ENVS10001 Lecture Notes - Lecture 18: Strip Farming, Flow Velocity, Soil Structure
17 May 2018
School
Department
Course
Professor
LAND DEGRADATION
Desertification
• The final stages of degradation of formerly productive dryland into unproductive desert
• Lack of trees, dry, poor soil quality/fertility + dusty
• Main step: topsoil reduced or degraded
o Soil depth
o Fertility
o Structure
o Biological activity
• Most vulnerable: semi-arid regions adjacent to desert biomes
• Extent: 3.6 billion ha of land, > 250mil people in 100 countries
• UN measures extent of desertification according to loss of
agricultural productivity
o None: less than 10%
o Moderate: 10 - 25%
o High: 25% - 50%
o Very high: more than 50%
Global Soil Erosion
• Movement of soil components, especially topsoil, from one place to
another by erosive elements (wind and water)
• Soil: non-renewable resource over human timescales, loss >
formation rate
• Types: splash, sheet and rill + gully
• Mitigation: (slow down flow velocity)
o Terracing, Contour planting + strip cropping, Alley
cropping, Windbreaks AND No till + residue retention !
retains soil moisture (mulching effect) + soil structure +
infiltration capacity
• Predicting soil erosion: RUSLE (Revised universal Soil Loss
Equation
o A (annual av soil loss) = R (rainfall – runoff erosivity) * K (
soil erodability) * LS (slope length + steepness) * C (
cover) * P (practice management factor)
Splash (Erosion by Rainfall)
• Water is 800 times heavier than air
• Beginning of erosion on hill slopes
• Significant impact on dislodging soil
Sheet Erosion
• The uniform removal of soil in thin layers by the forces of
raindrops + shallow overland flow
• Loss of fine particles (organic matter, silt + clay) ! hold most of
the nutrients + important for structure
Gully Erosion
• Greater velocity + concentration of water flow ! act like streams,
cut into soil + erode
Causes
Consequences
Overgrazing
Worsening drought
Deforestation
Famine
Erosion
Economic losses
Salinisation
Lower living
standards
Soil
Compaction
Environmental
refugees
All degraded
Lose ability of soil to hold
water + nutrients
Hjulstrom Diagram
•
What happens when flow
is concentrated + allowed
to reach certain velocities
•
To reduce erosion – must
reduce flow velocity
Document Summary
Desertification: the final stages of degradation of formerly productive dryland into unproductive desert, lack of trees, dry, poor soil quality/fertility + dusty, main step: topsoil reduced or degraded, soil depth, fertility, structure, biological activity. Equation: a (annual av soil loss) = r (rainfall runoff erosivity) * k ( soil erodability) * ls (slope length + steepness) * c ( cover) * p (practice management factor) Splash (erosion by rainfall: water is 800 times heavier than air, beginning of erosion on hill slopes, significant impact on dislodging soil. Sheet erosion: the uniform removal of soil in thin layers by the forces of raindrops + shallow overland flow, loss of fine particles (organic matter, silt + clay) ! hold most of the nutrients + important for structure. Gully erosion: greater velocity + concentration of water flow ! act like streams, Hjulstrom diagram: what happens when flow is concentrated + allowed to reach certain velocities, to reduce erosion must reduce flow velocity.
