Land Degradation Due to AgriculturePart 2:Soil Erosion

Fall 2012 , Lecture 6

Regolith• Rocks are broken into smaller and smaller

pieces by physical weathering – processes like the expansion of water when it freezes in cracks can break rock

• Regolith is composed of smaller and smaller pieces over time, but it is not soil

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Soil• Soil is regolith which has been altered by

weathering, and which may have had organic matter added to it

• Most of the weathering that creates soil is chemical or biochemical weathering

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Chemical Weathering• May involve a number of processes

Oxidation of minerals, which changes the chemical state of substances in the minerals and often affects solubility, making products more or less soluble

Hydration, the addition of water to minerals Conversion of common silicate minerals to clays,

which are important components of soil4

Clay Minerals• Clays have the ability to absorb, retain, and

later release substances important for plant growth Water is held and later released Nutrients, including nitrogen and phosphorous

compounds, which are important for plant growth

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Importance of Soil• Soil is essential to life as we know it, since

plants need soil to grow, and animals ultimately depend on plants for food

• The type of soil formed is dependent on the type of parent rock

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Parent Rock• The parent rock is the rock from which regolith is

derived

• Depending on their resistance to physical and chemical weathering, different parent rocks may form soil quickly or slowly

• The parent rock also determines how rich in nutrients the soil is

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Factors Influencing Soil Formation• Climate

• Topography

• Vegetation

• Time

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Climate• Warm, wet climates speed up chemical

weathering greatly In temperate regions, there is a balance between

development of soil, and chemical dissolution, or “leaching”

• In very cold climates, soil formation becomes very slow

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Leaching• Leaching may cause the loss of water-soluble plant

nutrients from the soil

• It can also help to avoid salt build-up in soils Excess salinity can greatly reduce plant growth, and may

eventually prevent certain types of plants from growing Improper irrigation can cause salt build-up

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Topography

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Vegetation• Plant roots are extremely important in

retention of soil

• Without vegetation, soil erosion becomes a very serious problem quickly

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Early Soil• Life on earth began in the oceans, at least 3.5

billion years ago

• Around 450 million years ago, the first land plants appeared

• Once land plants were present, soils developed and were retained

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Background Erosion• Prior to the evolution of terrestrial plants, soil

was removed at roughly the rate it formed, by the action of wind, water, and occasionally glacial ice

• This erosion rate is often called background erosion

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“Accelerated” Soil Erosion• When we speak of soil erosion today, we are

usually describing removal of soil at rates far faster than it is formed

• This is a recent problem in earth’s history

• It is almost always a result of mankind's unwise actions

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Man’s Actions • Overgrazing by animals that kills vegetation

• Unsuitable cultivation practices

• These leave the land unprotected and vulnerable

• During times of erosive rainfall or windstorms, soil may be detached, transported, and deposited, possibly after travelling a considerable distance

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Erosive Rainfall• Rain may move soil directly,

in a process known as splash erosion

• Splash is only effective if the rain falls with sufficient intensity

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• If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles short distances

Splash Erosion• Soil particles can only be moved a few centimeters so

the effects are solely on-site

• Considerable quantities of soil may be moved by rainsplash, but it is all redistributed back over the surface of the soil

• Thus a more descriptive term might be splash redistribution

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Topography Effects• The effects of topography can amplify the

process

• On steep slopes there will be a modest net downslope movement of splashed soil

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Runoff versus Infiltration• When rain strikes the soil, one of two things happens

It sinks into the soil, in a process known as infiltration Due of gravity, the fraction which does not infiltrate flows

downslope on the surface, in a process called runoff Runoff may occur because rainfall arrives faster than the

ground can absorb it, or because the near surface part of the ground is already saturated with water

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Rill and Gully Erosion• Runoff may occur in rills, small channels, or

gullies, which are larger channels, usually too big to be removed by tillage

• Rill and gully erosion is the dominant form of water erosion in many parts of the world

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Diffuse Flow• At first, runoff is a thin

diffuse film of water

• It has lost virtually all the kinetic energy which it possessed as falling rain

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• It moves slowly, with a low flow power, and is generally incapable of detaching or transporting soil particles

Development of Surface Runoff• If rain continues, the increasing depth of water will

eventually overtop microtopographic depressions and overland flow that is released in this way is likely to flow downhill more quickly and in greater quantities

• It will possess more flow power as a result of its kinetic energy

• Eventually, it will be able to begin transporting and even detaching soil particles

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Rill Development• Over time, microrills develop

• Some will fill due to deposition

• Others will develop and become rills

• Rills that intersect will grow larger, and eventually may flow into gullies

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Gully Formation• Rills have developed

and started to form gullies

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Thawing Snow• Meltwater from thawing snow operates in a broadly

similar way to rain-derived overland flow, detaching and transporting unfrozen soil in areas of concentrated flow

• This process has been less studied than rainfall erosion

• Even less studied is runoff from melting glaciers, where the amounts of runoff may be considerable

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Sediment Movement• As erosional channels increase in size, processes such as

gravitational collapse of channel walls increase in importance

• Runoff and sediment from rills and gullies may be moved into ditches, stream and rivers, and so transported well away from the point of origin

• However, sediment may also be deposited within the rill or gully, or beyond the rill or gully’s confines in a depositional fan, at locations where the gradient slackens

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Transport to Lake or Ocean• It may be stored for a variable period of time,

possibly being reworked by tillage activity, until a subsequent erosion event is of sufficient size to re-erode the stored sediment

• It may then be redeposited further downstream, or make its way into a permanent watercourse and thence to lake or ocean.

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Evidence for Accelerated Erosion• Archaeological

evidence from many parts of the world that accelerated erosion by water, and sometimes by wind, is often associated with early agriculture

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Kinderveld gully, central Belgium. (Source of photo: KU Leuven)

Scientific Observation• Scientifically, water erosion’s association with

unwise agricultural practices was first noted within during the early decades of the 20th century

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Imposed Agricultural Practices• During periods of colonialism, imposed

adoption of European agricultural methods frequently leads to accelerated erosion in developing countries

• This problem continues to the present day in many developing countries

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Intensive Agriculture

• Worldwide movement towards intensive agricultural technologies during the last few decades of the 20th century have frequently left the soil bare during times of heavy rainfall

• As a result, previously problem-free areas of the world, such as north-west Europe and parts of North America, began to experience notable increases in water erosion

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Gully development in Indiana, 1930’s

Erosion Scales• Soil erosion is affected by both common and rare events, and

so must be studied over both short and long timespans

• Erosion is also affected by factors on very small and very large spatial scales, and has its impacts over a similarly wide range of spatial scales

• These wide temporal and spatial variations makes soil erosion difficult to understand, to predict, and to control

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Erosion Timescales• Soil erosion occurs:

Incrementally, as a result of many small rainfall or wind-blow events

Dramatically, as a result of large but relatively rare storms

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Large vs. Small• It is the large storms which produce the big hard-to-miss

erosional features such as deep gullies

• Erosion due to small common events may appear insignificant on the field, the cumulative impact may, over a long timescale, be severe

• This is true in different spatial regimes• The eroding field

• And elsewhere

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Spatial Scales of Erosion• Splash redistribution and the initiation of microrills and rills

occur at a scale of millimeters

• Rill erosion on agricultural hillslopes operates at a scale of meters to tens of meters

• Gully erosion can occur on a scale of hundreds of meters to kilometers

• The offsite impacts of erosion can affect very large areas, hundreds or even thousands of square kilometers

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Patchiness of Erosion• Erosion is highly patchy at every spatial scale

The vagaries of topography and land use concentrate erosive flows on a wide range of spatial scales, even in areas of severe erosion

Obvious erosion in one field can be found side-by-side with virtually untouched areas

Within an eroded field, the severity of erosion can vary markedly

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Aeolian Erosion• Wind erodes the Earth's surface

Deflation - removal of small, loose, particles Turbulent eddies – swirling winds that temporarily suspend

and move larger particles, such as sand grains Abrasion - wearing down of surfaces by the grinding action

and sandblasting of windborne particles

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Arid Regions• Aeolian erosion is most effective in arid regions

Lack of moisture means that water is not available to act as a glue, holding sediment together

Lack of vegetation means there are few roots to hold sediment

Therefore, the wind becomes a very important agent of erosion

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Deflation and Desert Pavement• Winds blowing across

dry, treeless expanses lift and remove clay, silt and fine sand (dust storms)

• Landscape level is lowered, at rates of centimeters per century

Dust Storm in Senegal

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Initial Aeolian Erosion

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Desert Pavement Development

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Final Stage of Desert Pavement

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Desert Pavement• Desert Pavement is

formed when winds remove the sand and smaller particles, leaving gravel behind, via deflation

• Agriculture becomes impossible

Close up of desert pavement, Russell Spit, Nevada

Erosion Control on Farms

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Impact of Soil Erosion on Food• A 2006 study at Cornell found that, around the world,

soil is being swept and washed away 10 (in the United States) to 30-40 (in China and India) times faster than it is being replenished, destroying cropland the size of Indiana every year

• 99.7% of human food comes from cropland, which is shrinking by more than 10 million hectares (almost 37,000 square miles) a year due to soil erosion

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Second Biggest Environmental Problem?

• David Pimentel, retired professor of ecology at Cornell, said "Soil erosion is second only to population growth as the biggest environmental problem the world faces. Yet, the problem, which is growing ever more critical, is being ignored because who gets excited about dirt?"

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Economic Impact• Economic impact of soil erosion in the United States

costs the nation about $37.6 billion each year in productivity losses (2006)

• Soil erosion losses worldwide are estimated to be $400 billion per year

• As a result of erosion over the past 40 years, 30 percent of the world's arable land has become unproductive

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Worldwide Soil Loss $ Calculation• 50 billion tons of soil eroded worldwide each year

from agriculture

• Costs: $3 per ton of soil for nutrients $2 per ton for water loss $3 per ton for off-site impacts

• $8 per ton x 50 billion tons = $400 billion

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Comparison• Insured catastrophe losses in the United States

for 2011 totaled $35.9 billion, greatly surpassing the average of $23.8 billion for the years 2000 to 2010 (from Insurance Information Institute)

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Related Effects• About 60 percent of soil that is washed away ends up in

rivers, streams and lakes, making waterways more prone to flooding and to contamination from soil's fertilizers and pesticides

• Erosion increases the amount of dust carried by wind, which not only acts as an abrasive and air pollutant but also carries about 20 human infectious disease organisms, including anthrax and tuberculosis.

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Agricultural Cultivation Effect• Sediment from soil erosion is the single greatest

pollutant of the world's oceans, lakes and rivers

• Before intensive agricultural cultivation began, approximately 9 billion tons of topsoil was carried into our waterways annually through runoff

• Today the volume has tripled, exceeding 27 billion tons every year, and continues to increase

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Soil Erosion Map

• From the U.S. Department of Agriculture

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World Soil Degradation

Map• Author:  Philippe

Rekacewicz, UNEP/GRID-Arendal, 1997

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