Download Compressive stress

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)
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.
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 has
EC ~ 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