Soil erosion: sheet, gully and wind

Detail

What soil erosion is

• Soil erosion is when the soil cover is worn away (denudation) and then washed or blown away.
• A natural balance:
• Rocks slowly break down and make new soil (soil formation).
• Water, wind and ice slowly carry soil away (erosion).

• In nature these two processes normally balance each other.

• Natural agents of erosion: running water, wind and glaciers (slow-moving masses of ice).

• Human activities upset the balance:
• Deforestation: without roots to hold it, the soil is left bare.
• Overgrazing: animals eat the grass cover down to the ground.
• Construction and mining: these dig up and loosen the soil.

• Result: soil is lost faster than it can form. Soil takes hundreds of years to form, so a loss like this is effectively permanent.

• Faulty farming: ploughing up and down a slope cuts furrows that run downhill. These furrows act as channels, so rainwater runs off fast and carries the topsoil with it. The fix is contour ploughing (ploughing across the slope, along lines of equal height), which is covered under conservation.

• Class 11 NCERT lists soil erosion as a terrestrial hazard (a hazard that happens on land, Table 6.1). It sits alongside landslides and earthquakes, though it works slowly.

Types of erosion by water and wind

Stages of water erosion (sheet → rill → gully):

• Sheet erosion:
• Rain falls on a gentle slope and the water spreads out as a thin, even sheet.
• The sheet flows downhill over a large area.
• It removes a thin layer of topsoil (the fertile upper layer, rich in humus, the decayed plant and animal matter).

• It is hard to notice because no channel forms. This is why it is called the "silent" danger: the land just becomes less fertile year by year.

• Rill erosion:

• The sheet of water gathers into many small, shallow channels called rills, a few centimetres deep.
• Rills can be removed by normal ploughing.

• This is the stage between sheet and gully erosion.

• Gully erosion:

• Rills grow deeper and wider and join together.
• Running water cuts gullies (deep, steep-sided channels that a plough cannot cross).
• It is most common in clayey soils. These soils crack when dry and their sides collapse when wet.
• Result: the land becomes too cut up to farm. It is then called bad land.

Wind erosion:

• Wind lifts and blows loose, dry soil off flat or sloping land.
• Conditions that favour it: dry climate, sandy soil, little plant cover and strong winds.
• The soil moves in three ways:
• Suspension: fine dust is carried high in the air.
• Saltation: sand grains bounce along the ground.

• Surface creep: bigger grains roll along the surface.

• Where it happens in India: western Rajasthan (the Thar Desert) and Kachchh (Gujarat).

• The strongest winds blow in the hot, dry pre-monsoon months of April to June, which is when dust storms are common.

Type	Agent	Form	Typical soil or area in India	Can farming continue?
Sheet	Rainwater flowing as a thin sheet	Even loss of topsoil, no channels	Gentle slopes, Deccan and plateau farmlands	Yes, but fertility falls
Rill	Water gathered into small channels	Shallow channels a few cm deep	Ploughed slopes	Yes; ploughing removes rills
Gully	Concentrated running water	Deep channels, then badlands or ravines	Clayey and alluvial soils: Chambal, Yamuna, Mahi, Sabarmati	No; the land becomes bad land
Wind	Strong, dry wind	Deflation (soil blown away), sand drift and dunes	Western Rajasthan, Kachchh	Very limited

Badlands and ravines: the Chambal belt

• Badlands: land so cut up by gullies that it cannot be farmed.
• Ravines: the local name for these gullied badlands in the Chambal basin.
• Chambal river, to anchor the region on a map:
• Origin: the Janapao hills near Mhow in the Vindhya Range (Madhya Pradesh).
• Course: north, then north-east, through Rajasthan (past Kota). It forms part of the Madhya Pradesh–Rajasthan border.
• Mouth: it joins the Yamuna in Etawah district (Uttar Pradesh), so it is part of the Ganga system.

• Tributaries: Banas, Kali Sindh and Parbati.

• Three states hold the ravines: Madhya Pradesh (Morena, Bhind), Rajasthan (Dholpur) and Uttar Pradesh (Etawah, Agra).

• Why ravines form here:
• The soil is deep, soft alluvium that runs off easily.
• The river has cut down deep below the land around it.
• Heavy monsoon bursts fall on land with little plant cover.

• Result: gullies cut back headward (they grow at the top end, eating into the land). The farmland next to the ravines keeps shrinking.

• Other ravine belts: along the Yamuna, Mahi and Sabarmati (Gujarat).

• How big they are: ravines are widespread along the Yamuna and Chambal. FAO's 1994 South Asia land degradation assessment puts India's ravine land at 3.13–3.17 million ha [2].
• Human and ecological side:
• The maze of ravines long gave hiding places to dacoits (armed bandit gangs, locally called baghis).
• The same remote stretch now forms the National Chambal Sanctuary (set up in 1979). It covers Madhya Pradesh, Rajasthan and Uttar Pradesh and protects the gharial and the Gangetic river dolphin.

Water erosion hotspots beyond the ravines

• Himalayan and north-eastern slopes:
• Steep slopes, heavy rain and jhum (shifting cultivation: forest is cut and burnt, crops are grown for a few years, then the farmer moves on).

• The jhum cycle is getting shorter, so the bare soil does not get time to recover. The result is heavy sheet and gully erosion in Nagaland, Mizoram, Meghalaya and other north-eastern states.

• Western Ghats:

• Laterite soils (red, leached soils of high-rainfall areas) on steep slopes.

• Under 200–300 cm or more of monsoon rain, these soils lose their topsoil fast.

• Shivalik foothills (Punjab and Himachal Pradesh):

• Chos are seasonal torrents (streams that flow fast only after heavy rain) coming down from the bare Shivalik hills, for example in Hoshiarpur (Punjab).
• They cut gullies and spread sand over the farmland below.

How big the problem is (national data)

• Soil loss:
• Gross soil erosion by water in India is put at 5.1 ± 0.4 billion tonnes a year, about 15.6 tonnes per hectare a year (ICAR estimate, reported to Parliament in 2022) [3].
• An earlier government figure (2016) was 5,334 million tonnes a year, about 16.4 tonnes per hectare [4].

• (NCERT: India loses billions of tonnes of soil a year to water erosion.)

• Land degradation:

• The Desertification and Land Degradation Atlas of India is made by the Space Applications Centre (SAC), ISRO, Ahmedabad, from satellite data. It was released on 17 June 2021 [5].
• It found 97.84 million ha under land degradation and desertification in 2018-19, nearly 30% of India's total geographical area [5][6].
• The figure was 96.4 million ha (29.32%) in 2011-13 [5].
• The atlas also tracks change over 15 years, from 2003-05 to 2018-19 [5].

• Water erosion is the single largest process of land degradation in India. Wind erosion, salinisation and waterlogging follow [5][6].

• Area under erosion: FAO's 1994 assessment found about 144 million ha of Indian land hit by water or wind erosion [2].

Prelims Hooks

• Sequence of water erosion: sheet → rill → gully → badland or ravine.
• Sheet erosion removes topsoil evenly and leaves no channels. Gully erosion cuts deep channels, mostly in clayey soils.
• Ravines are the local name for gullied badlands in the Chambal basin, which is spread over Madhya Pradesh, Rajasthan and Uttar Pradesh.
• The Chambal rises in the Vindhya Range near Mhow (MP) and joins the Yamuna in Etawah (UP). Its tributaries are the Banas, Kali Sindh and Parbati.
• Ravines are also found along the Yamuna, Mahi and Sabarmati. They cover about 3.13–3.17 million ha in India (FAO, 1994) [2].
• Wind erosion hotspots: western Rajasthan and Kachchh. Chos erosion: the Shivalik foothills of Punjab and Himachal Pradesh.
• Ploughing up and down a slope speeds up erosion. Contour ploughing (across the slope) reduces it.
• The Desertification and Land Degradation Atlas is made by SAC-ISRO, Ahmedabad. It reports 97.84 million ha degraded in 2018-19 [5][6].
• Water erosion, not wind erosion, is the leading cause of land degradation in India [5].
• Trap: soil erosion is listed in NCERT Class 11 as a terrestrial (land-based) hazard. It is not counted as an atmospheric hazard, even though wind is one of its agents.

Mains Points

• Causes are both natural and man-made:
• Natural: monsoon downpours, steep Himalayan and Ghat slopes, dry winds in the Thar.
• Human: deforestation, overgrazing, mining, construction, a shorter jhum cycle and ploughing up and down slopes.

• Human pressure has broken the natural balance between soil formation and soil loss. (GS-I, GS-III)

• Effects are spread across the country:

• About 5.1 billion tonnes of soil is lost a year (ICAR, 2022) [3]. This lowers crop yields.
• The lost soil silts up reservoirs and rivers, which cuts their storage and raises flood risk.
• Ravines take land out of farming in the Chambal–Yamuna belt.

• Desert sand spreads in western Rajasthan and Kachchh.

• Regional pattern:

• Gully and ravine erosion: in the alluvial lands of central India.
• Sheet and gully erosion under jhum: in the North-East.
• Laterite topsoil loss: in the Western Ghats.
• Chos: in the Shivaliks.
• Wind erosion: in the arid west.

• Each region therefore needs its own conservation package, such as ravine reclamation, terracing, shelter belts and contour bunding.

• Policy link:

• SAC-ISRO's satellite-based atlas (2003-05 to 2018-19) gives the baseline for watershed programmes [5].
• It also feeds India's commitments under the UNCCD (UN Convention to Combat Desertification) [5][6].
• The Soil Health Card scheme (2015) tackles the fertility loss that follows topsoil erosion.