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Here is a compilation of essays on ‘Water Erosion’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Water Erosion’ especially written for school and college students.
Essay on Water Erosion
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Essay Contents:
- Essay on the Meaning of Water Erosion
- Essay on the Processes of Water Erosion
- Essay on the Forms of Water Erosion
- Essay on the Causes of Water Erosion
- Essay on the Factors Affecting Water Erosion
- Essay on the Estimation of Soil Loss
- Essay on the Losses Due to Water Erosion
- Essay on the Harmful Effects of Water Erosion
- Essay on the Control of Water Erosion
Essay # 1. Meaning of Water Erosion:
Water and wind are the main agencies responsible for soil erosion. Loss of soil from land surface by water, including runoff from melted snow and ice, is usually referred to as water erosion.
The major erosive agents in water erosion are impacting rain drops and runoff water flowing over the soil surface. Erosion and sedimentation embody the processes of detachment, transportation and deposition of soil particles.
Detachment is dislodging of soil particles from soil mass by the erosive agents. Transportation is movement of detached soil particles (sediment) from their original location. The sediment moves along the stream and part of it may eventually reach the ocean. Some sediment is usually deposited at the base of the slopes, reservoirs and flood plains along the way.
Essay # 2. Processes of Water Erosion:
i. Raindrop Impact:
Raindrop impact on bare soil is a powerful agent of particle detachment. Maximum effect of vegetative cover in controlling raindrop impact occurs when the cover is at ground surface. When rainfall is intercepted by plant canopy, its properties are altered.
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First, rainfall at the ground becomes concentrated beneath individual drip points. Second, raindrops coalesce on leaves to form larger drops. Typically, natural rainfall has a median drop size of about 2.0 mm; in contrast the median drop size of leaf drips is about 4.8-5.0 mm.
Although the energy of rainfall is absorbed by plant canopy, the drips gain energy again as they fall from leaves. With vegetation canopies on or close to the ground surface, this gain is very small, but once the canopy rises to 0.5 m or more above the ground, the drip energy is sufficient to detach soil particles.
Detachment rates on bare soil under forest canopies can be 2-4 times higher than those on open ground. Fortunately, in most forested areas, soil is protected by a litter layer of decaying plant matter.
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On agricultural land, however, no such protection exists and detachment rates under crops such as maize mean be two or more times that of un-cropped bare land. Since detachment is the first phase of water erosion process, its control is important. If few or no soil particles were detached, there would be no material for eroding agents to transport and erosion rates would therefore be low.
ii. Runoff:
Ground level vegetation imparts roughness to the surface, reducing the velocity of flowing water and therefore its capacity to erode the soil. Runoff will detach soil particles from the soil mass and entrain them in the flow when its velocity exceeds a critical value that depends upon the resistance of soil material.
The critical velocity is higher for coarse materials (sands, gravels), which weigh more and for fine particles (clay), which are held together by cohesion. It is lowest for silts and fine sands, which explains why silty and loamy soils are more prone to erosion than other soil types. Once a soil particle has been picked up by the flow, it can be carried at much lower velocities than those required for detaching it.
This is particularly true for clay particles which, once detached, can be transported considerable distances (tens of m to km) before coming to rest at a point in the landscape where velocity is reduced and deposition occurs.
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Process of water erosion can be divided into two stages: the basic processes of detachment and transport on source areas and transport and deposition in sink areas. Detachment of soil particles is by either raindrop impact or flowing water.
Individual raindrops strike the soil surface at velocities up to 9 m s-1, creating very intense hydrodynamic force at the point of impact leading to soil particle detachment. Overland flow detaches soil particles when its erosive hydrodynamic forces exceed resistance of the soil to erosion.
With increase in drop size and intensity of rain, kinetic energy and momentum increase. A 5 cm hr-1 storm dissipates energy at the rate of 5.6 kW ha-1 and a 7.5 cm hr-1 storm dissipates energy at the rate of 520 kV ha-1, nearly 100 times as fast. As a result, finer particles disperse in water and are carried away leaving the coarse particles behind. This coarsens the soil texture.
The steeper the slope and the greater the length of run the faster is the flow rate and greater is the turbulence and greater is the power to cut, disperse and transport the soil. Water running off a slope tend to concentrate into rills, which tends to coalesce resulting in torrents capable of cutting the soil deep and widening its flow. Water erosion starts as with a thin suspended film of finer particles in the form of sheets, becomes rills and then gullies causing spectacular soil erosion.
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Detached soil particles are transported by raindrop splash and runoff. The amount of soil transported by runoff is more than that due to raindrop splash. Sediment is deposited when the sediment load exceeds the total flow transport capacity or flow losses capacity to transport coarser particles in the sediment load.
Thus, the falling raindrops break down the soil aggregates and detach soil particles from each other. The finer particles (silt and clay) block the soil pores and increase the rate of runoff and hence the soil and water loss.
Essay # 3. Forms of Water Erosion:
Water erosion occurs in stages identified as sheet, rill, gully, ravine, landslide and stream bank erosion.
i. Sheet Erosion:
It is the uniform removal of surface soil in thin layers by sheet, or overland flow of water. The breaking action of raindrop combined with surface flow is the major cause of sheet erosion. It is the first stage of erosion and is least conspicuous (Fig. 4.1).
ii. Rill Erosion:
When run-off starts, soil is lost from small but well defined channels or streamlets (rills) by water when there is concentration of surface flow. Raindrop impact does not directly detach any particles below flow line in rills but it increases the detachment and transportation capacity of the flow.
Rill erosion starts, when the runoff exceeds 0.3 to 0.7 mm s-1. Most of the upland sediment load is transported down slope in the rills. This is the second stage of erosion. Rills are small channels which can be removed by timely normal tillage operations.
iii. Gully Erosion:
It is the advanced stage of water erosion. Size of the unchecked rills increase due to runoff. Gullies are formed when channelized runoff from vast sloping land is sufficient in volume and velocity to cut deep and wide channels. Gullies are the spectacular symptoms of erosion. If unchecked in time, no scope for arable crop production. (Fig. 4.2)
iv. Ravines:
Prolonged process of gully erosion leads to ravines, typically found in deep alluvial soils. They are deep and wide gullies indicating advanced stage of gully erosion. The side slopes of gullies fail and blocks of soil mass slough into the flow. The slope stability depends on soil strength and subsurface moisture regime.
v. Land Slides:
Land-slides and land slips occur during wet season in mountain slopes. The quantity of pediment moved from hill sides by mass movement is far higher than that due to gullies. Debris from land-slides causes obstruction to roads, choking of bridges etc., thus blocking traffic and communications. Land slide debris deposited in fields make them unfit for cultivation.
vi. Stream Bank Erosion:
Stream banks are eroded by water either flowing over the sides of a stream or scouring at the base. Erosion of the bank is due to surface flow leading to scouring and under cutting the soil below the water surface. Small streams, torrents, etc. are subjected to stream bank erosion.
a. Pedestral Erosion:
Protection by tree roots or stone against splash results in pedestral capped by resistant material. Height of such pedestarlas is a good indication of the magnitude of soil loss due to erosion.
b. Pinnacle Erosion:
High pinnacles in gully side and bottom are associated with deep vertical rills in the gully sides, cutting back rapidly to join one another to leave isolated pinnacles. A more resistant soil layer and gravel often caps a pinnacle.
c. Piping:
It is caused by infiltration of surface water through porous layers when encountered by less permeable layers. It carries with it fine material as well.
d. Slumping:
It is a geological process significantly contributing to gully formation. When head of the gully worked backup to the crest and beyond and there in no inflow at the gully head, flood flow causes slumping of sides and head and erosion continues by slumping, leading to bank collapse.
Essay # 4. Causes of Water Erosion:
Water erosion of soil starts when raindrops strike bare soil peds and clods, resulting the finer particles to move with the flowing water as suspended sediments. The soil along with water moves downhill, scouring channels along the way. Each subsequent rain erodes further amounts of soil until erosion has transformed the area into barren soil.
Water erosion is due to the dispersive action, and transporting power of water—water as it descends in the rain and leaves the land in the form of run-off. Water erosion caused by people who remove protective plant covers by tillage operation, burning crop residues, overgrazing, over cutting forests etc. inducing loss of soil.
i. Raindrop Splash Erosion:
Rain drop splash erosion results from soil splash caused by the impact of falling rain drops.
There are four factors that determine the rate of rain drop erosion namely:
(i) Climate (mostly rainfall and temperature),
(ii) Soil—its inherent resistance to dispersion and its infiltration rate,
(iii) Topography particularly steepness and length of slope, and
(iv) Vegetative cover—either living or the residues of dead vegetation.
The continued impact of raindrops compacts the soil and further seals the surface—so that water cannot penetrate into the soil and as a result causing more surface run-off. The impact of rain drops per unit area is determined by the number and size of the drops, and the velocity of the drops.
ii. Sheet Erosion:
Sheet erosion is the removal of a fairly uniform layer of surface soil by the action of rainfall and run-off water. This type of erosion, though extremely harmful to the land, is usually so slow that the former is not conscious of its existence. It is common on lands having a gentle or mild slope, and results in the uniform “skimming off of the cream” of the top soil with every hard rain. In this erosion, shallow soils suffer greater reduction in productivity than deep soils. Movement of soil by rain drop splash is the primary cause of sheet erosion.
iii. Rill Erosion:
Rill erosion is the removal of surface soil by running water, with the formation of narrow shallow channels that can be leveled or smoothed out completely by normal cultivation. Rill erosion is more apparent than sheet erosion.
Rills develop when there is concentration of run-off water and if neglected, they grow into large gullies. Rill erosion is more serious in soils having a loose shallow top soil. This type of soil erosion may be regarded as a transition stage between sheet and gully erosion.
iv. Gully Erosion:
Gully erosion is the removal of soil by running water, with the formation of channels that cannot be smoothed out completely by normal agricultural operation or cultivation. Gully erosion is an advanced stage of rill erosion.
Unattended rills get deepened and widened every year and begin to attain the form of gullies. During every rain, the rain water rushes down these gullies, increasing their width, depth and length. Gully erosion is more spectacular and therefore, more noticeable than any other erosion.
The development of gully occurs due to following four stages:
a. Formation Stage:
With channel erosion by a downward scour of the surface soil.
b. Development Stage:
Consisting of upstream movement of the gully head and enlargement of the gully in width and depth.
c. Healing Stage:
Beginning with the growing of vegetation in the gully.
d. Stabilization Stage:
The gully reaches a stable gradient, gully walls reach a stable slope, and vegetative cover spreads over the gully surface.
Gully erosion can be classified as follows:
v. Stream Channel Erosion:
Stream channel erosion is the scouring of material from the water channel and the cutting of banks by flowing or running water. ‘This erosion occurs at the lower end of stream tributaries and to streams that have nearly continuous flow and relatively flat gradients. Stream but erode either by run-off flowing over the side of the stream bank, or by scouring or undercutting. Scouring is influenced by the velocity and direction of flow, depth and width of the channel and soil texture.
Essay # 5.
Factors Affecting Water Erosion:
i. Rainfall:
Erosion is directly a function of rainfall and runoff. Amount, intensity, duration and distribution of rainfall influence runoff and erosion.
High intensity rains of longer duration cause severe erosion. Greater the intensity larger the size of raindrop. Velocity of falling raindrop varies from 4-5 m s-1 for one mm drop to about 9 m s-1 for 5 mm drop. Rainfall intensity more than 5 cm hr-1 is considered severe.
Total energy of raindrops falling over a hectare land with rainfall intensity of 5 cm hr-1 is equal to 625 HP. Two-thirds of the energy is used in sealing soil pores. Runoff may occur without erosion, but there is never water erosion without runoff.
The most suitable expression of rainfall erosivity is an index based on kinetic energy of the rain. The erosivity of rainstorm is a function of its intensity and duration and of the mass, dia and velocity of raindrop.
Erosion index (EI30), the product of kinetic energy of the storm times the maximum 30 minute intensity has been found to be the good index of erosivity of rainfall and have good correlation with the soil loss.
ii. Soils:
A highly erodable soil may erode 10 times faster than a less susceptible soil to the same moderate to intense rainfall. Soil texture, structure, organic matter, infiltration and permeability influence runoff and soil loss. Fine soils are more susceptable to erosion than coarse soils.
In India, deep lateritic soils of Ootakamund (TN) and red soils of Deochanda (Bihar) have the lowest rate of runoff, the alluvial soils of Vasad (Gujarat) and Dehra Dun (Uttaranchal) have very high rate of runoff and black soils have an intermediate runoff, but this rate of runoff is still high.
Soil erodability index (K) representing the soil loss per unit of El30 as measured on standard bare soil plot of 22 m long with 9 per cent slope is used to predict soil erodability by water.
The values of soil erodability factor for different locations in India are given in Table 4.4:
iii. Topography:
Water flows slowly over a gentle slope and faster over a steeper one. If the velocity of a stream is increased, its eroding power is increased to the square of its velocity (if the velocity is doubled, the erosive power will increase 4 times that of the original velocity). A rapid current carries more quantity of material of a given size than a slow current.
If the velocity is doubled, the quantity of material of a given size that can be carried is increased about 32 times the quantity of material that the original current was capable of carrying (the amount varies as the fifth power of velocity).
Not only does the quantity of material increase with increase in velocity but the size of particle also increases. If the velocity is doubled, the size of particles that can be transported is increased about 64 times the size of those particles that the current carried at its original velocity (size of particles varies as the sixth power of velocity).
Length of slope is also an important factor of water erosion. If the slope is long, large quantity of rain will fall on it and if the rate of rainfall exceeds the rate of infiltration, there will be large accumulation of water at the base leading to problems. If the slope is short, there will be less accumulation of water at the base.
iv. Soil Surface Cover:
Cover include plant canopy and material like mulches and plant residues in contact with soil surface reducing erosion. The canopy intercepts raindrops and reduces its impact on soil. Soil surface covers also reduce the velocity of runoff as they act as obstruction to the flow. Plant roots bind the soil particles and addition of organic matter through organic surface covers improves soil aggregation to resist break down.
v. Biotic Interference:
The natural balance is disturbed and destructive forces of nature operate only when the man with his plough and cattle enter the scene. Removal of vegetal cover for cultivation, disturbances of slopes, misuse of forest area by burning, over-felling, over-grazing, etc. result in accelerated erosion whose extent depends on the degree and nature of human interference.
Essay # 6. Estimation of Soil Loss:
Several methods are available for measuring soil loss from different land units. Several equations are also available to estimate soil erosion. The universal soil loss equation (USLE) developed by Wischmeier and Smith is most useful. The USLE, an empirical equation, estimates average annual soil loss per unit area as a function of major factors affecting sheet and rill erosion.
It enables determination of land management erosion rate relationships for a wide range of rainfall, soil slope, crop and management conditions and to select alternative cropping and management combinations that limit erosion rates to acceptable limits.
A = R x K x LS x C x P
where, A = soils loss in t ha-1 yr-1
R = rainfall erosivity factor
K = soil erodability factor
L = slope length factor
S = slope steepness factor
C = soil cover and management factor
P = erosion control factor.
i. Rainfall Erosivity Factor (R):
The R factor takes into account the erosive effects of storms. The total kinetic energy of each storm (intensity and total rainfall) plus the average rainfall during the 30 minute period of greatest intensity is considered. Sum of the indices for all storms during a year provides an annual index. Location value of this index is the rainfall erosivity factor (R) in the USLE.
ii. Soil Erodability Factor (K):
Soil erodability factor (K) in USLE is related to the rate at which different soils erode. Under the conditions of equal slope, rainfall, vegetative cover and soil management practices, some soils will erode more easily than others due to inherent soil characteristics. Its value is low for soils with high infiltration rate. Soil erodability factors for different soil types have been given in Table 4.5. The only factor in USLE with dimensions is K.
iii. Topographic Factor (LS):
Slope length factor (L), the ratio of soil loss from field slope length to that from 22.13 m length plots under ideal conditions and slope steepness factor (S), the ratio of soil loss from field slope gradient (steepness) to that from 9 per cent slope under otherwise identical conditions can be determined separately. However, the procedure has been further simplified by combining L and S factors as topographic factor (LS).
The LS is the expected ratio of soil loss per cent area from field slope to that from 22.13 m length of 9 per cent slope. Nomographs for determining LS factor are developed for convenience.
The LS factor for different slope lengths and steepness are given in Table 4.5:
iv. Soil Cover and Management Factor (C):
It is the expected ratio of soil loss from land cropped under specific conditions to soil loss from clean tilled fallow on identical soil and slope and under the same rainfall. The C values decrease with increase in soil cover (Table 4.6).
v. Erosion Control Factor (P):
Crops on lands subjected to erosion are usually protected by practices that will reduce runoff and erosion. Contour cultivation, strip-cropping, terracing etc., are some of the erosion control practices for crops on lands subjected to erosion.
Factor P in USLE is the ratio of soil loss with a specified erosion control practice to the corresponding loss with up-and-down cultivation.
The P values for contour terraced fields are given in Table 4.7:
Expected soil loss in any location can be calculated using the USLE.
Given the values as:
R = 150
K = 0.3
LS = 0.5
C = 1.0
P = 0.5
The expected soil loss would be:
A = R x K x LS x C x P
= (150) x (0.3) x (0.5) x (1.0) x (0.5)
= 11.25 t ha-1
Run-off and soil loss studies in India indicate maximum soil loss (64.5 t ha-1) in cultivated black soil region followed by northeastern region (41.0 t ha-1), ravine region (33.0 t ha-1) and Assam valley (28.0 t ha-1). Total estimated annual loss in the country is 5,333,000,000 t with an average of 16.35 t ha-1.
Essay # 7. Losses Due to Water Erosion:
When once the soil is lost by erosion, none of the management practices including inputs can mitigate the loss of soil. Dryland agriculture will not be remunerative when once the rain water is lost by runoff.
The adverse effects of water erosion are:
i. Land Degradation:
Nearly 65 per cent of the degraded land in India is due to water and wind erosion. Loss of fertile top soil along with plant nutrients is the major cause of agricultural land degradation.
ii. Floods and Flood Plains:
Floods and landslides cause large scale damage to humans as well as animal life and property. Productive land and might go out of cultivation in floodplains due to silt deposition.
iii. Productivity of Agricultural Lands:
Light to moderately degraded lands yield around 10 per cent less than non-degraded lands. Yield loss on strongly degraded lands go up to 20 per cent and may be .ion-productive in extreme cases.
iv. Loss of Scarce Rainwater:
Rainwater which otherwise percolates into the soil for use by rainfed crops or to recharge groundwater is lost as surface runoff leading to failure of dryland crops and depletion of groundwater resources.
v. Environmental Pollution:
Sediment is one of the major pollutants. If erosion continues at the present rate, we shall be left with reclamation of soil rather than its conservation and management.
vi. Changing Forest Cover:
The forest cover is declining due to degradation of permanent pastures and open forests in desert and arid regions.
vii. Loss of Biodiversity:
Loss of seeds and propagules in erosion fluxes, washing away of nutrients runoff etc., deplete the biodiversity. In the last few decades, India has lost at least half of its forest, polluted over 70 per cent of water bodies and degraded most of its coasts.
Essay # 8. Harmful Effects of Water Erosion:
Water erosion causes various damages to the lands as follows:
i. Loss of Top Fertile Soil:
The surface soil lost as run-off consists of fertile soils and fresh or active organic matter. The eroded soil deposited in a river bed or reservoir is not only unavailable for agricultural use but is definitely harmful.
ii. Accumulation of Sand or Other Unproductive Coarse Soil Materials on Other Productive Lands:
In the plains, fertile lands have been made unproductive by the deposition or accumulation of soil material brought down from the hills by streams and rivers.
iii. Silting of Lakes and Reservoirs:
Soil erosion from the catchment areas of reservoirs results in the deposition of soil, thus reducing their storage capacity and minimising their useful life.
iv. Silting of Drainage and Water Channels:
Deposition of silt in drainage ditches in natural streams and rivers reduces their depth and capacity to handle run-off and at the same time the demand on these outlets is increased. As a result, overflows and flooding of downstream areas increase with damage to agricultural crops and also disaster to man-made structures.
v. Decreases Water Table:
With the increase in run-off, the amount of water available for entering the soil is decreased. This reduces the supply of water to replenish the ground water in wells, the yield of well is reduced.
vi. Fragmentation of Land:
Water erosion especially gully erosion may divide the land into several valleys and ridges and thus fields become smaller and more numerous. Crop rows are shortened, movement from field to field is obstructed and a result the value of land is decreased.
Essay # 9. Control of
Water Erosion:
Water erosion occurs simultaneously in two steps; detachment of soil particles by falling raindrops and transportation of detached soil particles by flowing water. Therefore, water erosion can be minimised by preventing the detachment of soil particles and their transportation.
Principles of water erosion control are:
1. Maintenance of soil infiltration capacity.
2. Soil protection from rainfall.
3. Control of surface runoff.
4. Safe disposal of surface runoff.
For a sound soil conservation programme, every piece of land must be used in accordance with land capability classification.
Land Capability Classification:
It is a systematic arrangement of different kinds of land according to properties that determine the ability of land to produce common cultivated crops and pastures on a permanent basis.
Land capability is determined by:
1. Inherent Soil Characteristics:
Texrure of top soil, effective soil depth, permeability, soil reaction, salinity, alkalinity, nutrient supplying capacity.
2. External Land Features:
Slope, erosion, water-logging, overland flow and flooding.
3. Environmental Factors:
Rainfall, temperature etc.
Based on the intensity of hazards and the limitations, there are 8 land capability classes. These classes are of two broad groups: arable and non-arable lands.
a. Arable Land:
Suitable for cultivation and other land uses (class I to IV lands).
Class I (Green):
Soils have very few or no limitations that restrict their use. The soils may need one or more management practices such as fertilisers, green manuring and crop rotation to maintain the productivity. These soils are ideal for all the crops.
Class II (Yellow):
This class has some limitations that reduce the choice of crops or require simple conservation practices. Most common management options include strip-cropping, contour cultivation, water disposal area, vegetative cover, crop rotation, green manuring, mulching, fertiliser application etc. These soils can be used for food crop production, pastures, forests and wild life.
Class III (Red):
These soils have moderate limitations, which reduce the choice of crops and require special conservation practices. For cultivated crops, the conservation practices are usually more difficult to apply and to maintain. These soils can be used for growing cultivated crops, pastures, forests and wild life.
Class IV (Blue):
Soils have severe limitations that restrict the choice of crops and require more careful management, when cultivated. Restrictions for using these soils are greater than those of class III and choice of crops is more limited. Conservation practices are more difficult to apply and maintain. Soils may be used for crops, pastures, forests and wild life.
b. Non-Arable land:
Not suitable for cultivation, but suitable for other land uses (class V to VIII lands).
Class V (Dark Green or Uncoloured):
Soils have little or no erosion hazard, but have other limitations, whose removal is not practicable. They are used largely for pastures, forests and wild life, food and cover crops. These are not ideal for cultivated crops. Pastures can be improved through management practices such as seeding, fertiliser use, contour furrows, drainage ditches etc.
Class VI (Orange):
Soils have very severe limitations that make them unsuitable for cultivation and limit their use largely to pasture, forests or wild life. There is no scope for correcting several limitations.
Class VII (Brown):
Soils have very severe limitations that makes them unsuitable for cultivation and restrict their use largely to grazing, forester and wild life. Soils are subjected to severe limitations under either grazing or forestry use. It is not practicable to adopt pasture improvement practices.
Class VIII (Purple):
Land limitations preclude their use for commercial plat production and restrict their use to recreation, wild life and aesthetic purposes. No scope for correcting the limitations with reasonable investment.
Soil and water conservation measures can be broadly grouped into three categories: agronomic, mechanical/engineering and forestry measures.
Agronomic Measures:
In soil and water conservation programme, agronomic measures (conservation agronomy) have to be considered in coordination with others for their effectiveness. These measures are effective on gentle slopes up to 9 per cent. Reduction in runoff is achieved by choice of crops, land preparation, contour cultivation, strip-cropping, mulching, application of manures and fertilizers and appropriate cropping systems.
1. Choice of Crops:
Row crops (erosion permitting crops) such as sorghum, maize, pearl-millet etc., are not as effective as soil conserving crops (erosion resistant crops) such as cowpea, groundnut, green-gram, black-gram etc. Generally, legumes (smothering crops) provide better cover and protection to soil by way of minimizing the impact of raindrop and acting as obstruction to runoff.
2. Land Preparation:
Land preparation including postharvest tillage influence intake rate of water, obstruction to surface flow and consequently the rate of erosion. Deep ploughing or chiseling have been found effective in reducing erosion. Rough cloddy surface is also effective in controlling erosion.
3. Contour Cultivation (Contour Farming):
All the cultural practices such as ploughing, sowing, inter-cultivation, etc. across the slope reduce the soil and water loss. By ploughing and sowing across the slope, each ridge and plough furrow and each row of the cop act as obstruction to the runoff and provide more time for water to enter into the soil leading to reduced soil and water loss.
4. Strip-Cropping:
It involves growing of few rows of erosion resisting crops and erosion permitting crops in alternate strips on contour with the objective of breaking long strips to prevent soil loss and runoff. Erosion resisting crops reduce the transporting and eroding power of water by obstructing runoff and filtering the sediment from the runoff to retain in the field.
5. Mulching:
It reduces soil loss considerably by protecting the soil from direct impact of raindrop and reducing the sediment carried with runoff. A minimum plant residue ground cover of 30 per cent is necessary to keep runoff and soil loss within acceptable limits. As most of the plant remains are used as cattle fodder, availability of organic residues for mulching is the major limitation under conditions of scarce fodder.
6. Conservation Tillage:
Conservation tillage, encompassing residue management, is well- tested strategy of soil and water conservation in USA: Low intensity tillage favours consolidation of soil and imparts erosion resistance through better consolidation, structure, infiltration, pore distribution and depression storage. Zero tillage with mulching appears ideal in several situations for soil and water conservation.
7. Organic Manures and Fertilisers:
Organic manures, besides supplying nutrients, improve soil physical conditions. Fertilisers improve vegetative growth (canopy) which aids in erosion control. Improvement in soil structure improves rate of infiltration leading to reduced runoff.
8. Cropping Systems:
Mono-cropping of erosion permitting crops accelerate soil and water loss year after year. Intercropping erosion permitting and erosion resistant crops or their rotation have been found effective for soil and water conservation.
As the legumes (cowpea, greengram, horsegram, blackgram) are effective for soil conservation due to their smothering effect, they should be sown in time to develop adequate canopy by the time of peak rate of runoff.
Cover crops such as greengram, blackgram, cowpea, soybean, sunnhemp, groundnut etc., restore soil fertility besides reducing soil erosion. Such crops as intercrops with widely spaced crops can give continuous cover of ground protection against erosion (Table 4.8). Mixed cropping is equally effective for conserving soil and water.
Mechanical Measures (Engineering Measures):
When agronomic measures alone are not adequate, mechanical measures are adopted to supplement the agronomic measures. Thus, the mechanical and agronomic measures are not the two alternatives, but are complementary which should be used together.
Important principles to be kept in view while planning for mechanical measures according to Rama Rao (1962) are:
1. Increasing the time of concentration to allow more runoff water to be absorbed and held by the soil.
2. Interrupting a long slope into several short ones to maintain a critical velocity of runoff.
3. Protection against damage due to excessive runoff.
4. Interrupting a long slope into several short ones to maintain a critical velocity of runoff.
5. Protection against damage due to excessive runoff.
Implements like basin listing and sub-soiler can be effectively used for soil and water conservation.
1. Basin Listing:
Basin listing consists in making of small interrupted basins along the counter with a special implement, called a basinlister. Basin listing helps to retain rainwater as it falls and is especially effective on retentive soils having mild slopes.
2. Sub-Soiling:
This method consists in breaking with a subsoiler the hard and impermeable subsoil to conserve more rainwater by improving the physical conditions of a soil. This operation, which does not involve soil inversion and promotes greater moisture penetration into the soil, reduces both runoff and soil erosion. The subsoiler is worked through the soil at a depth of 30- 60 cm at a spacing of 90-180 cm.
Mechanical measures consist of construction of mechanical barriers across the direction of the flow of water to retard or retain the runoff for reducing soil and water loss. These measures include contour bunding, graded bunding, bench terracing, construction of grade stabilisation structures, etc.
A bund or terrace is an earthen embankment or a depression or a combination of both constructed across the land slope to control runoff and minimise soil erosion by reducing the length of slope. By reducing the slope, the velocity of runoff is not allowed to attain a critical value which initiate scouring. In the case of bench terracing, however, both the length and degree of slope are reduced.
Forestry Measures:
Perennial vegetation such as trees and grasses can be successfully used for economic utilisation of degraded lands besides reducing soil and water loss. Effectiveness of perennial vegetation in reducing the impact of raindrop on soil surface and minimising the velocity of runoff of surface flow has been well established.
A large number of tree species were introduced for evaluating their performance under different agro-climatic conditions of the country.
Tree species found economical are:
1. Pinus patula (Pinus).
2. Pinus kesia.
3. Eucalyptus camaldulensis (Eucalyptus).
4. Moliodora.
5. Leucaena latisilqua (Subabul).
6. Acacia mearnsii (Acacia).
7. Acacia nilotica.
8. Acacia tortilis.
The desirable characters of grass for soil and water conservation are perennial nature, drought resistance, rhizomniferous, good canopy, deep root system, prostrate habit and usefulness in cottage industry.
Some of the useful grasses and legumes are:
Grasses:
1. Cenchrus ciliaris
2. Chloris guyana
3. Cynodon dactylon
4. Dicanthium annulatum
5. Heteropogon contortus
6. Iseilema laxum
7. Panicum antidotale
8. Andropogan haltei
9. Panicum virgatum
10. Eragrostis curvula
Legumes:
1. Atylosia scarbaceoides
2. Centrosema pubescence
3. Stylosanthus hamata
Gully Control:
Generally, gullies are formed by an increase in surface runoff. Therefore, minimising surface runoff is essential in gully control. Watersheds deteriorate because of man’s misuse of land, short intensive rainstorms, prolonged rains of moderate intensity and rapid snow melts.
In gully control, the following three methods must be applied according to the order given:
(i) Improvement of gully catchments to reduce and regulate the peak runoff rates.
(ii) Diversion of surface water above the gully area.
(iii) Stabilisation of gullies by structural measures and re-vegetation.
When the first and/or second methods are applied in some regions of countries with temperate climates, small or incipient gullies may be stabilised without having to use the third method. On the other hand, in tropical and subtropical countries with heavy rains (monsoons, typhoons, tropical cyclones etc.), all the three methods must be carried out for successful gully control.
Development of Gullies:
Gullies are formed where many rills join and gain more than 30 cm depth. Rate of gully erosion depends on runoff producing characteristics of the watershed: drainage area, soil characteristics, the alignment, size and shape of gully and gradient of the gully channel.
A gully develops in three distinct stages; waterfall erosion; channel erosion along the gully bed and landslide erosion on gully banks. Correct gully control measures must be determined according to these development stages.
(i) Waterfall Erosion:
It can also be broken down into three stages:
First Stage:
First, sheet erosion develops into rills and then the rills gain depth and reach the B-horizon of the soil.
Second Stage:
Gully reaches C-horizon and the weak parent material is removed. A gully head often develops where flowing water plunges from upstream segment to the bottom of gully.
Third Stage:
Falling water from the gully head starts carving a hollow at the bottom of gully by direct action as well as by splashing. When the excavation has become too deep, steep gully head wall collapses.
(ii) Channel Erosion along Gully Beds:
It is a scouring away of soil from the bottom and sides of gully by flowing water. Length of gully channel increases as waterfall erosion causes the gully head to advance backwards. At the same time, gully becomes deeper and wider because of channel erosion.
(iii) Landslide Erosion on Gully Banks:
Channel erosion along gully beds is the main cause of landslides on gully banks. During rainy season, when the soil becomes saturated and gully banks are undermined and scoured by channel erosion, big soil blocks start sliding down the banks and are washed away through the gully channel.
The three stages of gully development (waterfall erosion, channel erosion along the gully bed and landslides on gully banks) will continue unless the gully is stabilised by structural control measures and re-vegetation.
Classification of Gullies:
Gullies are classified under several systems based on their characteristics:
(i) Gully Classes Based on Size:
Based on size (depth and drainage area), gullies are classified into small, medium and large gullies (Table 4.10).
(ii) Gully Classes Based on Shape:
This system classifies gullies according to the shape of their cross sections.
U-Shaped gullies are formed where both topsoil and subsoil have the same 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 gullies develop where subsoil has more resistance than topsoil against erosion. This is the most common gully form.
Trapezoidal gullies can be formed where gully bottom is made of more resistant material than topsoil.
(iii) Gully Classes Based on Continuation:
Continuous gullies consist of many branch gullies. A continuous gully has a main gully channel and many mature or immature branch gullies. A gully network (gully system) is made up of many continuous gullies. A multiple gully system may be composed of several gully networks.
Discontinuous gullies may develop on hillsides after landslides. They are also called independent gullies. At the beginning of its development, a discontinuous gully does not have a distinct junction with the main gully or stream channel.
Flowing water in a discontinuous gully spreads over a nearly flat area. After some time, it reaches the main gully channel or stream. Independent gullies may be scattered between the branches of a continuous gully or they may occupy a whole area without there being any continuous gullies.
Gully Management:
Prevention of gully formation is much easier than controlling it once it has formed. Prevention is also more economical because structural measures are considerably more expensive than preventive measures. Therefore, in erosion control or gully control, emphasis should be given to proper land management practices and retention and infiltration of surface water.
(i) Proper Land Management Practices:
1. Prevention of forest fires and illegal wood cutting in plantations and natural forests.
2. Prevention of grass fires.
3. Control of grazing and re-vegetation of open and grass lands.
4. Maintenance of soil fertility and stability on land which is under agro-forestry or agriculture.
5. Control of road construction and mining.
6. The immediate stabilisation of moderate sheet and rill erosion and incipient gullies in forest, rangeland and cultivated areas.
(ii) Retention and infiltration of surface water, in addition to proper land management practices, include specific slope treatment measures such as retention and infiltration ditches, terraces, wattles, fascines and staking, should be carried out above the gully area and in the eroded area between the branch gullies, to reduce the rate and amount of surface runoff.
(iii) Diversion of surface water above the gully area direct runoff away from gully heads and discharges it either into natural waterways or vegetated watercourses or onto rock outcrops and stable areas which are not susceptible to erosion. Surface water must not be diverted over unprotected areas or it will cause new gullies.
The basic aim of diversions is to reduce the surface water entering into the gully through gully heads and along gully edges and to protect critical planted areas from being washed away. Diversion ditches should be large enough to carry all the water that is discharged from the gully catchment area during periods of maximum runoff.
Gully Treatment Measures:
Once gullies have begun to form, they must be treated as soon as possible to minimise further damage and restore stability.
Treatment measures include:
(i) Filling and Shaping in Regions with Low Intensity Rainfall:
Gullies with very little water flow can be stabilised by filling and shaping. Steep gully heads and gully banks should be shaped to a gentler slope (one-to-one slope). Rills and incipient branch gullies may be filled in by spade, shovel or plow.
In general, weeds will grow first and then hardy plants capable of surviving in a gullied area. After one or two years, the region’s predominant vegetation will cover the gully as part of this natural process.
(ii) In Regions with High Intensity Rainfall:
In regions with heavy rains, filling, shaping and diversions alone will not suffice to control gullies. Additional gully control and slope stabilisation measures, such as check dams, stone terraces, wattles and re-vegetation, should be undertaken.
(iii) Gully Treatment by Grade Stabilisation Structures:
In gully control, temporary structural measures such as woven-wire, brushwood, logs, loose stone and boulder check dams are used to facilitate the growth of permanent vegetative cover. Check dams are constructed across the gully bed to stop channel and lateral erosion.
Temporary check dams, which have a life span of three to eight years, collect and hold soil and moisture in the bottom of the gully. Tree seedlings, as well as shrub and grass cuttings planted in gullies can grow without being washed away by flowing water. Thus, a permanent vegetative cover can be established in a short time.
To obtain satisfactory results from structural measures, a series of check dams should be constructed for each portion of the gully bed (Fig 4.7). Stabilised watershed slopes are the best assurance for the continued functioning of gully control structures. Therefore, attention must always be given to keeping the gully catchment well vegetated. If this fails, the structural gully control measures will fail as well.
Masonry check dams are permanent structures used for both controlling gully erosion and water harvesting (Fig 4.8).
These masonry structures are popular in India. These structures are preferred at sites where velocity of runoff water flow in gullies is high and stable foundation considerations are encountered and where construction of embankment is costly and unstable.
These structures check the velocity of water flowing in gullies, affects deposition of flood load, decreases the erosive power of water, moderate bed slope and increase the contact time of water with land surface and thus increase the groundwater.
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