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 Crisis Since 1980’s Falling prices Spiraling overproduction Bankruptcies, foreclosures

1. Water erosion a. 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 = RKLSCP R : rainfall erosivity (intensity, quantity) K : soil erodibility (erosion rate per unit of R; in Soil Survey) L : slope length S : slope gradient C : 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 ( 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 factor L : width of field factor V : 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 Soil and Water Conservation Districts 3. Shelterbelt Program 218 million trees

USDA’s “tolerable soil loss” 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 channel guide runoff, prevent gullies, reduce soil loss