Tropical Agro-Ecosystem Function
• Soil erosion by wind & shelterbelts
Dr. Ronald F. Kühne; [email protected]
Georg-August-University Göttingen
Department for Crop Sciences – Tropical Agronomy
Grisebachstr. 6, 37077 Göttingen, Germany
Soil erosion by wind - Contents
1. Soil detachment and transport by forces
generated by wind
2. Erosivity vs. erodibility
3. Modes of movement
4. Fundamental processes
5. Factors affecting wind erosion
6. Control
7. Modeling of wind erosion
Wind erosion forms
Dust storm
Wind erosion on crop land
crops buried in the sand
Dust
bowl
disaster
1930s
USA
Factors of erosivity and erodibility
with wind erosion
Modes of movement
Footage from wind tunnel experiments:
saltation & creep movement of dune sand
Close-up of saltation and
suspension in wind tunnel
Plant damage by wind erosion (abrasion)
on wheat and maize seedlings
Wind speed profiles as affected by
roughness
Threshold velocities of wind speed
as affected by grain size
Factors affecting wind erosion
Methods to assess soil erosion -1
Indirect methods (erosion potential or hazard):
• based on the properties of soil and climate (wind
resistance of soil aggregates, macro- and microrelief,
estimating ground cover, wind velocity, climatic aridity
etc. either direct or by statistical techniques) to assess
the factors related to wind erosion;
• empirical formulas;
• comparing soil properties of eroded and uneroded
profiles such as texture (clay, gravel) and indicator
element contents (organic matter, N, P, radioactive
isotopes (fall out of atmospheric A-bomb testing);
• surveys and satellite imagery;
• wind tunnels (wind resistance of soil aggregates etc.).
Methods to assess soil erosion -2
Direct methods are based on measurement of drift (amount of
soil blown away):
a) measurements of lost profile depth;
– using exposed roots of trees (rough estimate, over very
long time) or
– rods or markers embedded in soil (equivalent to water
erosion research, over long time);
a) sand deposited along
– fence lines, shelterbelts, ridges and other obstructions
(problems: estimation of source area,measurement
interferes with process);
a) trapping the solid particles entrained by different
mechanisms
– suspension, saltation, rolling and measuring by air density
or by collecting and weighing the sediments (most direct
method).
5 Principles of control
1. Produce STABLE CLODS or AGGREGATES on
the surface (increasing the size of aggregates
means that it takes a stronger wind to move the
soil)
2. ROUGHEN the soil surface to reduce wind
velocity and trap drifting material
3. REDUCE FETCH along the prevailing wind
direction with barriers or crop strips to reduce
wind velocity and trap particles
4. LEVEL OR BENCH land to reduce field widths or
to reduce erosion rates on slopes
5. Establish and maintain vegetative or nonvegetative COVER to protect the surface
Measures to control soil erosion by wind
A. VEGETATIVE COVER AND BARRIERS
1) Integration of trees in arable land: examples
for semi-arid Africa: Neem (Azadirachta
indica), Eucalyptus spp., Acacia albida, Acacia
tortilis, Prosopis juliflora, Albizzia lebbek,
Cassia (Senna) siamea etc.
2) Planting windbreaks or shelterbelts with living
trees and shrubs (s. A1) perpendicular to the
prevailing erosive winds.
3) Strip cropping of erosion-susceptible and
erosion-resistant crops (such as grass and
small-grain crops (wheat etc.), rapid soil
cover).
4) Planting of dry-season cover crops
Measures to control soil erosion by wind
A. CONSERVATION TILLAGE
1) No-till system and crop residue mulch.
2) If ploughed, then rough cloddy seedbed (reduces
saltation and creep).
3) Ploughing perpendicular to the erosive wind direction.
4) Emergency tillage: ripping of rough strips at right
angles to the wind direction (rough surface and ridges
slows the intensity of saltation and surface creep,
sandfighter
5) Ridge-furrow seedbed, sowing in furrows, ridges
protect the seedling from sandblast
6) Stubble mulch tillage (chisel ploughs).
7) Weeding without disturbing the surface mulch
(subsurface tillage: duck-foot cultivator, rod weeder
Ripping (left) on clay soil/listing (right) on sandy
soil for emergency tillage
Stable clods
Furrows/troughs to trap
Sand fighter
• break the surface,
• trap windblown
sand,
• reduce crop
damage by sand
blasting and burial
Measures to control soil erosion by wind
A. IMPROVE SOIL STRUCTURE (soil
amendments)
1) Increase soil organic matter content
2) Mulching
3) Natural and synthetic soil conditioners
a) Sealing the soil surface with a soil-polymer crusts through
application of synthetic resin emulsions polymers (K-4, K-9,
PAM, bitumen emulsion, „uresol“ (80-100 kg/ha !!) -> Initial
phase of dune stabilisation to facilitate re-vegetation)
b) Improvement of soil aggregation (> 0.84 mm) - natural and
synthetic polymers
B. CONSERVING SOIL MOISTURE
a) Moist soil can not be blown away by wind!
b) Promoting practices:
•
•
mulching with harvest residues,
planting windbreaks and cover crops (competition, difficult to
establish in dry regions)
Soil conditioner
• defined as synthetic organic chemical or chemically - modified
natural substance that stabilises soil aggregates, and/or
favourably modifies the soils’ structural or physical properties
• natural polymers, e.g. polyuronic acids, alginic acids,
polysaccharides, and humus. Problems encountered were:
– microbial attack and biodegradation,
– short time effectivity
• synthetic polymers e.g. hydrolysed polyacrylonitrile and
polyacrylamide (PAM).
– Stabilisation of the soil structure derived from aggregate formation
(indirectly associated with properties such as porosity, ability of water
penetration, rhizosphere aeration,
– Increase in water-holding capacity of the soil as a result of the addition
of swellable hydrophilic polymers
• failed in the market place because of excessive cost, difficulty
in use, and inconsistent results. Further problems are toxicity
and environmental fate.
Shelterbelts – reduction of wind speed f(H)
Shelterbelts – reduction of wind speed
f(permeability)
Shelterbelts – micrometeorological changes
Limitations of shelterbelts/wind barriers
• area of downwind influence is limited =>
very narrow spacings of not more than
ten times the barrier height would be
required in most erosion areas;
• during establishment and growth of
vegetative belts protection is limited for
many years (dry areas !) => combination
with other control practices is necessary;
• growth of the belt is limited by the lack of
moisture
Shelterbelts – examples
Wind erosion equation (WEQ)
E = f (I’, K’, C’, L’, V)
where E is the potential average annual erosion,
I’ is the soil erodibility index,
K’ is the soil-ridge roughness factor,
C’ is the climatic factor,
L’ is the median unsheltered travel distance across a field,
V is the equivalent quantity of vegetative cover.
I’ is defined as the potential soil loss per acre per annum
from a wide, unsheltered, isolated field with a bare, smooth,
noncrusted surface.
Woodruff and Siddoway (1965)