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Raindrop Impact

Raindrop Impact. Erosivity. “Rainfall erosivity is the potential…for rainfall to cause soil loss.” ( da Silva, 2003). Rainfall Intensity in in./hr for a 2-year, 1-hour Storm Event (FHWA, 1992). Erosivity and Erosion Prediction. USLE and RUSLE: A = R * K * LS * C * P

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Raindrop Impact

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  1. Raindrop Impact

  2. Erosivity • “Rainfall erosivity is the potential…for rainfall to cause soil loss.” (da Silva, 2003)

  3. Rainfall Intensity in in./hr for a 2-year, 1-hour Storm Event (FHWA, 1992).

  4. Erosivity and Erosion Prediction • USLE and RUSLE: A = R * K * LS * C * P • A = average annual soil loss • R = rainfall erosivity factor • K = soil erodibility factor • L = slope length factor • S = slope steepness factor • C = cover/management factor • P = support practice factor

  5. Rainfall erosivity map for Brazil

  6. Soil splash • Soil material splashed into the air by raindrop impact • A more important cause of soil detachment than overland flow prior to rill and gully formation (Salles and Poesen, 2000) • A key mechanism for transport of plant pathogens (Madden, 1997)

  7. Ds = 8.3MD +0.09 Ds = mass rate of sand detachment M = drop momentum D = drop diameter Salles, C., and J. Poesen. 2000. Rain properties controlling soil splash detachment. Hydrological Processes 14:271-282.

  8. Stress and strength • Compressive stress is the “normal” inward force per unit area • Shear stress is the tangential force per unit area. • Soil strength is the maximum stress that a particular soil body can bear without failing. ASTM D 2166: Standard Test Method for Unconfined Compressive Strength of Cohesive Soil http://www.test-llc.com/strength.htm

  9. Reading assignment • The electrostatic double layer, p. 58-61 • Infiltration into crust-topped soils, p. 274-276

  10. Soil crust • A thin, less permeable layer at the soil surface characterized by higher bulk density, smaller pores, and lower hydraulic conductivity than the underlying soil.

  11. Rainfall intensity = 42.5 mm hr-1 Soil texture is silty clay loam Published in Soil Sci Soc Am J 54:1117-1123 (1990)

  12. Electrostatic double layer

  13. Effect of cation valence

  14. Effect of solute concentration

  15. Factors promoting chemical dispersion • relatively high percentage of monovalent cations on the exchange sites • low solute concentration, i.e. low electrical conductivity • irrigation water with high sodium adsorption ratio (SAR)

  16. ESP = exchangeable sodium percentage EC = electrical conductivity (of the applied water) For comparison saline soil has EC > 4 dS m-1 And rain in OK has EC ~ 0.04 dS m-1 (1 mmho cm-1 = 1 dS m-1)

  17. Biological soil crust • The community of organisms living at the surface of desert soils. Major components are cyanobacteria, green algae, microfungi, mosses, liverworts and lichens. • http://www.soilcrust.org/

  18. http://picasaweb.google.com/lh/photo/Ahmg2Yu4msHm4qikNHRjcw

  19. http://www.extension.iastate.edu/CropNews/2008/Issues/20080526.htmhttp://www.extension.iastate.edu/CropNews/2008/Issues/20080526.htm

  20. Reading assignment • Review sections on contact angle, capillarity, and viscosity: p. 30 – 34

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