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 the amount of water that flows downslope along the surface – overland flow – channel flow or stream flow

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

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

Runoff reduction

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)

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: NSERL, West Lafayette, Indiana

Reading assignment Soil erosion, p & p An Urgent Appeal for Soil Stewardship

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

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

Photo source:

Oklahoma State University, 315 Student Union, Student Union Theater 6:45 p.m., Monday, November 5, 2012 Ken Burns’ The Dust Bowl: A Special Advanced Screening & Community Conversation Featuring Congressman Frank Lucas D R. B OB S TEWART D IRECTOR D RYLAND A GRICULTURE I NSTITUTE W EST T EXAS A&M U NIVERSITY “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. Plant and Soil Sciences Distinguished Speaker Series

Soil erosion stages Detachment Transport Deposition

Detachment Water – Raindrop impact – Runoff scour Wind – gusts

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

Transport Water – Sheet – Rill – Gully

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

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Creep in Barnes Co., ND Slump near Bismark, ND Same slump nine months later. Source: /index_mass_wasting.htm /index_mass_wasting.htm

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,

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

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

Reading assignment “Mechanical Analysis” p.45-47

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 (r 2 ) 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)

Drag force  = viscosity Force of gravity At terminal velocity Solve for velocity Stokes Law

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 mm (fine sand) – 5 x mm (silt) – 5 x mm (clay)  = 1 x kg m -1 s -1  s = 2650 kg m -3

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

Reading assignment Redistribution of soil moisture – p