soil erosion
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Soil Erosion
Soil Erosion
Most important problem associated with agricultural and other land use practices
Soil Erosion
Soil Erosion
According to 1987 National Resources Inventory, USDAo 4 billion metric tons of soil are lost every year due to soil erosion
(wind and water) in 1970so 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 lando Economics of soil erosion: 44 billion dollars/year, $100/acre
It takes 100-600 years to form one inch topsoil
In Illinois,o 40% (9.6 million acres) cropland suffer from erosiono Average soil erosion rate: 6.18 tons in 1987. (max 50 tons/acre)
Soil Erosion
Soil Erosion
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
Soil Erosion
Mississippi River The river carries roughly 550-500 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
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 (1934 -1936) in the
Great Plains
Frost erosion Particularly effective in mountai nous 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 = S kinetic energy (EK = mv2/2) and momentum (M = mv)
Fog ~ 0.05 mm/h Light rain ~ 1.02 mm/h Heavy rain ~ 15.24 mm/h Torrential rain ~ 101.60 mm/h
P. Gary White
Hillel, 1998; Selby, 1993
Brook et al, Fig 7.2Notes:
- Soil aggregates destroyed- Saltation- Erosivity threshold ~25 mm/h
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 intensityo Water content of the soilo Density of the soilo 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 impactt0 = f (density, water depth, slope)
Sheetflow GullyRill
Soil Erosion
Rill and Gully Formation
Brooks et al., Fig 8.1
Soil Erosion
Erosion and Transport Processes
Soil Erosion
Interrill Erosion
Di = Ki i q Sf Cv
Di = Interrill erosion rate (kg/m2-s)Ki = Interrill erodibility of soil (kg-s/m4) (Table 7.1)i = Intensity of rainfall (m/s)q = Runoff rate (m/s)Sf = Interrill slope factor = 1.05 – 0.85 e(-4sinθ) where θ = slope angle (degree)Cv = Cover adjustment factor (0 - 1)
(Equation 7.1)
Soil Erosion
Rill Erosion Rate
1 sr r c
c
qD KT
t t
Dr = Rill detachment rate (Kg/m2-s)Kr = Rill erodibility, due to shear (s/m) = Hydraulic shear of flowing water (Pa) = γ R S (Equation 7.3)
γ = Specific weight of water (9810 N/m3)R = Hydraulic radius of rill (m)S = Hydraulic gradient or slope (m/m)
c = Critical shear below which no erosion occur (Pa)qs = Rate of sediment flow in rill (Kg/m-s)Tc = Sediment transport capacity of rill (Kg/m-s) = B 1.5 (Equation 7.4)
B = Transport coefficient based on soil and water properties generally between 0.01 and 0.1 (dimensionless)
(Equation 7.2)
Soil Erosion
Rill Erosion Rate
If Tc > qs : erosion takes place If Tc < qs : deposition takes place
If > c : erosion takes place If < c : no erosion
1 sr r c
c
qD KT
t t
: defines erosion or deposition
The interrill and rill erosion processes are used in several process-based erosionprediction computer models, including the Water Erosion Prediction Project (WEPP) model
Soil Erosion
Example: Rill Erosion
A rill channel is observed at a location 20 m from the top of the hill during a storm. The slope is 5%, the rill width is 150 mm, and the hydraulic radius is estimated to be 0.01 m, respectively. The sediment transport rate (qs) is measured and found to be 0.2 kg/m-s. The rill erodibility (Kr) is estimated to be 0.004 s/m, critical shear (c) to be 2.5 Pa, and transport coefficient (B) to be 0.1. Assume that the specific weight of water is 9810 N/m3. Calculate the sediment transport capacity and erosion rate for the location. If the hillslope is 120 m long and rainfall duration is 1 hour, how much soil will be eroded from the hillslope by rill erosion?
= γ R S (Pa) Tc = B 1.5 (Kg/m-s)
(Kg/m2-s) 1 sr r c
c
qD KT
t t
= 4.905 PaTc = 1.086 Kg/m-s Dr = 0.00785 Kg/m2-s
= 0.1629 Kg/s
= 423.8 Kg
= 0.001177 Kg/m-s
Soil Erosion
Wind erosion
Important especially in arid regions
Dependent on o Wind speed and exposureo Soil particle- and aggregate sizeso Surface roughnesso 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
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