1. Soil Mapping and Erosion
LECTURE 19
Soil Mapping and Erosion

2. Soil Mapping

3. Soil Mapping…
Why would we want to map soils?

4. Soil Mapping… Why would we want to map soils?
Communication of geographic information
Tools for land management and planning
Many soil scientists specialize in the mapping of soils.

5. Steps in mapping the soil of a given area
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.

6. Soil sampling in the landscape
Soil description
Use is made of a soil pit or augered samples
Horizons are identified and characteristics of the soil systematically described
Soil sampling in the landscape
It is important to understand the way in which landscape, vegetation etc. affects or indicates soil properties to ensure efficient sampling techniques.

7. Soil surveys:
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)

8. How could soil classification and mapping aid those involved in:
Engineering and construction?
Landcare and conservation?

9. Soil Erosion

10. What is soil erosion?
“The wearing away of the land surface by running water, wind, ice or other geological agents, including such processes as gravitational creep.”
“Detachment and movement of soil or rock fragments by water, wind, ice or gravity.”

11. Different types of water erosion…
Accelerated erosion
Geological or natural erosion
Donga or gully erosion
Normal erosion
Rill erosion
Sheet erosion
Splash erosion
Tunnel erosion or piping

12. Effects of erosion… On site: Off site: Loss of fertile topsoil
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:
Buildup of sediment and water elsewhere in the system

13. The mechanics of soil erosion…
Detachment
Transportation
Deposition

14. The effects of raindrops:
Detach soil
Destroy granulation
Can lead to crusting
Transport of particles in some cases

15. Transport Deposition Rainsplash Running water Sheetwash Gully erosion
NB Infiltration capacity
Sheetwash
Gully erosion
Deposition
Can occur over long distances or short distances
Amount of soil delivered to stream divided by the amount eroded = delivery ratio

16. Prediction and modelling of soil erosion…
Why would we want to predict soil erosion?
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…

17. What do we need to understand before we can predict erosion?
What factors affect soils’ susceptibility to erosion?
Erosivity of erosion agents.
Erodibility of soils.
Length of slope.
Gradient of slope.
Land cover and management.
NB vegetation, plant residues, soil tillage
Erosion control practices.

18. Erosivity… Total rainfall
Intensity and seasonal distribution of the rain
Why is intensity important?
Intense rains have large drop size
Higher rate of rainfall = more runoff

19. Erodibility… Indicates a soil’s inherent susceptibility to erosion
Infiltration capacity
Structural stability
Properties that tend to result in high erodibility
High fine sand and silt content
Expansive clay minerals
Impervious soil layers
Blocky, platy or massive soil structure

20. Properties that lead to low erodibility
High organic matter content
Nonexpansive clays
Strong granular structure

21. Some important principles in erosion control…

22. Some important principles in erosion control…
Keeping disturbed soil covered
Controlling runoff
Trapping sediment
Altering soil properties (more difficult)