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