Dryland Degradation by Wind Erosion

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1 GEO Land Degradation Dryland Degradation by Wind Erosion Geert Sterk Department of Physical Geography Utrecht University Dryland degradation by wind erosion and its control Paper by G. Sterk, M. Riksen and D. Goossens Published in: Annals of Arid Zone 40: (2001) Available on????? Dryland Degradation by Wind Erosion Contents: General aspects of drylands Wind erosion processes Agricultural damage due to wind erosion Management options for wind erosion control Drylands Global population growth From 2 to 6 billion people in 20 th century Increasing demand for food Since 1950, improved agricultural practices Increased yield per unit of area Favorable environments only In future more pressure on drylands Drylands Drylands - global distribution P/ETo < 0.65 (UNEP, 1991) Hyper-arid, arid, semi-arid, and dry sub-humid Approximately 41% of continental area Many developing countries Poverty, high population growth Agricultural-based economies 1

2 Global distribution of drylands Nomads in the Sahel Farmer in Niger managing his pearl millet field Cattle herding in the Sahel Drylands Drylands - general characteristics 2.5% irrigated agriculture 7.4% rainfed cropland 74% rangeland Insufficient and variable rainfall Low soil fertility 70% of drylands affected by soil degradation Drylands Drylands - soil degradation according to GLASOD map: Physical degradation on 35 mha Chemical degradation on 111 mha Water erosion on 478 mha Wind erosion on 513 mha Most important degradation process! Desertification 2

3 Great Plains, USA, during the 1930 s Great Plains, USA, during the Dust Bowl (1930 s) A mother with her children on their way from the Great Planes to California. (Dorothea Lange) The novel The Grapes of Wrath by John Steinbeck describes the faith of a farming family moving from Oklahoma to California. They were forced to move away due to drought, wind erosion, and changing agricultural practices. Because of the many migrants, things weren t much better in California. Wind erosion mainly in areas with: Sandy soils Flat topography Dry conditions (semi-arid and arid zones) Balance between: Wind force Soil resistance Wind erosion processes Wind erosion processes Wind erosion: Removal of soil material (soil particles, nutrients and organic matter) by wind Sedimentation: deposition of wind-blown material Transport by: creep, saltation and suspension 3

4 Wind erosion processes Wind erosion processes Creep transport 5-25% of total mass transport Particles with d > 500 µm Coarse sand No nutrients Short-distance transport Wind erosion processes Wind erosion processes Saltation transport 50-75% of mass transport Particles with 63 < d < 500 µm Very fine to medium sand Low concentration of nutrients Intermediate transport range Wind erosion processes Suspension transport 3-40% of total mass transport Particles with d < 63 µm Clay + silt Dust storm in Burkina Faso 4

5 Convective thunderstorm causing Wind erosion processes severe wind erosion in Senegal Suspension transport 3-40% of total mass transport Particles with d < 63 µm Clay + silt High concentration of nutrients Long-distance transport Agricultural damage by wind erosion Sedimentation at undesired places dune formation filling of irrigation and drainage canals blockage of roads Dust transport from Western Sahara over Atlantic Ocean Agricultural damage by wind erosion A sand dune in the Sahara desert Sediment filled up a ditch in Texas 5

6 Agricultural damage by wind erosion Sedimentation at undesired places dune formation filling of irrigation and drainage canals blockage of roads Crop damage abrasion or sand blasting burial of seedlings Agricultural damage by wind erosion Agricultural damage by wind erosion Soil degradation physical: deterioration of soil structure chemical: selective removal of nutrients Saltation: local redistribution of nutrients Suspension: loss of nutrients Main methods: Creating resistance to wind forces Reducing wind speed at surface Major wind erosion control measures: Soil roughening Maintaining soil cover Wind barriers Nutrient export with suspended sediment 6

7 Soil roughening: Tillage operations that leave: non-erodible aggregates at the surface ridges perpendicular to prevailing wind Stabilizing effect Effect limited on sandy soils Tillage must be repeated frequently Nutrient export with suspended sediment Maintaining soil cover Post-harvest crop residues: Flat mulch (widely applied) Standing stubble (more effective) Cover crops: Maintaining cover after cropping season Reduces wind speed and stabilizes soil surface Nutrient export with suspended sediment Pearl millet seedlings protected by post-harvest crop residue 7

8 Wind barriers Strips of trees or crops: perpendicular to prevailing wind direction effect depends on porosity and height Wind speed is reduced in the lee of the barrier Strip cropping in the US Limitations to erosion control in drylands Tillage often not applied Sandy soils (e.g. the Sahel) Mulch quantity is limited: multiple uses low biomass production Competition (cover crops and wind barriers) Variable wind direction during storms Competition between wind barrier and pearl millet Potential for erosion control in drylands Management of natural vegetation of scattered trees and shrubs for wind erosion control So far, no scientific research in this area Management strategies are non existing Rinaudo (1996) reported successful wind erosion control by trees and shrubs in the Sahel 8

9 Effects of vegetation on wind erosion Recent research project in Burkina Faso: Quantification of wind speed reduction by natural vegetation at multiple scales Quantification of wind erosion control by natural vegetation at multiple scales Development of a wind erosion model that predicts the effect of natural vegetation on wind erosion control Main findings: Trees are effective in slowing down wind speed Shrubs are effective in trapping sediment Farmers are interested in vegetation management But, competition with crop remains an issue Wow! 9