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Runoff and Erosion

Runoff and Erosion. Surface water excess. the free water on the soil surface whenever the water supply rate exceeds the infiltration rate. Surface storage capacity. the volume of water per unit area which can be held on the soil surface before runoff begins. Surface runoff.

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Runoff and Erosion

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  1. Runoff and Erosion

  2. Surface water excess • the free water on the soil surface whenever the water supply rate exceeds the infiltration rate

  3. Surface storage capacity • the volume of water per unit area which can be held on the soil surface before runoff begins

  4. Surface runoff • the amount of water that flows downslope along the surface • overland flow • channel flow or stream flow

  5. Good or bad? • p. 286 “Uncontrolled runoff is never desirable…” • Quantity vs. quality • Runoff inducement • mechanical treatments • chemical treatments

  6. ElMustaqbal is a Bedouin school located c 10km southeast to Beer Sheva. The school maintains a small garden (Bustan) based on rain water harvesting.

  7. Runoff reduction

  8. Runoff Prediction • Curve Number Method • Developed by Soil Conservation Service (now NRCS) • Purely empirical • Widely used • Mechanistic models • Rainfall, soil properties, and land use must be known • First, simulate infiltration • Second, simulate overland flow process • Example: Water Erosion Prediction Project (WEPP)

  9. Water Erosion Prediction Project (WEPP) • Developed by the Agricultural Research Service, National Soil Erosion Research Laboratory • Infiltration simulation driven by the Green-Ampt model • Hydraulic conductivity in the model is influenced by tillage, crusting, surface cover, and storm precipitation amount. • Runoff predicted by surface water excess • Online version prototype: http://milford.nserl.purdue.edu/ NSERL, West Lafayette, Indiana

  10. Reading assignment • Soil erosion, p.287-295 & p. 359-361 • An Urgent Appeal for Soil Stewardship

  11. A farmer and his two sons during a dust storm in Cimarron County, Oklahoma, 1936. Photo: Arthur Rothstein

  12. Goodwell, Oklahoma, June 4, 1937 Lubbock, Texas, October 17, 2011

  13. Photo source: http://www.greatmirror.com

  14. Plant and Soil Sciences Distinguished Speaker Series Dr. Bob Stewart Director Dryland Agriculture Institute West Texas A&M University “Global Agricultural and Environmental Issues” Monday, November 5, 2012 FAPC, Room 201 Welcome reception with refreshments, 3:00 p.m. Lecture, 3:30 p.m. Ken Burns’ The Dust Bowl: A Special Advanced Screening & Community Conversation Featuring Congressman Frank Lucas Oklahoma State University, 315 Student Union, Student Union Theater 6:45 p.m., Monday, November 5, 2012

  15. Soil erosion stages • Detachment • Transport • Deposition

  16. Detachment • Water • Raindrop impact • Runoff scour • Wind • gusts

  17. Detachment • Water • Raindrop impact • Runoff scour • Wind • gusts • Depends on: • surface cover, soil strength, rain intensity, wind or water flow velocity, etc…

  18. Transport • Water • Sheet • Rill • Gully

  19. Source: Soil Erosion and Its Control, Q.C. Ayres, 1936, McGraw-Hill

  20. Source: http://cst.cmich.edu/users/Franc1M/2GEO334/lectures/erosion.htm

  21. Source: http://www.dot.ca.gov/hq/esc/geotech/photos/south/erosion.htm

  22. Slump near Bismark, ND Creep in Barnes Co., ND Source: http://www.ndsu.edu/nd_geology/nd_mass_wasting/index_mass_wasting.htm Same slump nine months later.

  23. http://landslides.usgs.gov/research/other/centralamerica.php A massive landslide occurred in the Las Colinas neighborhood of Santa Tecla, El Salvador, Central America as a result of the M=7.6 earthquake of January 13, 2001.

  24. Transport • Water • Sheet • Rill • Gully • Wind • Surface creep (d > 0.5 mm) • Saltation (0.1 < d < 0.5 mm) • Suspension (d < 0.1 mm)

  25. Transport • Water • Sheet • Rill • Gully • Wind • Surface creep (d > 0.5 mm) • Saltation (0.1 < d < 0.5 mm) • Suspension (d < 0.1 mm) • Depends on: • flow velocity, particle size, particle density

  26. Reading assignment • “Mechanical Analysis” p.45-47

  27. Deposition • Initiated by a decrease in the flow velocity • Approximated by Stokes’ Law: • the settling velocity of a spherical particle is proportional to its radius squared (r2) • To apply we assume: • soil particles are spherical • the suspension is dilute enough that the particles do not interact with each other • the fluid flow is laminar (not turbulent)

  28. Stokes Law • Drag force  = viscosity • Force of gravity • At terminal velocity • Solve for velocity http://www.answers.com/topic/stokes-law

  29. Deposition example • A pulse of sediment laden runoff is delivered to a pond. About how long will it take for soil particles with diameters of the following sizes to settle to a depth of 1 m? • 5 x 10-2 mm (fine sand) • 5 x 10-3 mm (silt) • 5 x 10-4 mm (clay)  = 1 x 10-3 kg m-1 s-1 s = 2650 kg m-3

  30. Sediment transport and deposition • Major issue in streams, reservoirs, and coastal areas • Major driver for spatial variability in floodplain soils • Management concern for agriculture, construction, and engineering http://blackwarriorriver.org/siltation-sedimentation.html

  31. Reading assignment • Redistribution of soil moisture • p. 297-303

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