Soil Erosion Caused by Water

After reading this article you will learn about:- 1. Types of Soil Erosion Caused by Water 2. Factors Affecting Soil Erosion Caused By Water 3. Conservation Measures.

Types of Soil Erosion Caused by Water:

Soil erosion caused by water may be of the following kinds:

(i) Splash erosion:

The raindrops which fall at an approximate speed of 75 cms per second on ground strike it with a force of about 14 times its own weight.

The impact of this great force causes the soil particles to detach each other to form flowing mud. This mud is then splashed as much as 30 cms high and 12 cms down the land slope.

Fine sandy soil and salty soils are dislodged easily. Such a removal of the soil by the impact of the fast falling raindrops is known as splash erosion.

(ii) Sheet Erosion:

Thin uniform layers of soils are removed from the surface of the land by rain water flowing over the land during the initial stage of soil erosion called Sheet erosion when muddy water flows over the land.

(iii) Rill Erosion:

If sheet erosion is not controlled immediately, the down­ward flow of muddy rain water concentrates to form minute finger shaped channels called rills over the entire field during the later stage of soil erosion called rill erosion.

(iv) Gully Erosion:

If rill erosion is not controlled in time, then rills are deepened by the downward flow of water. Ultimately rills are converted to deep channels called gullies, which may be “U” or “V” shaped “U” shaped gullies are formed if the subsoil has a coarse texture whereas “V” gullies are formed if the sub-soil has a fine texture. Gullies become deeper and wider during heavy rains. Large fields are gradually fragmented and ultimately destroyed.

(v) Slip Erosion:

Large masses of rocks and soil materials are detached from the sides of mountains during the rainy season and are carried down by the action of gravity. This phenomenon is knows as slip erosion.

(vi) Stream Bank Erosion:

Rivers e.g. Kosi in Bihar frequently changes its course when one bank is cut and the soil materials are carried downstream. The finer soil particles are deposited along the other bank. This phenomenon is known as stream bank erosion.

Factors Affecting Soil Erosion Caused By Water:

1. Rainfall:

Intensity of rainfall is the volume of rainfall recorded per unit time. Whenever the intensity of rainfall exceeds the infiltration capacity of the soil, the excess rain water runs down the slope of the land and carries away considerable amounts of soil with it.

It has been observed on black soil. That rain water ran off the slope of the land when it rained, more than two inches per day at Manjri and Sholapur in Maharashtra and Bijapur in Karnataka. Rainfalls of between one to two inches per day and between half to one inch per day caused a run off in 80 and 40 per cent of the cases respectively.

2. Land Slope:

According to the law of falling bodies, if the vertical drop is increased four times, the velocity of run-off water is doubled and the erosive power of the run-off water is increased by four times, the amount of soil particles of a particular size that is carried away by run-off water, in increased by 32 times and the size of soil particles that are carried away by the run-off water increases 64 times.

It has been observed at the Deochanda Experiment station of the Damodar Valley Corporation that in red and yellow soils, the percentage of rainfall that runs off increases from 6 to 29 when the land slope is increased from 2 to 5 per cent.

The corresponding soil loss in tons per hectare has increased from 3.3 to 23.6. More plant nutrients are lost from lands when the soil loss is increased due to the increase in the degree of the slope of the land.

For example, it has been observed at Kanpur that when the slope of lands are increased from 0.5 to 3.0 per cent, the loss of organic matter, nitrogen, phosphoric acid, potash, lime and magnesia from the alluvial soils increases from 87,9,11,43,53 and 41 kilogram per hectare to 180,11, 23,118,203 and 212 kilogram per hectare respectively.

If the land slope length is increased, the velocity of run-off water gradually increases as the run-off water flows down the long slope. This means that the erosive power of the run-off water gradually increases. Hence more soil is lost from the lower portion of the land than the upper portion of the land.

3. Nature of Soil:

Water infiltrates into the soil and percolates down the soil through the micropore spaces, which are more in the sandy soil than in clayey soils. Hence less rain water would run off the sandy land soil than the clayey land soil.

Red loam, red and yellow soils are much more resistant to soil erosion than black soils because kaolinite and hydrous oxides of iron and aluminum dominate the clay mineralogy of the former soils whereas the smectite group of clay minerals dominates the clay mineralogy of the latter soils which expand and contract with the changed in soil moisture content. Consequently soil masses are easily dislodged and carried away by rain water.

4. Crop Management Practices:

The fast falling raindrops strike the bare surface of soil with great force to make a soil water suspension, which clogs the pore spaces of the surface soil and reduces the infiltration capacity of the surface soil. Hence more rain water runs off the sloping land if soil exposing crops like maize, are grown than if the soil covering crops like groundnut are grown.

For example, it has been observed that more soil is lost from the red and yellow soil of the Deochanda experiment station of the Damodar Valley Corporation if maize is cultivated (soil loss is 24 tons per hectare) than if groundnut is cultivated (soil loss is 14 tonnes per hectare).

Conservation Measures for Soil Erosion Caused by Water:

Soil conservation measures basically aim at increasing the infiltration of rain water in soils of fields and decreasing the run off and soil loss from fields. All agricultural lands must be used only for that purpose for which they are best suited, in order to get the maximum possible return from them without any physical damage or decrease in their productivity.

Agronomic soil conservation measures may be adopted to control soil erosion up to land capability class II beyond which Agronomic soil conservation measures must be supplemented with mechanical soil conservation measures.

1. Agronomic Soil Conservation Measures:

Agronomic soil conservation measures basically aim at keeping the land under vegetative cover at least during the period or season when the land most susceptible to soil erosion. The vegetative cover of the land protects it against the beating action of fast falling raindrops and provides organic matter to the soil that decomposes, and improves the physical condition of the soil.

The agronomic soil conservation measures include:

(i) Crop rotation,

(ii) Mulching,

(iii) Contour farming,

(iv) Strip cropping, and

(v) Growing grasses and forest trees.

(i) Crop Rotation:

It is the growing of a set of crops in a regular succession on the same field within a certain period. A good crop rotation comprises of a soil exposing crop like maize, a densely grown small grain like upland Paddy (Dular Paddy, Gora Paddy in Bihar), a spreading legumes like groundnut and a deep rooted crop like pigeon pea. In these ways, nutrients are removed by different crops from different depths and the land is kept covered.

(ii) Mulches:

Organic materials like leaves, straw etc. which have been left on the surface of the land to cover it are called mulches. They protect the surface of the land against the beating action of rain drops. Later they decay to form humus.

(Iii) Contour Farming:

All the points on a contour line are on the same elevation. Contour Farming means growing crops in lines parallel to contour lines. If reduces the speed of the rain water flowing down the soil and consequent soil loss.

(iv) Strip Cropping:

It means the growing of soil exposing crops like maize, and soil covering crops like groundnut in alternate patches or strips across the slope of the land. It is suitable for gently sloping land.

It may be of the following four kinds:

(a) Contour strip cropping: Strips of crops are parallel to the contour lines.

(b) Field strip cropping: Strips of crops are parallel to the general slope of the land.

(c) Buffer strip cropping: Severally eroded portions of land are permanently kept under grasses and contour strip cropping is practiced in the rest of the area.

(d) Wind strip cropping: Strips of crops are across the direction of the wind.

(v) Growing Of Grasses and Forest Trees:

Grasses completely cover the surface of the land. Their root system binds the soil particles to form soil aggregates. Hence the physical condition of the soil is improved, so more rain water infiltrates, into the soil and less rain water flows the slope.

Hence the rate of soil erosion is decreased. Grasses should be planted on the grasses waterway and terraces in order to stabilize them. Perennial grasses like Pennisetum pedicellatum and Pennisetum polystachyon and annual grasses like Panicum antidotale, Pennisetum purpurium. Chloris gayanan, Dicantheum nodosum, and Cenchrus ciliaris may be grown permanently on class V to class VI land, to reduce soil erosion from them.

2. Mechanical Soil Conservation Measures:

Mechanical control measures aim at dividing a long slope of the land into a series of shorter ones in order to reduce the velocity of runoff water.

Water is impounded over the land for long periods. Hence more water infiltrates in the soil and less water flows down the slope of the land at non erosive velocity.

The important mechanical soil conservation measures are as follows:

(i) Basin listing:

The land is ploughed to make furrows parallel to the contour lines. Than these furrows are closed at short intervals to make a series of shallow basins on the contour line. This process is called Basin Listing. It is effective on gently sloping land in low rainfall areas.

(ii) Sub-soiling:

The impermeable layers at a depth of 30 to 60 cms are broken at an interval of 90 to 180 cms apart with a sub-spoiler across the slope of the land in order to increase the permeability of the land.

(iii) Terracing:

Low earthen embankments which have been constructed across the slope of the land are called terraces.

These are of the following kinds:

(a) Channel Terrace:

Shallow channels are made across the slope of the land either exactly on the contour line or with a slight grade (0.1 to 0.2%). The excavated soil is placed on the lower side of the channel.

(b) Broad Based Terrace:

Wide based, very low earthen embankments are made on the contour lines by excavating the soil from both the sides of terraces. This is practiced in areas with a relatively low rainfall.

(c) Narrow Based Terraces:

Narrow based low earthen embankments are made across the slope of the land at suitable intervals in high rainfall areas. The spacing between Terraces may be calculated as follows:

Vertical Interval = 0.3 (S + 2) Where S = Slope percentage. A water way must be provided for terraced field so that the excess rain water in the field may drain at a non-erosive velocity. It should be designed by using following formulae: Q = CIA where C = Run off coefficient. I =Intensity of Rainfall. A= Catchment area. C is the ratio of the rates of run off and rainfall.

If the soil texture becomes finer or any impermeable layer is present in the soil profile, then the proportion of rainwater percolating down the soil profile decreases and therefore the value of the run off coefficient C increases. For example, the value of C is 0.29, 0.40 and 0.5 for sandy soils, loamy soils without any impermeable layer, and clayey soils with clay pan respectively, in the case of the cultivated land soil.

More rain water percolates down the profile of the cultivated land soils because excessive cultivation of the soil destroys the soils structure. Hence the value of C is less in grass land soil and in the forest land soils than the cultivated land soils.

For example its value for sandy soils, loamy soils without any impermeable layer and clayey soils or soils with clay pan is 0.15, 0.35 and 0.45 respectively in the case of grassland soils, and 0.10, 0.30 and 0.40 respectively in the case of the forest land soils.

The value of the intensity of rainfall “I” varies in different geographical locations.

I = KT^a/(t+b)ⁿ where T = return period of rain in years, usually ten and t = duration of rain in hours, i.e. time of concentration of the rain in the watershed.

The values of a, b, m and k have been calculated for the eastern (Orissa, Bihar, West Bengal and Assam), southern (Andhra Pradesh, Karnataka, Tamil Nadu and Kerala), western (Maharashtra and Gujarat), northern (Uttar Pradesh, Haryana, Punjab, Rajasthan, Kashmir and Himachal Pradesh) and central (Madhya Pradesh) zones of India as mentioned below.

The value of “K” is 0.6933,6.311,3.974,3.914 and 7.4645 for eastern, southern, western, northern and central zones of India respective­ly. The value of “a” is 0.1353, 0.1523, 0.1647, 0.1623, 0.1712 for eastern, southern, western, northern and central zones of India respectively.

The value of “b” is 0.50 for eastern, southern, and northern zones of India and 0.15 and 0.75 for western and central zones of India respectively. The value of “n” is 0.8801, 0.9465, 0.7327,1.0127 and 0.9599 for eastern, southern, western, northern and central zones of India “respectively”.

where L = maximum length of travel in metres and H is the difference in elevation between the most remote point and the outlet in metres.

The area of cross-section of the channel a is calculated by using the formula

Q = a V. Where V is the velocity in metres per second of the run-off water. It usually varies from 0.3 to 0.6 metres per second depending on the availability of the grass sod. E.g. 0.6 metres /sec if good grass is available and 0.3 metres/sec if the land is bare, calculating a, the channel is tentatively designed by choosing the arbitrary top width, bottom width and depth after calculating “a”.

For example, if a = 3 square feet, then top width, bottom width and depth of the trapezoidal channel would be 8,4 and 1/2 foot respectively, because

If the value of V thus calculated by using the Menning’s Formula, closely agrees with the value of V chosen from designing the channel, then the channel has been correctly designed, otherwise another value of V is to be chosen and the channel I to be reigned.

(d) Bench terraces:

They consist of a series of platforms with a suitable vertical drop along the contour lines. The vertical drop may vary from vary 2 to 6 ft. The soil materials that have been excavated from the top portion have been used to fill the lower portion.

The depth of the soil must be at least 15 cms deeper than the vertical interval to ensure sufficient soil depth on the inner side after leveling.

The vertical interval for the bench terrace is calculated by using the following formula:

VI = WS/100

VI = where W = width of the bench in metres and S = slope Percentage of the land.

The boundary of benches should be kept parallel to the contour line, to reduce the earthwork to the minimum. The earthwork in cubic metres per hectare is 12.50 WS for level bench terrace. It is 12.50 W (S-s) for outwardly sloping benches and 12.5 (S + s) for inwardly sloping benches where s is the inward or outward slope of finished benches.

(iv) Contour Trenching:

A series of Trenches two feet wide and one foot deep are excavated on the contour lines. The excavated soil is kept on the lower edge of the trenches where forest trees are planted.