Soil Erosion TSM 352 Land and Water Management Systems

Soil Erosion Most important problem associated with agricultural and other land use practices

Soil Erosion According to 1987 National Resources Inventory, USDA o 4 billion metric tons of soil are lost every year due to soil erosion (wind and water) in 1970s o 2 billion metric tons in 1997 due to increased used of conservation and best management practices (BMPs) o 70% of the total soil loss is from agricultural land o Economics of soil erosion: 44 billion dollars/year, $100/acre It takes years to form one inch topsoil In Illinois, o 40% (9.6 million acres) cropland suffer from erosion o Average soil erosion rate: 6.18 tons in (max 50 tons/acre)

Soil Erosion

Soil Erosion by Water T - maximum average annual permissible soil loss without decreasing productivity T-values ranging from 4.5 to 11.2 metric tons/ha per year (2 to 5 tons/acre per year).

Soil Erosion Mississippi River  The river carries roughly million tons of sediment into the Gulf of Mexico each year.  It brings enough sediment to extend the coast of Louisiana by 91 m (300 ft) each year.

Soil Erosion Soil Transport Mechanism  Micro-scale soil transport: Surface erosion  Macro-scale soil transport: Mass movements (Landslides) Erosion is the detachment and transport of soil particles (natural and accelerated) Sediment is relocated from the source (= soil) to streams and eventually into reservoirs or the sea Sedimentation: deposition of sediments in streams or on fans and floodplains

Soil Erosion Effects of Soil Erosion On site  Loss of top fertile soil  Loss of nutrients, OM  Decreased productivity Off site  Non-point source pollution  Filling of reservoirs and dams  Air quality problem  Effects on aquatic organisms  Effects on drinking water quality  Redistribution of pollutants and toxics

Soil Erosion Erosion is caused by: Water  Primary water erosion (Splash Erosion)  Secondary water erosion (Surface flow induced erosion) Wind erosion  The Dust Bowl of 1930s ( ) in the Great Plains Frost erosion  Particularly effective in mountai­nous areas  Example: NW USA

Soil Erosion Factors Affecting Soil Erosion Climate Soil Topography – slope length and steepness Vegetation Land-use practices

Soil Erosion Water Erosion Processes Raindrop/splash Erosion Sheet Erosion Rill Erosion Gully Erosion Stream channel Erosion Shore line Erosion Interrill Erosion

Soil Erosion Water Erosion processes - Hillslope View Hillel, 1998

Soil Erosion Splash Erosion - Raindrop Impact Erosive power =  kinetic energy (E K = mv 2 /2) and momentum (M = mv) Fog ~ 0.05 mm/h Light rain ~ 1.02 mm/h Heavy rain ~ mm/h Torrential rain ~ mm/h P. Gary White Hillel, 1998; Selby, 1993 Brook et al, Fig 7.2 Notes: - Soil aggregates destroyed - Saltation - Erosivity threshold ~25 mm/h

Soil Erosion Consider two hypothetical 2 hr storms: Rain A: Light shower, steady 5 mm/h, mean drop diameter 2 mm (v T = 6 m/s) Rain B: Heavy downpour, steady 20 mm/h, mean drop diameter of 4 mm (v T = 9 m/s) Rain ARain B Mean drop radius, r (cm) Mean drop volume, V (cm 3 )4.2× ×10 -2 Mean drop mass, m (g)4.2× ×10 -2 Energy per drop, E d (J)7.56× ×10 -3 Volume of rain per m 2 (cm 3 )10 4 4×10 4 Number of drops per m × ×10 6 Energy of rain per m 2 (J)1.8× ×10 3 Power of rain per m 2 (W)2.5× ×10 -1 Conclusion: The heavier rain storm, 4 times the intensity and 1.5 times the drop diameter, strikes the soil surface with 9 times the energy and power. Hillel, 1998

Soil Erosion Surface flow induced erosion Requires overland flow (thin surface films or concentrated) and often is intensified by raindrop impact The generation of surface flow depends on: o Rain intensity o Water content of the soil o Density of the soil o Surface roughness of the soil SCS CN method: “Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these…After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow” Important in range and agricultural systems

Soil Erosion Overland flow Exceeds detention storage Begins with detachment: requires a force (shear) created by small eddies in flow or raindrop impact  0 = f (density, water depth, slope) SheetflowGully Rill

Soil Erosion Rill and Gully Formation Brooks et al., Fig 8.1

Soil Erosion Erosion and Transport Processes

Soil Erosion Interrill Erosion D i = K i i q S f C v D i = Interrill erosion rate (kg/m 2 -s) K i = Interrill erodibility of soil (kg-s/m 4 ) (Table 7.1) i = Intensity of rainfall (m/s) q = Runoff rate (m/s) S f = Interrill slope factor = 1.05 – 0.85 e (-4sinθ) where θ = slope angle (degree) C v = Cover adjustment factor (0 - 1) (Equation 7.1)

Soil Erosion Rill Erosion Rate (Equation 7.2)

Soil Erosion Rill Erosion Rate : defines erosion or deposition The interrill and rill erosion processes are used in several process-based erosion prediction computer models, including the Water Erosion Prediction Project (WEPP) model

Soil Erosion Example: Rill Erosion = Kg/s = Kg = Kg/m-s

Soil Erosion Wind erosion Important especially in arid regions Dependent on o Wind speed and exposure o Soil particle- and aggregate sizes o Surface roughness o Tillage Surface roughness creates turbulence in the surface-near air layer → Under pressure sucks particles in the air Form of movement: < 0.1 mm → Suspension (“dust storm”) < 1 mm → Saltation (“jumping”) > 1mm → Creep (“rolling”)

Soil Erosion