Topic 8: Soil Erosion Flashcards
What is soil erosion?
A natural process where soil particles are removed by wind or water
What is the rate of soil erosion like in sois that haven’t been affected by human activity?
The same as the rate of soil formation
When is soil erosion an issue?
When the rate is faster than that of soil formation
Name the 2 types of soil erosion
Wind erosion
Water erosion
When is soil most vulnerable to wind erosion?
When it is a dry soil or has a low clay content
This is because they are likely to be loose and there is little cohesion between the particles that would hold the soil together
So the soil blows away
Where is wind erosion a problem?
In the area where the soil has been removed
In the area where the soil is deposited- it may cover crops or land in urban areas
Name the types of water erosion
Rain splash erosion
Surface runoff erosion
Slumping and landslides
What increases all forms of water erosion?
Steeper gradients
Rain splash erosion
When soil particles are dislodged by a rain droplet
Soil particles are dispersed in different directions
Those going downhill are more likely to go further
Surface runoff erosion
Caused when the infiltration capacity of the soil has been exceeded
Can occur in heavy or prolonged rainfall or if soil is impermeable
Leads water to flow over the ground surface
Slumping and landslides
When soil on slopes becomes wet, increased mass and lubrication of water makes it more likely that large amounts of soil will move downwards
Often occurs when deep soil on slopes becomes less stable because of deforestation
Why are landslides more common because of deforestation?
The roots of trees that held the soil together decompose
The soil isnt held together as strong
Landslides are more likely after heavy rain
What areas are susceptible to both types of soil erosion?
Areas with seasonal rainfall, like Sub-Saharan Africa
Why may areas like sub-Saharan Africa suffer from both types of soil erosion?
Wind erosion in the dry season
Rainfall erosion in the wet season
How does vegetation reduce the rate of soil erosion?
-it acts as a natural windbreak, reduces wind velocity/kinetic energy that could carry away soil particles
-that and leaf litter reduce the impact of raindrops-soil particles are less likely to be dislodged
-soil organic matter helps to bind soil particles together
-plant roots hold the soil together
-plants held infiltration of water into soil high reduces the rate of runoff, reducing water erosion
What human activities increase the rate of soil erosion?
Vegetation removal
Ploughing vulnerable soils
Overgrazing
Reducing soil biota
Soil compaction
Cultivating steep slopes
How can vegetation removal increase soil erosion?
Removes protection from erosion
How can ploughing vulnerable soils increase soil erosion?
It breaks up soil structure
Soil particle are exposed to erosion as a result of
how can overgrazing increase soil erosion?
Vegetation eaten faster than it can grow
Soil is exposed to erosion
Disturbance & root damage by hooves can increase erosion risk
How can reducing soil biota increase soil erosion?
Less dead organic matter is broken down
How does soil compaction increase soil erosion?
Use of heavy machinery
Reduction in soil detrivores
Makes smaller interstitial spaces which reduces the infiltration rate
How does cultivating steep slopes increase soil erosion?
Surface runoff water flows more rapidly down steeper slopes
More kinetic energy to pick up more soil particles
Ploughing e.g. adds to erosion risk
What are the affects of accelerated soil erosion?
Reduced productivity
Sedimentation in rivers & reservoirs
Increased atmospheric particulates
Desertification
How is productivity reduced because of accelerated soil erosion?
Fertile top soil is usually what is eroded
Remaining soil will be less fertile
Plant growth is reduced
Smaller harvests
Shallower soil may also make root penetration difficult
How is sedimentation in river and reservoirs an affect of accelerated soil erosion?
Soil particles in water may be deposited if they slow down-less kinetic energy
Sediments in rivers reduces flow capacity so river is more likely to overflow and cause flooding after heavy rain
How is increased atmospheric particulates an effect of accelerated soil erosion ?
Wind erosion increases the amount of atmospheric particulates
Can make health problems like asthma worse
Explain desertification as an affect of accelerated soil erosion
Soil erosion makes it more difficult for vegetation to grow
Contributing to further soil erosion and reduced rainfall
Leading to the expansion of deserts
What are the methods of reducing soil erosion?
Long term crops, zero-tillage cultivation, contour ploughing, tied ridging, terracing, rows of stones, windbreaks, multi cropping,increased soil organic matter, livestock management
How are long term crops a method of reducing soil erosion?
The growth of crops that dont need frequent replanting reduces soil disturbance and erosion rate
E.g. permanent grassland or bush
Tree crops like fruit,tea,coffee,cotton
Explain zero-tillage cultivation as a method of reducing soil erosion
Cultivation that reduces soil disturbance and therefore erosion
Direct drilling of seeds into the ground- reduces ploughing- reduces disturbance
Explain contour ploughing as a method of reducing soil erosion
Ploughing on contours at a 90 degree angle to the slope reduces erosion as the water flow is stopped by ploughed furrows and loses its kinetic energy so it cant carry particles
Works until the rainfall is so heavy that the furrow overflows
Explain tied ridging as a method of reducing soil erosion
Used on almost flat land
Divide a field into crisscross sections for intersecting ridges
They retain water when it rains and increase filtration and reduce runoff
Explain terracing as a method of reducing soil erosion?
Sloping land is cultivated into a series of narrow fields
Soil is held in place by retaking walls along the contours
Water flowing over the walls is quick but slows down
Most water will infiltrate the soil as a result
Explain rows of stones as a method of reducing soil erosion
Used if there’s a gentle gradient
Laid along contours to slow runoff and reduce erosion
Any disturbed soil will be deposited
Explain how windbreaks can be used as a method of reducing soil erosion
Growing crop reduces wind velocity and wind erosion
Field will be exposed when crops re,over
Protection maintained by hedgerows or trees around a field
Hedgerows will reduce the wind velocity for a distance of 30 x its height
Explain multi cropping as a method of reducing soil erosion
Can redice exposure to wind if they are harvested at different times
Done by human labour rather than machinery
Explain increasing soil organic matter as a method of reducing soil erosion
Anything that adds organic matter
Will protect soil from wind and rain impact
As it decomposes it will help soil particle adhesion
How will livestock management reduce soil erosion?
Reduce trampling damage
Reduce livestock near a riverbank where it is more likely to cause erosion into the river
Remove livestock at high-risk times like heavy rain
What does the USLE do?
Estimate soil loss rates and assess the impact of changes to farming techniques
USLE formula
A=R x K x L x S x C x P
A in the soil loss equation
Rate of soil erosion
R in the soil loss equation
Rainfall erosivity
(The potential ability of soil to be eroded by rain)
K in soil loss equation
Soil erodability
L in soil loss equation
Slope length
S in the soil loss equation
Slope gradient
C in the soil loss equation
Cropping management
(Impact of vegetation)
P in the soil loss equation
Erosion control
(Effect of type of ploughing)
Under what value will soil loss be considered sustainable ?
9 t ha-1 year-1
Which is the natural rate of soil formation
