raindrop impact erosivity “rainfall erosivity is the potential…for rainfall to cause soil...
TRANSCRIPT
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– 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
Rainfall erosivity map for Brazil
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)
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
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
Reading assignment
• The electrostatic double layer, p. 58-61• Infiltration into crust-topped soils, p. 274-276
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.
Rainfall intensity = 42.5 mm hr-1
Soil texture is silty clay loam
Published in Soil Sci Soc Am J 54:1117-1123 (1990)
Electrostatic double layer
Effect of cation valence
Effect of solute concentration
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)
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)
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/
http://picasaweb.google.com/lh/photo/Ahmg2Yu4msHm4qikNHRjcw
http://www.extension.iastate.edu/CropNews/2008/Issues/20080526.htm
Reading assignment
• Review sections on contact angle, capillarity, and viscosity: p. 30 – 34