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 Rain properties controlling soil splash detachment. Hydrological Processes 14: D s = 8.3MD D s = 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

Reading assignment The electrostatic double layer, p Infiltration into crust-topped soils, p

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: (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.

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