Soil Erosion and Conservation. “Erosion” a natural leveling process that wears down high places;...
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Transcript of Soil Erosion and Conservation. “Erosion” a natural leveling process that wears down high places;...
Soil Erosion and Conservation
“Erosion”
• a natural leveling process that wears down high places; fills in low places
• agents: running water, ice, wind, gravity, waves
“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
• Deposition or sedimentation is flip side of erosion.
• the soil that is removed has to go somewhere: wetlands, lakes, streams, atmosphere
Minnesota
154 million tons of topsoil / year
96% cropland
water erosion 42%
wind erosion 58%
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
United States• In past 200 yrs, 30% of US
farmlands have been abandoned due to erosion, salinization and waterlogging
• wind erosion increasing• water erosion decreasing• 90% US cropland losing soil above
sustainable rate• croplands: lose 17 tons/ha/yr• pastures: lose 6 tons/ha/yr
In U.S….
• In past 50 yrs, average farm size change:– 90 to 190 ha (225 to 475 acres)
• to create larger fields: remove shelterbelts, grass strips, hedgerows
• use of heavier machinery damages soil
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
Farm CrisisSince 1980’s
• Falling prices• Spiraling overproduction• Bankruptcies, foreclosures
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
b. sheet erosion
Water flows smoothly in a thin film over surface; detached soil moves with the water
c. rill erosion
• Sheet flow concentrates water into channels
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
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
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
• 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
Wind Erosion Model (WEQ)
E = ƒ( ICKLV)I : soil erodibility (slope angle, soil
moisture, structural stability)C : climate factor (wind speed , soil
temp., ppt.)K : roughness factorL : width of field factorV : vegetative cover
Colorado 1935
S. Dakota 1936
Rhode Island
35 mph wind
loess
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
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
Prevention practices:
1. Windbreaks
Plant trees on windward side of crops
30 mph --> 13 mph
Senegal
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)
4. Terracing
on sloping land ; to check water flow
Inca
Bolivia
5. Reclamation of gullies
build dams (manure and straw, concrete, stones, sticks) to collect silt; plant gully
6. Cover crops or surface mulch (in orchards or vineyards)
protects ground surface between rows or during non-growing season
6. Conservation tillage
eliminates or restricts tilling
• In traditional tilling, surface soil is inverted,
• plant residue buried
• bare soil exposed
In conservation tillage:· leave plant residue on at least 30% of
surface
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
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
Open-top culvert on logging road
lead runoff water off of road
Sedimentation pond
Catch sediment
bioengineering
Riprap channelguide runoff, prevent gullies, reduce
soil loss