Transcript LECTURE 19 - Rhodes University

LECTURE 19
Soil Mapping and Erosion
Soil Mapping
Soil Mapping…
 Why would we want to map soils?
Soil Mapping…
 Why would we want to map soils?
Communication of geographic information
 Tools for land management and planning
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 Many soil scientists specialize in the
mapping of soils.
 Steps in mapping the soil of a given area
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Define the scale and level of detail that is
required.
Study existing information regarding the soil,
geology, topography, vegetation of the area.
Define soil units to be mapped.
Compile information about the nature of each
soil.
Mark boundaries of where each soil unit
occurs.
 Soil description
Use is made of a soil pit or augered samples
 Horizons are identified and characteristics of the soil
systematically described
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 Soil sampling in the landscape
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It is important to understand the way in which
landscape, vegetation etc. affects or indicates soil
properties to ensure efficient sampling techniques.
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Soil surveys:
1.
2.
3.
4.
5.
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Mapping of the soils
Characterization of mapping units.
Classification of mapping units.
Correlation with other soil surveys.
Interpretation of soil suitability for various land uses.
Soil surveys can be done at very different scales
(1st order to 5th order; intensive to reconnaisance)
 How could soil classification and mapping
aid those involved in:
Engineering and construction?
 Landcare and conservation?
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Soil Erosion
What is soil erosion?
1. “The wearing away of the land surface by
running water, wind, ice or other geological
agents, including such processes as
gravitational creep.”
2. “Detachment and movement of soil or rock
fragments by water, wind, ice or gravity.”
Different types of water
erosion…
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Accelerated erosion
Geological or natural erosion
Donga or gully erosion
Normal erosion
Rill erosion
Sheet erosion
Splash erosion
Tunnel erosion or piping
Effects of erosion…
 On site:
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Loss of fertile topsoil
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Selective removal of organic matter and fine material
Loss of seeds and seedlings
Change in slope topography (unsafe conditions, more
difficult remediation, high expense)
Change in soil characteristics like permeability,
infiltration rate, etc.
 Off site:
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Buildup of sediment and water elsewhere in the system
The mechanics of soil
erosion…
 Detachment
 Transportation
 Deposition
 The effects of raindrops:
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Detach soil
Destroy granulation
Can lead to crusting
Transport of particles in some cases
 Transport
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Rainsplash
Running water
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NB Infiltration capacity
Sheetwash
Gully erosion
 Deposition
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Can occur over long distances or short
distances
Amount of soil delivered to stream divided by
the amount eroded = delivery ratio
Prediction and modelling of
soil erosion…
 Why would we want to predict soil erosion?
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Optimal resource management
Evaluation of consequences of different land use
Compliance with environmental requirements
Development of sediment control plans
(particularly for construction projects)
Prediction of dam infiltration rates…
What do we need to understand
before we can predict erosion?
 What factors affect soils’ susceptibility to
erosion?
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Erosivity of erosion agents.
Erodibility of soils.
Length of slope.
Gradient of slope.
Land cover and management.
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NB vegetation, plant residues, soil tillage
Erosion control practices.
Erosivity…
 Total rainfall
 Intensity and seasonal distribution of the rain
 Why is intensity important?
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Intense rains have large drop size
Higher rate of rainfall = more runoff
Erodibility…
 Indicates a soil’s inherent susceptibility to erosion
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Infiltration capacity
Structural stability
 Properties that tend to result in high erodibility
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High fine sand and silt content
Expansive clay minerals
Impervious soil layers
Blocky, platy or massive soil structure
 Properties that lead to low erodibility
High organic matter content
 Nonexpansive clays
 Strong granular structure
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Some important principles in
erosion control…
Some important principles in
erosion control…
 Keeping disturbed soil covered
 Controlling runoff
 Trapping sediment
 Altering soil properties (more difficult)