"The threat of nuclear weapons and man's ability to destroy the environment are really alarming. And

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Soil Erosion

• "The threat of nuclear weapons and man's ability
  to destroy the environment are really alarming.
  And yet there are other almost imperceptible
  changes - I am thinking of the exhaustion of our
  natural resources, and especially of soil erosion
  - and these are perhaps more dangerous still,
  because once we begin to feel their repercussions
  it will be too late." (p144 of The Dalai Lama's
  Little Book of Inner Peace 2002, Element Books,
  London)

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• Soil erosion is the 1 source of pollution to
  surface water in most states. Each year
  rainstorms and snowmelt wash tons of dirt off the
  land.
• How could something so natural be so bad? Soil
  erosion is natural after all. However, when we
  change the landscape from forest to yards,
  streets, farm fields, shopping centers and roads,
  we accelerate soil erosion.
• In the USA, soil is eroding at about seventeen
  times the rate at which it forms.

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Soil is naturally removed by the action of water
or wind such 'background' (or 'geological') soil
erosion has been occurring for millions of years.
In general, background erosion removes soil at
roughly the same rate it formed. But
'accelerated' soil erosion is a far more recent
problem that results from overgrazing or
unsuitable cultivation practices since the first
European settlers arrived. Accelerated soil
erosion by water or wind may affect both
agricultural areas and the natural environment,
and is one of the most widespread of today's
environmental problems. It has impacts which are
both on site and off site. More recently still,
the use of powerful agricultural implements has,
in some parts of the world, led to damaging
amounts of soil moving downslope merely under the
action of gravity this is so-called tillage
erosion.
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The GLASOD project has produced a world map of
human-induced soil degradation in three sheets at
an average scale of 110M (Mercator projection).
The map was digitized afterwards and stored in
GIS format with attribute database and
supplementary statistics on the extent and degree
of degradation
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Loss of soil usually means loss of topsoil
(first). Topsoil contains the most Organic
matter and nutrients, and has the most desirable
soil structure. Bad for the land and bad for the
final resting place. First concerns were
post-WWI. Soil Conservation Service was formed
in 1935 to study this problem. Annual loss can be
as high as 300 metric tons per hectare per year
(2.5 cm), which means the whole plow layer would
be lost in 6-7 years. (Geological erosion is
about 0.2-0.5 t/ha/yr.) Anything greater than 10
t/ha/yr is considered to be serious.
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Despite the global nature of the problem, we do
not have good information regarding the global
extent or severity of erosion by water. The
GLASOD study estimated that around 15 of Earth's
ice-free land surface is afflicted by land
degradation. Of this, accelerated soil erosion by
water is responsible for 56 (11 million square
km) and wind erosion - 28 (5.5 million square
km). The area affected by tillage erosion is
currently unknown. Because soil is formed
slowly, it is essentially a finite resource. The
severity of the global erosion problem is only
now becoming widely appreciated.
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• Erosion results in 
• higher project costs,
• (2) damage to aquatic habitat,
• (3) reduced water quality,
• (4) elimination of trout and salmon fisheries,
• (5) lower shorefront property values,
• (6) higher property taxes, and
• (7) loss of business and jobs.  Soil is a
  valuable resource on the land, but when,

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Environmental and Economic Costs of Soil Erosion
and Conservation Benefits excerpts from Science
Magazine Vol. 267, February 1995 In the article,
Pimentel et al say Soil erosion is a major
environmental threat to the sustainability and
productive capacity of agriculture. During the
last 40 years, nearly one-third of the worlds
arable land has been lost by erosion and
continues to be lost at a rate of more than 10
million hectares per year. With the addition of a
quarter of a million people each day, the world
population's food demand is increasing at a time
when per capita food productivity is beginning to
decline. "In the United States, an estimated 4
to 5 x 109 tons of soil and 130 x 109 tons of
water are lost from the 160 x 106 ha of cropland
each year. This translates into an on-site
economic loss of more than 27 billion each year,
of which 20 billion is for replacement of
nutrients (50) and 7 billion for lost water and
soil depth. The most significant component of
this cost is the loss of soil nutrients."
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Erosion processes

• Soil may be detached and moved by water, wind or
  tillage. These three processes, however, differ
  greatly in terms of
• where and when they occur
• what happens to the area that is being eroded
• how far the eroded soil is moved, and
• if the soil is moved away from the place where it
  was eroded, what happens as a result.

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Rain erosion

• Detachment
• Transport
• Deposition

Soil erosion by water is the result of rain
detaching and transporting vulnerable soil,
either directly by means of rain splash or
indirectly by rill and gully erosion
http//www.public.asu.edu/mschmeec/rainsplash.htm
l
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Rainsplash Rain may move soil directly
'rainsplash erosion'. The rain must fall with
sufficient intensity on bare soil to detach and
move soil particles a short distance. This is
solely an on-site effect. Because rainsplash
requires high rainfall intensities, it is most
effective under convective rainstorms in the
worlds equatorial regions. Rainsplash is
relatively ineffective where rain falls with low
intensity, such as in the north USA or in
northern Europe.
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The microtopography (i.e. small-scale pattern of
irregularities) of the soils surface tends to
cause this overland flow to concentrate in closed
depressions, which slowly fill this is known as
detention storage or ponding. Both the
flowing water, and the water in detention
storage, protect the soil from raindrop impact,
so that rainsplash redistribution usually
decreases over time within a storm, as the depth
of surface water increases. There are, however,
complex interactions between rainsplash and
overland flow.
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Erosion is a process that involves the detachment
of soil particles from within the soil surface
FOLLOWED by the transport of these detached
particles away from the site of detachment.
Detachment versus transport (or actual loss) Well
aggregated soil versus non-coherent sand
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That fraction of the rainfall which does not
infiltrate the soil will flow downhill under the
action of gravity it is then known as runoff or
overland flow. Runoff may occur for two reasons.
First, if rain arrives too quickly (i.e. with too
high an intensity) for it to infiltrate. Second,
runoff may occur if the soil has already absorbed
all the water it can hold (i.e. because it is
fully saturated, or frozen). As runoff moves
downhill, it is at first a thin diffuse film of
water which has lost virtually all the kinetic
energy which it possessed as falling rain. Thus
it moves only slowly, has a low flow power, and
is generally incapable of detaching or
transporting soil particles.
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You need runoff to have transport. Rainfall must
exceed the infiltration capacity of the soil.
Sheet erosion
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Rill and gully erosion
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These check dams are inadequate to prevent soil
from leaving the site.   The hay bales are not
wide enough across the swale  water and silt are
flowing around the bales.   There is no mulch,
although a fairly broad area of ground is open. 
However, on the bright side, the swale itself
looks pretty good -- it is broad and flat,
allowing runoff to move slowly.   Water that
moves slowly is less likely to erode soil.
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Rills form when the concentration and velocity of
water running over ground exceeds what the
stabilization method is able to support.    Rills
can also form when groundwater leaches out of
slopes. This haybale check dam is ineffective
because it doesn't extend across the entire ditch
line.  Water can run around it. 
Though mulching has been done, providing a
vegetative cover would have better prevented
erosion.
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Universal Soil Loss Equation

• There are five factors of rainfall erosion
• Nature of the rainfall (frequency, intensity,
  seasonality)
• Soil characteristics (infiltration,
  susceptibility to detachment and transport)
• Steepness and length of slope
• Cover
• Soil Management Practices

A R x K x SL x C x P
A is the predicted average soil loss in metric
tons/ha/yr
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R is based on the number of heavy rains per year,
including the total energy of the storm (size of
raindrops, number of raindrops and total amount
of water) and the maximum 30 minute intensity.
Only storms with 1.25 cm.
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Slope Factor
Erosion increases as the length and/or steepness
of the slope increases by affecting the volume
and velocity of water flow. The slope determines
the total area for erosion. As speed ?,
infiltration ?, runoff ?, and velocity ?. If
velocity ?x2, water can move particles 64 x
larger and can carry 32x more in suspension.
Erosive power is 4x greater. As length ?,
concentration of water ?. If length of slope ?x2,
soil loss ? x2.6, and runoff ? x1.8
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Unity plot 22 meters long, 9 slope. Assume
fallow conditions.
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Soil Erodibility Factor, K

• Texture
• Structure
• Organic Matter content
• Subsoil conditions (affects internal drainage)
• expressed as t/ha per unit of rainfall erosion
  index.
• measured on Unity plots under clean till fallow
  conditions.

Inherent ability of the soil to erode.
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• Can be estimated using information in the Soil
  Surveys
• silt and very fine sand (0.002 0.1 mm)
• sand, excluding very fine (0.1 2.0 mm)
• OM
• Structural class
• Permeability class

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K Factor Data ( Organic Matter Content)

etc.
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Crop Type Factor
C The crop/vegetation and management factor is
used to determine the relative effectiveness of
soil and crop management systems in terms of
preventing soil loss. The C factor is a ratio
comparing the soil loss from land under a
specific crop and management system to the
corresponding loss from continuously fallow and
tilled land.

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Tillage Method Factor

• Amount and duration of cover they provide
• Quantity and type of residue left on the field
• Nature of tillage practices

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P - Management Practices Factor
P reflects the effects of practices that will
reduce the amount and rate of the water runoff
and thus reduce the amount of erosion. The P
factor represents the ratio of soil loss by a
support practice to that of straight-row farming
up and down the slope. The most commonly used
supporting cropland practices are cross slope
cultivation, contour farming and stripcropping.
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Table 5. P Factor Data

http//www.iwr.msu.edu/cgi-bin/rusle/rusle_constr.
cgi
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Management Strategies to Reduce Soil Losses

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Coutour tillage
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Contour strip cropping
P ½ P for contour tillage alone.
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Terracing
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Minimum tillage
Zero tillage
We used to plow the field to incorporate old
plants, then disk the field to break up the clods.
Strip plant
Seed drill
Must use pesticides
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R, SL and K are inherent properties of your
location. e.g. R 100, SL 1 and K 0.6
(A 60 metric
tons/ha). We need to adjust C and P to reduce A
to
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If you still choose to plant potatoes following
row crops with fall plowing, you need to consider
your management practices. Contour tillage P
0.6 (A 17 t/ha) This is still not
acceptable.
Contour strip cropping P ½ P for contour
tillage alone. Therefore, A 8.5 t/ha. This is
just barely acceptable and does not address the
recommended T value.