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