Transcript Land and Water Degradation Due to Agriculture

Land Degradation
Due to Agriculture
Part 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 soil
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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
•
•
•
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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
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•
•
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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
• If it does, then as the raindrops hit bare soil, their kinetic
energy is able to detach and move soil particles short
distances
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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
• It moves slowly, with a low flow power, and is generally
incapable of detaching or transporting soil particles
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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
Kinderveld gully, central
Belgium. (Source of photo: KU
Leuven)
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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
Gully development in
Indiana, 1930’s
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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
Dust Storm in Senegal
• 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 century4040
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
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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/GRIDArendal, 1997
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