soil erosion

Accelerated Soil erosion

Agricultural productivity depends largely on the topsoil (up to

20 cm thickness), as it serves many functions such as – support for

rooting, supply of plant nutrients, storage and release of soil moisture etc.

The loss of the topsoil usually instigated by the activities of man, termed

as accelerated soil erosion, is the most serious form of land degradation.

Some erosion, however, takes place through natural processes slowly but

some natural processes of 11decay and regeneration replace this loss. A

balance is maintained between soil loss and soil formation, but when

vegetation is stripped away, as in deforestation and shifting cultivation,

the soil becomes vulnerable to damage by wind and water and the slow

rate of natural erosion is accelerated. (Under accelerated erosion, soil

loss takes place much faster than new soil can be formed as a result soil

productivity goes down (Table 2.3).

  India is one of the few countries in the World, which has an audit

of land degradation due to soil erosion; soil erosion estimates in India are

given below in the Table 2.4.

 Soil Erosion estimates in India

Parameter Erosion

(million tonnes)

Total soil loss 5334 100

Transported from one place  3282 61

Deposited in the reservoirs 480 10

Lost into the sea 1572 29

Per cent

  The analyses of the existing soil loss date indicate that soil

erosion takes place at an average rate of 16.35 tonnes/ha/year totaling

5.334 million tones/year. Nearly 29% of the total eroded soil was

permanently lost to the sea; and nearly 10% was deposited in

multipurpose reservoirs, resulting in the reduction of their storage

capacity by 1-2% annually. The remaining 61% of the eroded soil was

transported from one place to another. Along with the eroded topsoil,

three major plant nutrients viz., Nitrogen, phosphorus, and potassium

ranging from 4.4 to 8.4 tonnes are also lost. Crop yields are reduced by

erosion through less supply of plant nutrients, shallow depth of soil, poor

structure, surface sealing and crusting, which leads to reduced water

infiltration and poor seedling establishment , In addition to this on-site

effects, the off-site effects of erosion are also of serious concern for the

sustainability of Agricultural productivity.

Siltataion of Reservoirs

Siltation of reservoirs is the major off-site effect of soil erosion

by water. The process of deposition of soil particles carried by water is

called sedimentation or siltation. Sedimentation or siltation is both a

serious and growing problem, but its severity varies from one reservoir to

another. Usually, every reservoir is provided with certain storage

capacity to accommodate for the natural sedimentation rate, which

capacity is called its dead storage. Sediment accumulation is not

controlled in most of the reservoirs and they are irreversibly getting filled

with sediment and constitute the most non-sustainable water resource

system in India today. 

 Soil loss in different catchments of India

Catchment Loss of soil (t/ha/year)

Snow clad deserts 5.0

Shiwalik hills 80.0

Ravines of Chambal and other river 4.0

Western Ghats 20.0-30.0

Shifting cultivation of N.E. states 740.0

 Source: Kanwar(2000)

  The life of many irrigation reservoirs has been reduced by

accelerated soil erosion in the catchment’s area (Table 2.5) and increased

the rates of siltation more than the assumed rates at the of planning stage

(Kanwar,2000). These increased rates of siltation are adversely affecting

capacity of the reservoirs to sustain the gains in productivity achieved

over the past decades. In addition to the drastic reduction in life of the

projects involving huge investments, these accelerated rates of siltation

of reservoirs and other water storage sites are causing floods that effect

life and property quite often. The area  annually are causing floods that

effect life and property quite often. The area annually flooded, including

cropland, has increased from an average of 6.86 m ha in fifties to 16.57

m ha in the eighties (Table 2.7). Published data on 116 large dams

showed that by the year 2020 over 20% of India’s reservoirs will lose

50% of their storage capacity due to fast rates of silting up.

 Trends in flood havoc in India

Decade Av.Annual

area

Affected(m ha)

Av. Annual flood

Affected population

Av. Annual total

flood damage

(m Rs)

1950s 6.86 2.08 923

1960s 5.86 2.47 1041

1970s 11.19 5.55 6741

1980s 16.57 6.91 15904

Source: Centre for Science and Environment, 1991

 Efficient reservoir maintenance calls for thorough survey of the

factors affecting sediment load from the catchment’s area and to take up

measures to prevent sedimentation. Soil type and degree of vegetative

cover in the catchment’s area mostly influence the rate of siltation

(Table2.8). The rate of sedimentation in some reservoirs of Andhra

Pradesh is less if forest  cover mostly covers the catchment’s area. In

general, the erosion hazard of cropland is estimated to increase sharply

from class I through class IV soils.

 Sedimentation rate of reservoirs as influenced by

soil type in the catchments

Name of the

reservoir

Sedimentation rate

(ha-m/100 km

2

/ year

Area covered

(1% of the catchment)

  Forest Clay soil Red soil

Kalyani 0.57 98 1 1

Kinnerasani 0.62 90 2 3

Wyra 4.51 30 10 60

Osman sagar 5.02 20 10 70

Kotapally 5.43 30 5 65

Nizam sagar 6.67 22 57 21

Pampa 6.70 24 57 19

Himayatsagar 6.94 20 57 23

Upper Manair 8.53 8 80 12

Sriramsagar 9.07 3 94 1

   The conditions indicative of high sediment yield potential from

cropland and other sources in the catchment’s area are given below:

 Cropland

• Long slopes farmed without terraces or run-off diversions

• Crop rows planted up and down on moderate or steep slopes

• No crop residues on soil surface after seeding a new crop

• No crop cover between harvest and establishment of new crop

canopy

•  Poor crop stands or poor quality of vegetation

 Other sources

• Gullies

• Residential or commercial constructions

• Road or railway tract construction

• Poorly managed range, wastelands or wooded lands

• Un-stabilized road or railway tract banks

• Surface mining areas, etc

Usually, it is better to take steps that prevent sedimentation, either

engineering or agronomic, rather than clearing the sediments

mechanically, which is not only very expensive but also highly

impractical. Some engineering and agronomic measures suggested for

prevention of salutation reservoirs are given below:

 Engineering Measures

• Provision of scour sluices in the body of dam and whose

opening from to time clear of the sediments

• Construction of small impounding tanks in the valley upstream

to break up water flow rate and shared silt load before the flow

reaches final storage

Agronomic Measures

• Cover crops

• Afforestation

• Pastures

• Contour cultivation

• Contour bunding

• Live bunding

• Grassed waterways

 Effects- Social, economic and crop production

Although India has successfully achieved self-sufficiency in food

grain production, the problem of resource degradation poses a serious

threat and challenge to our ability to do so in future. The tragedy of the

depletion of our natural resource and its impact can be shown below

(Fig. 2.8). Although precise and quantitative estimates of the impacts of

degradation are lacking, there are several pointers to the overall effects.

Population 

growth

Resource 

Degradation

Poverty

Reduced 

returns / unit 

***