m above particle were carried about 1m above on the surface.( i.e. above 3ft)on the surface.( i.e. above 3ft)

saltation saltation This is caused by the pressure of the This is caused by the pressure of the wind on the soil particle and its collision with other wind on the soil particle and its collision with other particles.particles.

surface creep surface creep This movement is mostly This movement is mostly pronounced on the surface of the soil.pronounced on the surface of the soil.

Mechanics of wind erosionMechanics of wind erosion

wind erosion process may also be broken into the three simple wind erosion process may also be broken into the three simple but distinct phases:but distinct phases:

1.1. Initiation of movement.Initiation of movement.2.2. Transportation.Transportation.3.3. Deposition.Deposition. 1.1. Initiation of movement.Initiation of movement. Initiation of movement as a result of turbulence and wind Initiation of movement as a result of turbulence and wind

velocity. velocity. Fluid threshold velocityFluid threshold velocity The main velocity The main velocity required to produce soil movement by direct action of wind.required to produce soil movement by direct action of wind.

Impact threshold velocity Impact threshold velocity the minimum velocity the minimum velocity required to initiate movement from the impact of soil required to initiate movement from the impact of soil particle carried in saltation.particle carried in saltation.

2.2.Transportation Transportation the quantity of soil moved is the quantity of soil moved is influenced by the particle size gradation of particle, wind influenced by the particle size gradation of particle, wind velocity and distance across the erodindg area.velocity and distance across the erodindg area.

The quantity of soil moved varies as the cube of The quantity of soil moved varies as the cube of the excess wind velocity over and above the constant the excess wind velocity over and above the constant threshold velocity directly as the square of the particles threshold velocity directly as the square of the particles diameter and increases with the gradation of the soil.diameter and increases with the gradation of the soil.

Six shape bulk densitySix shape bulk density

consider them as groups. we use equivalent diameter to test the consider them as groups. we use equivalent diameter to test the level of compa:level of compa:

Standard particleStandard particle is any spheres with bulk density of 2.65.this is any spheres with bulk density of 2.65.this has certain erodibility. has certain erodibility.

Soil particle Soil particle this also has a diameter shape and bulk density. this also has a diameter shape and bulk density. Equivalent diameterEquivalent diameter is the diameter of standard particle that is the diameter of standard particle that

has an erodibility which is equal to the erodibility of soil particle.has an erodibility which is equal to the erodibility of soil particle. Ed = bxd/2.65 Ed = bxd/2.65 diameter of soil particle. diameter of soil particle.

Q x ( V-Vc )Q x ( V-Vc )

Vc=threshold velocity.Vc=threshold velocity.

V=wind velocityV=wind velocity

when V = Vc =no movement.when V = Vc =no movement.

Deposition:Deposition: deposition of sediment occurs when the deposition of sediment occurs when the gravitational force is greater then the force holding the gravitational force is greater then the force holding the particles in the air.particles in the air.

this generally occurs when there is a decrease in wind this generally occurs when there is a decrease in wind velocity.velocity.

Soil Soil physical factorsphysical factors played also a major roll. played also a major roll. Mechanics of wind.Mechanics of wind. soil moisture condition.soil moisture condition. effect of organic matter.effect of organic matter. i.e. various climate factors.i.e. various climate factors.

Control of wind erosion.Control of wind erosion.

Two major types of wind erosion control consist ofTwo major types of wind erosion control consist of1.1. Those measures that reduce surface wind velocity(vegetation tilling soil after rain)Those measures that reduce surface wind velocity(vegetation tilling soil after rain)2.2. Those that affect soil characteristics such as:Those that affect soil characteristics such as:

conservation of moisture and tillage.conservation of moisture and tillage. contouring (teracing) contouring (teracing) generally generally vegetative measure vegetative measure tillage practices tillage practices mechanical mechanical

methods.methods.

1.1. Those that affect soil characteristics such as:Those that affect soil characteristics such as:

Damages done by windDamages done by wind

1.1. crop damagecrop damage particularly at seeding stageparticularly at seeding stage .. expose of land use.expose of land use.2.2. The change in soil textureThe change in soil texture3.3. Health.Health.4.4. Damage to properties (road and building).Damage to properties (road and building).

Contouring,stip cropping and tillage.Contouring,stip cropping and tillage.

One of the base engineering practices in conservation One of the base engineering practices in conservation farming is the adjustment of tillage and crop farming is the adjustment of tillage and crop management from uphill to downhill to contour management from uphill to downhill to contour opertions contouring,strip cropping and terracing are opertions contouring,strip cropping and terracing are important conservation practices for controlling water important conservation practices for controlling water erosion.erosion. Surface Surface roughness, ridges, depression and related physical roughness, ridges, depression and related physical characteristics influenceing depression storage of characteristics influenceing depression storage of precipitation.precipitation.

contouringcontouring

When plow furrows, planter furrows,and cultivation When plow furrows, planter furrows,and cultivation furrows run uphill and downhill then forms natural furrows run uphill and downhill then forms natural channels in which runoff accumulates. As the slope of channels in which runoff accumulates. As the slope of these furrows increases the velocity of the water these furrows increases the velocity of the water movement increases with resulting destructive erosion.movement increases with resulting destructive erosion.

In contouring In contouring tillage operations are carried out as nearly as practicals tillage operations are carried out as nearly as practicals on the contour. a guide line is laid out for eash plow on the contour. a guide line is laid out for eash plow land and the back furrows or dead furrows are plowed land and the back furrows or dead furrows are plowed on these lines.on these lines.

Disadvantage: is used alone on steeper slopes or Disadvantage: is used alone on steeper slopes or under conditions of high rainfall intensity and soil under conditions of high rainfall intensity and soil erodibility, these is an increased hazard of gullying erodibility, these is an increased hazard of gullying because row breaks may release the stored water.because row breaks may release the stored water.

Strip cropping: strip cropping consist of a series of Strip cropping: strip cropping consist of a series of alternate strips of various types of crops laid out so that alternate strips of various types of crops laid out so that all tillage and crop management practices are all tillage and crop management practices are performed across the slope or on the contour.performed across the slope or on the contour.

The three general types of strip The three general types of strip cropping are:cropping are:

1.1. Contour strip cropping with layout and tillage held Contour strip cropping with layout and tillage held closely with the exact contour and with the crops closely with the exact contour and with the crops following a definite rotational sequence.following a definite rotational sequence.

2.2. Field strip cropping with strips of a uniform width Field strip cropping with strips of a uniform width placed across the general slope.placed across the general slope.

3.3. Buffer strip cropping with strips of some grass or Buffer strip cropping with strips of some grass or legume crop laid out between contour strips of crops legume crop laid out between contour strips of crops in the regular rotations, then may be even or irregular in the regular rotations, then may be even or irregular in width.in width.

when contour strip cropping is combined with contour tillage or when contour strip cropping is combined with contour tillage or teuracing, it effectively divides the length of the slope,checks checks teuracing, it effectively divides the length of the slope,checks checks the velocity of runoff,filters out soil from the runoff water and the velocity of runoff,filters out soil from the runoff water and facilitates absorbtion of rain.facilitates absorbtion of rain.

strip cropping layoutstrip cropping layout The three general methods of laying out strip cropping are:The three general methods of laying out strip cropping are:1.1. Both edges of the strips on the contourBoth edges of the strips on the contour2.2. One or more strips of uniform width laid out from a key or base One or more strips of uniform width laid out from a key or base

contour line.contour line.3.3. Alternate uniform width and variable width correction or buffer strips.Alternate uniform width and variable width correction or buffer strips.

methods of layout vary with topography and with each methods of layout vary with topography and with each individual’s preference.individual’s preference.

Tillage practicesTillage practices: tillage is the mechanical : tillage is the mechanical manipulation of the soil to provide soil conditions suited manipulation of the soil to provide soil conditions suited to the growth of crops, the control of weeds and for the to the growth of crops, the control of weeds and for the maintenance of infiltration capacity and aeration. maintenance of infiltration capacity and aeration. Indiscriminate tillage, tillage without thought of Indiscriminate tillage, tillage without thought of topography , soil climate and crop conditions will lead topography , soil climate and crop conditions will lead to soil deterioration through erosion and loss of to soil deterioration through erosion and loss of structure.structure.

TERRACINGTERRACING This is a method of erosion control accomplished by constructing This is a method of erosion control accomplished by constructing

broad chennels across the slope of rolling land.broad chennels across the slope of rolling land. Reasons for constructing teraceReasons for constructing terace If surface runoff is allowed to flow unimpeded down the slope of If surface runoff is allowed to flow unimpeded down the slope of

arable land these is a danger that its volume or velocity or both arable land these is a danger that its volume or velocity or both may build up to the points where I is not only carries the soil may build up to the points where I is not only carries the soil dislodged by the splash erosion but also has a scouring action of dislodged by the splash erosion but also has a scouring action of its own.its own.

various names given to this techniques are:various names given to this techniques are: terraces (U.S.A).terraces (U.S.A). ridge or bund (common wealth countries).ridge or bund (common wealth countries).

functionsfunctions

To decrease the length of the hillside slope , thereby To decrease the length of the hillside slope , thereby reducing sheet and rill erosion.reducing sheet and rill erosion.

Preventing formation of gullies and retaining runoff in Preventing formation of gullies and retaining runoff in areas of inadequate precipitation.areas of inadequate precipitation.

In dry regions such conservation of moisture is In dry regions such conservation of moisture is important in the control of wind erosionimportant in the control of wind erosion

Types of terraces ( terrace classification) Types of terraces ( terrace classification) a . two major types of terraces are:a . two major types of terraces are: 1.bench terrace .1.bench terrace . 2.broad base terrace.2.broad base terrace.

Bench terraceBench terrace reduces land slope reduces land slope Broad base terrace Broad base terrace removes or retain water on sloping land. removes or retain water on sloping land. broad base terrace broad base terrace has its primary functions classified as: has its primary functions classified as: gradedgraded level.level. graded terrace graded terrace primary purpose of thus type is to remove primary purpose of thus type is to remove

excess water in such a way as to minimize erosion.excess water in such a way as to minimize erosion. level terrace level terrace primary purpose of thus type of terrace is primary purpose of thus type of terrace is

moisture conservation erosion control is a secondary objective.moisture conservation erosion control is a secondary objective.

soil profile .soil profile .the embankment for thus type of terrace is the embankment for thus type of terrace is usually constructed of soil of soil taken from both sides usually constructed of soil of soil taken from both sides of the ridge. This is necessary to obtain a sufficiently of the ridge. This is necessary to obtain a sufficiently high embankment to prevent over topping and breaking high embankment to prevent over topping and breaking through by the entrapped runoff water.through by the entrapped runoff water.

TERRACE DESIGN TERRACE DESIGN The design of terrace system involves the proper The design of terrace system involves the proper

spacing and location of terraces, the design of channel spacing and location of terraces, the design of channel with adequate capacity and development of a farmable with adequate capacity and development of a farmable x-section. x-section.

Terrace spacingTerrace spacing

spacing is expressed as the vertical distance between spacing is expressed as the vertical distance between the channels of successive terraces. The vertical the channels of successive terraces. The vertical distace is commonly known as the V.I .distace is commonly known as the V.I .

V.I = as + b = V.I =0.3( as + b)V.I = as + b = V.I =0.3( as + b) a = constant for geographical location.a = constant for geographical location. b = constant for soil erodibility.b = constant for soil erodibility. s = average land slope above the terrace in %s = average land slope above the terrace in %

Terrace gradesTerrace grades

Terrace grades : Terrace grades : refers back apart from the fact that it refers back apart from the fact that it must provide good drainage ,it must also remove must provide good drainage ,it must also remove runoff at non erosive velocity.runoff at non erosive velocity.

Terrance length: Terrance length: size and slope of the field outlet size and slope of the field outlet possibilities, rate of runoff as affected by rainfall and possibilities, rate of runoff as affected by rainfall and soil in filtration and chemical capacity are factors that soil in filtration and chemical capacity are factors that influence terrace length.influence terrace length.

Planning the terrace systemPlanning the terrace system

a. a. selection of outletsselection of outlets: or disposal area:: or disposal area: vegetated outlets ( preferable)vegetated outlets ( preferable) The design runoff for the outlet is determined by The design runoff for the outlet is determined by

summation of the runoff from individual terrace. outlets summation of the runoff from individual terrace. outlets are of many types such as : are of many types such as :

natural draws.natural draws. constructed channels.constructed channels. sod flumes.sod flumes. permanent pasture or meadow.permanent pasture or meadow. road ditches.road ditches.

b. b. Terrace locationTerrace location : factors that influence terrace location includes; land slope, soil : factors that influence terrace location includes; land slope, soil conditionsconditions..

Lay out procedure : Lay out procedure : determine the predominant slope above the terrace.determine the predominant slope above the terrace. obtain a suitable vertical interval.obtain a suitable vertical interval. stakes the Wight channel is stakes the Wight channel is terrace construction : terrace construction : a variety of equipment is available for terrace construction a variety of equipment is available for terrace construction

which necessitated a classification of four machine according to thuds of moving soil.which necessitated a classification of four machine according to thuds of moving soil.

Equipment Equipment soil moisturesoil moisture crop and crop residuescrop and crop residues degree and regularity of land degree and regularity of land soil tilthsoil tilth gullies and other obstructiongullies and other obstruction terrace lengthterrace length terrace cross-sectionterrace cross-section experience and skill of operatorexperience and skill of operator

Factors affecting rate of constructionFactors affecting rate of construction