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Earth Surface Environments

Earth Surface Environments. Surface Processes in Drylands. The Namib. When/where are my practicals and fieldtrip? A summary. Unique times and locations for all….so please make a careful note. Learning Outcomes. Provide an informed definition of drylands

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Earth Surface Environments

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  1. Earth Surface Environments Surface Processes in Drylands The Namib

  2. When/where are my practicals and fieldtrip? A summary Unique times and locations for all….so please make a careful note

  3. Learning Outcomes • Provide an informed definition of drylands • Locate major deserts & explain their occurrence • Outline the major generic earth surface processes typical in dryland environments (particularly the relationship between grain size and erosion) • Outline how and why degraded soils can be used as a major carbon sink

  4. Ripples on a Saharan Sand Dune

  5. Typical Kalahari View

  6. Communal Grazing in the southern Kalahari

  7. Baobab Tree, Central Botswana

  8. The Tsodilo Hills, North West Botswana

  9. Van der Post’s panel, Tsodilo Hills

  10. What are Drylands? Aridity Index (UNEP, 1992) Dry sub-humid 0.5 - < 0.65 Semi-arid 0.2 - <0.5 Arid 0.05 - <0.2 Hyper Arid <0.05 AI = P PET 40-50% earth’s surface

  11. Temperature Variability Hot - Central Sahara, Arabia Mild - Kalahari, Simpson Cool - Atacama, Mojave Cold - Gobi

  12. Global Extent of Drylands

  13. Global Extent of Drylands Central Sahara Arabian Kalahari Namib Atacama Mojave Gobi Simpson

  14. Selected Environmental Characteristics 1. Dry 2. Sandy & crusted soils 3. Dust storms 4. Floods Saharan dune ripples

  15. Why are they dry? Low annual and high inter-annual variability of rainfall Four factors: 1. Atmospheric - descending stable air 2. Continentality - too far from rain-bearing winds 3. Topography - rain shadow 4. Cold Ocean Currents - low evaporation & precipitation

  16. Select Environmental Characteristics 1. Dry 2. Sandy & crusted soils 3. Dust storms 4. Floods Kalahari Sands

  17. Why are they sandy? Predominance of physical and absence of chemical weathering • Insolation Temperature changes Volumetric coefficient thermal expansion 1.6 - 3.0 x 10-5 K Granite: 50oC  vol 0.12% Spalling

  18. Why are they sandy? Insolation increased in presence of moisture Salt weathering significant Fines blown by wind Namib fog (NASA) Chott el Jerid, Tunisia

  19. Soils Entisols (c. 13% globe) Aridisols (c. 11% globe)

  20. Soils • Upward movement of limited water: • Few horizons • Few organics & nutrients • Salts • Old • Crusts • Vulnerable to degradation • Chemical • Biological • Physical Kalahari, Botswana

  21. Physical & Biological Soil Crusts • Physical Crusts: • Compaction • Raindrop impact • Clay dispersal x950 USGS • Biological Crusts: • Bacteria & Cyanobacteria • (Microcoleus, Nostoc spp.) • Succession

  22. Select Environmental Characteristics 1. Dry 2. Sandy & crusted soils 3. Dust storms 4. Floods Sahara, Tunisia

  23. Particle entrainment by wind 1. Forces: • Wind produces shear forces on bed • Expressed as shear velocity: u* = o / a (m s-1)  o = shear force (g cm-1 sec -2) a = density of air (g cm-3)

  24. Particle entrainment by wind Resistance: 1. Gravity (related to d3) 2. Interparticle cohesion

  25. Grains in motion 3 major processes: Creep, saltation and suspension

  26. Thresholds • Size limit of particles carried by wind of given v or u* • Distinct relationship between particle-size and u*

  27. The Hjulstrom Curve

  28. Selected Environmental Characteristics 1. Dry 2. Sandy & crusted soils 3. Dust storms 4. Floods

  29. Floods High magnitude, low frequency rainfall Low infiltration rates: Shallow soils Stony surfaces Crusts 100mm rainfall in 3 hours, nr. Las Vegas, USA

  30. Learning Outcomes • Provide an informed definition of drylands • Locate major deserts & explain their occurrence • Outline the major generic earth surface processes typical in dryland environments (particularly the relationship between grain size and erosion)

  31. Journal Review • 4 spread through term • Complements lectures/practical/fieldwork • Good habit • Key to successful degree

  32. Dr Kate Rowntree, Institute For Water Research, Rhodes University, South Africa Publications:32 articles in international journals, books & conference proceedings Rowntree, K., Duma, M., Kakembo, V. and Thornes, J., 2004, Debunking the Myth of Overgrazing and Soil Erosion. Land Degradation and Development, 15 (3), 203-214. http://www3.interscience.wiley.com/cgi-bin/jtoc/6175/

  33. The Assignment Full reference Aims and objectives Brief methods (if applicable) Main findings and conclusions (or use the paper headings) Be succinct Work over 1 page of A4 will not be marked, 11pt font minimum In words understandable to an interested 14 year old Use bullet points where/if appropriate Paper 1 Summary due in on the 14th February

  34. Introduction Summary • Overgrazing is often said to be a major cause of land degradation and soil erosion • Erosion rates are affected by vegetation cover (and thus the interaction between climate, soils, topography and grazing levels) • To understand how grazing affects erosion need to understand the link between ecology of grazing lands and erosion proceses

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