Raindrop Impact
description
Transcript of Raindrop Impact
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Raindrop Impact
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Erosivity
• “Rainfall erosivity is the potential…for rainfall to cause soil loss.” (da Silva, 2003)
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Rainfall Intensity in in./hr for a 2-year, 1-hour Storm Event (FHWA, 1992).
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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
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Rainfall erosivity map for Brazil
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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)
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Salles, C., and J. Poesen. 2000. Rain properties controlling soil splash detachment. Hydrological Processes 14:271-282.
Ds = 8.3MD +0.09
Ds = mass rate of sand detachment
M = drop momentum
D = drop diameter
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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
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Reading assignment
• The electrostatic double layer, p. 58-61• Infiltration into crust-topped soils, p. 274-276
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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.
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Rainfall intensity = 42.5 mm hr-1
Soil texture is silty clay loam
Published in Soil Sci Soc Am J 54:1117-1123 (1990)
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Electrostatic double layer
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Effect of cation valence
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Effect of solute concentration
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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)
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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 hasEC ~ 0.04 dS m-1
(1 mmho cm-1 = 1 dS m-1)
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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/
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http://picasaweb.google.com/lh/photo/Ahmg2Yu4msHm4qikNHRjcw
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http://www.extension.iastate.edu/CropNews/2008/Issues/20080526.htm
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Reading assignment
• Review sections on contact angle, capillarity, and viscosity: p. 30 – 34