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
( 0.5 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
2. 3000 Soil and Water Conservation
Districts
3. Shelterbelt Program
218 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 channel
guide runoff, prevent gullies, reduce soil
loss