1 Soil Erosion h Chapter 17 p. 740‐795 1 THE PROBLEM Land Degradation and Loss • Erosion • Salinization • Waterlogging • Waterlogging • Urbanization • Deforestation to replace productivity lost • Over 15 million acres/year 2 3 3 4 4 On‐site Effects: Soil Productivity • Reduction in topsoil depth – Exposes less desirable (less productive) subsoil – Higher acidity & clay, lower fertility & SOM • Reduced rooting zone; soil surface is closer to: Reduced rooting zone; soil surface is closer to: – Bedrock – Restrictive horizons (e.g., fragipans) • Lower nutrient supply and available water holding capacity • Loss of fines (silt & clay) 5 Off‐site Effects: Sediments • Yellow River ‐ China – 1.6 billion tons of sediment per year – China loses ≈ 18 tons/acre/year • Ganges River ‐ India Ganges River India – 1.5 billion tons/year • Mississippi River ‐ USA – 300,000,000 tons/year – BEST SOILS IN THE US (and world) 6
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Soil Erosion
hChapter 17p. 740‐795
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THE PROBLEM
Land Degradation and Loss
• Erosion
• Salinization
• Waterlogging• Waterlogging
• Urbanization
• Deforestation to replace productivity lost
• Over 15 million acres/year
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On‐site Effects: Soil Productivity
• Reduction in topsoil depth– Exposes less desirable (less productive) subsoil– Higher acidity & clay, lower fertility & SOM
• Decreases available oxygen due to organic matter decomposition
i l l h l• Increases nutrient levels that accelerate eutrophication
• Transports organic chemicals and metals into the water column
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Effects of SedimentsPhysical
• Reduces or prevents light penetration
• Changes temperature patterns
• Decreases the depth of ponds and lakesDecreases the depth of ponds and lakes
• Changes flow patterns
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Magnitude of the Problem – USA2003 Data
• 368 million acres of cropland (19 % of total area)
/ /• Average soil loss ≈ 2.63 tons/acre/year from all cropland
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Magnitude of the Problem – Tennessee 2003 Data
• 4.75 million acres cultivated and non‐cultivated cropland ≈ 17.6 % of total land area
• Average cropland soil loss ≈ 3.6 ton/acre/year
h 60 0 il i• There are 60,507 stream miles in Tennessee
• 24,808 stream miles are impaired (41 %)
• 5,212 stream miles are impaired by sediments (21 % of impaired; 8.6 % of total)
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Magnitude of the Problem – Tennessee 2003 Data
9.1
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7.1
5.6
3.6
Other Erosion Problem Areas• Poor forest management• Poor pasture and range management– Too many animal units/acre– Animal access to streams– Tramped & poorly vegetated forage areas
• Urban construction– 100 to 200 ton/acre/year common
• Surface Mining– 200+ ton/acre/year common
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What do tons/acre mean?
• 1 acre, 6‐inches deep ≈ 2,000,000 lbs
• Thus, 6 inches ≈ 1,000 tons of soil
• 1 inch of soil over an acre = 167 tons
• 16.7 tons/acre/year = 0.1 inch soil loss
• At 2003 erosion rates, it would take 46 years to lose 1 inch of soil evenly from the 4.75 million acres of cropland in Tennessee
[Tennessee was losing 1 inch of soil every 18 years in 1982]
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One inch of soil may take 200 to 10,000 years to form
Once eroded the loess derivedOnce eroded, the loess-derivedsoil in west Tennessee is lostforever
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2020
2121
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The Erosion Process
• Erosion: the movement of soil from desirable to undesirable locations
• A combination of three sequential processesD t h t– Detachment
• T = maximum rate of annual soil loss that will permit crop productivity to occur economically and indefinitely.
• Ranges from 1 to 5 ton/acre/year• Ranges from 1 to 5 ton/acre/year
• 2 to 5 ton/acre/year for most soils
• Soil loss tolerance “T” is assigned according to properties of root limiting subsurface soil layers.
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Soil Loss Tolerance ‐ T Value
• Criteria for assigning “T” are estimated from:– The severity of physical or chemical properties of subsurface layers.
– The climatically influenced properties of soilThe climatically influenced properties of soil moisture and temperature.
– The economic feasibility of utilizing management practices to overcome limiting layers or conditions.
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Soil Loss Tolerance Factor ‐ T ValueDepth to limiting layer (inches) Group 1 Group 2 Group 3
0 ‐ 10 1 1* 3
10 ‐ 20 1 2 3
20 ‐ 40 2 3 4
40 ‐ 60 3 4 4
>60 5 5 5
• Group 1 ‐‐ The limitations are significant or have permanent layers of root limitation.
• Group 2 ‐‐ The limitations are of moderate root restriction, or have a less than permanent loss to productivity in a given climate.
• Group 3 ‐‐ The limitations can be overcome in a given climate, through natural or managed processes to achieve the productivity level of the non‐eroded soil.
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* Some soils are assigned with soil loss tolerance of 2
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Prediction of Soil Erosion by Water RUSLE
• Revised Universal Soil Loss Equation
• Equation to predict loss (t/ac/yr) as function of rainfall soil type landscape vegetationof rainfall, soil type, landscape, vegetation, and conservation practices
• A = R × K × LS × C × P
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Definition of FactorsRUSLE
• A = Soil loss in tons/acre/year
• R = Rainfall factor ‐ energy/acre/year term
• K = Soil erodibility factor ‐ tons/energy
• LS = Slope length and steepness factor
• C = Crop/vegetation management factor
• P = Conservation structures factor
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Rainfall Factor
• Average annual energy from rainfall events
• In TN, ranges from 150 to about 300– Knox County: R = 180Sh lb C 300– Shelby County: R = 300
• Where is the most erosion going to occur?
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Soil Erodibility (K) Factor
• Range from about 0.2 to 0.5
• Sandy loam, sandy surface texture– K ≈ 0.2 to 0.3 (Hartsells SL = 0.20)
• Loam to Clay loam– K ≈ 0.3 to 0.4 (Etowah SiCL = 0.32)
• Silt loam in loess (very high silt %)– K ≈ 0.5 (Memphis SiL = 0.49)
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Slope Length and Steepness Factor
• Values of LS are relative and represent how erodible the particular slope length and steepness is relative to the 72.6 ft long, 9 % steep unit plot (LS = 1)steep unit plot (LS = 1)
• Thus, some values of LS are less than 1 and some values are greater than 1
• The LS factor ranges from 0.05 to 52.70
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Cover and Management Factor
• Depends on the amount of tillage and the amount of surface residue
• Ranges from 0.001 (undisturbed forest) to 1 (fallow with no residues)(fallow with no residues)
• We control this one
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Practices Factor
• The ratio of soil loss with contouring and/or stripcropping to that with straight row farming up‐and‐down slope (P = 1 is worst‐case)
• Terraces; P = 0.5;• Contour tillage (perpendicular to slope)– P varies from 0.5 to 0.9 depending on slope, cover management, and hydrology
• Stripcropping ‐ alternating strips of grasses and row crops across slope (P generally < 0.5)
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Example
• Memphis sil, 6% slope, 200 ft long, no vegetation, no conservation practices– R = 300 K = 0.49
LS = 0 95 C = 1 P = 1– LS = 0.95 C = 1 P = 1
– A = 300 × 0.49 × 0.95 × 1 × 1– A = 95.6 tons/acre/year
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Example 2
• Same soil under permanent forest: C = 0.001– A = 300 × 0.49 × 0.95 × 0.001 = 0.14 t/ac/yr