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 informationTools for land management and planning

Many soil scientists specialize in the mapping of soils.

Steps in mapping the soil of a given area1. Define the scale and level of detail that is

required.2. Study existing information regarding the soil,

geology, topography, vegetation of the area.3. Define soil units to be mapped.4. Compile information about the nature of each

soil.5. Mark boundaries of where each soil unit

occurs.

Soil descriptionUse is made of a soil pit or augered samplesHorizons 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.

Soil surveys:1. Mapping of the soils

2. Characterization of mapping units.

3. Classification of mapping units.

4. Correlation with other soil surveys.

5. 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?

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…

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:

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

The mechanics of soil erosion…

Detachment Transportation Deposition

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

Transport Rainsplash Running water

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

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…

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.

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

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

Properties that lead to low erodibilityHigh organic matter contentNonexpansive claysStrong granular structure

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)