Soil Mapping and Erosion 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 Many soil scientists specialize in the mapping of soils.

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.

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.

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)

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

Soil Erosion

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.”

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: 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

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 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

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 erodibility High organic matter content Nonexpansive clays Strong 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)