18. soil erosion ans soil conservation by allah dad khan
TRANSCRIPT
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Soil Erosion and Conservation ByAllah Dad Khan
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“Erosion”
• a natural leveling process that wears down high places; fills in low places
• agents: running water, ice, wind, gravity, waves
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“accelerated erosion”
• Process by which soil particles are removed, transported and deposited; rate of removal of soil greater than rate of formation
• 500 yrs / inch topsoil
– Caused by removal of vegetation• agents: wind, water
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Deposition
• Deposition or sedimentation is flip side of erosion.
• the soil that is removed has to go somewhere: wetlands, lakes, streams, atmosphere
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Worldwide
75 billion metric tons soil lost / year(predominantly cropland)
80% cropland: moderate - severe erosion
10% cropland: slight - moderate erosion
highest rates in Asia, Africa, South America
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Short History of Agriculture
Post WWII: – Increase in chemical/mechanical
intensive production practices• Decrease in number of farms• Increase in size of farms
– Production of commodities/export crops• Top 5 commodities (2003)
– Cattle, dairy, corn, soybeans, broilers
– Cheap food policy• Over-production, cost-price squeeze,
consolidation of farms
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Farm CrisisSince 1980’s
• Falling prices• Spiraling overproduction• Bankruptcies, foreclosures
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1. Water erosiona. rainsplash erosion
• Raindrops accelerate as fall until they reach speed at which friction balances gravity– for large raindrops: 30 km / hr– transfer kinetic energy to soil:
• detach soil• destroy structure• transport soil (as much as 0.7 m vertically and
2 m horizontally)• Only in intense rain events; soil stays local
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b. sheet erosion
Water flows smoothly in a thin film over surface; detached soil moves with the water
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c. rill erosion
• Sheet flow concentrates water into channels
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d. gully erosion
• Water cuts deeper into soil, rills coalesce into deep troughs
• cannot (easily) be removed by tillage
• most dramatic, but most soil loss is due to sheet and rill erosion
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Universal Soil Loss Equation (USLE)
A = RKLSCPR : rainfall erosivity (intensity, quantity)
K : soil erodibility (erosion rate per unit of R; in Soil Survey)
L : slope lengthS : slope gradientC : cover and management (ratio of soil loss compared to fallow)
P : erosion-control practices
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2. Wind erosion
• Arid and semi-arid climates
• Dry soil; loss of structure; wind can remove soil particles
• Damage is on-site and off-site
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• Smallest detach into suspension (<0.1 mm)
• medium move by saltation (0.1 - 0.5 mm)
• large move by rolling and sliding (creep) > 0.5 mm
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loess
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Soil Conservation Measures
US gov’t response to Great Dust Bowl:1. SES---SCS---NRCS
2. 3000 Soil and Water Conservation Districts
3. Shelterbelt Program218 million trees
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USDA’s “tolerable soil loss”
2 - 11 metric tons / ha / yr.
(11 = 5 tons/acre/yr)
not sustainable : soil formation rate = 0.5 tons / acre/yr
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Prevention practices:
1. Windbreaks
Plant trees on windward side of crops
30 mph --> 13 mph
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2. Contour plowing
Cultivate with the contour of the slope (rather than parallel to it); lessens water runoff
3. Strip cropping
Plant strips of alternating crops.(Contour strip cropping)
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4. Terracing
on sloping land ; to check water flow
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5. Reclamation of gullies
build dams (manure and straw, concrete, stones, sticks) to collect silt; plant gully
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6. Cover crops or surface mulch (in orchards or vineyards)
protects ground surface between rows or during non-growing season
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6. Conservation tillage
eliminates or restricts tilling
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• In traditional tilling, surface soil is inverted,
• plant residue buried
• bare soil exposed
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In conservation tillage:· leave plant residue on at least 30% of
surface
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No-till:
no plowing, seeds are planted in narrow slits or directly drilled into holes
17.5% US cropland in 2000
increases need for herbicide
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Conservation methods in construction:
• schedule during low rain• work one area at a time• cover soil immediately (vegetation,
straw,etc)• control runoff to prevent gullies• trap sediment
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Open-top culvert on logging road
lead runoff water off of road
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Sedimentation pond
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Catch sediment
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bioengineering
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Riprap channelguide runoff, prevent gullies, reduce
soil loss