Soil Erosion Flashcards
(35 cards)
1
Q
Geological Erosion
A
Natural, not human induced
2
Q
Accelerated Erosion
A
Not natural, induced by human activities
-10 to 1000 time as destructive as geological erosion
3
Q
Estimated rates of erosion on agricultural lands:
A
Africa, Asia, South America: 30-40 Mg/ha/yr
United States, Europe: ~12 Mg/ha/yr
Undisturbed humid region forest and grassland: <0.1 Mg/ha/yr
4
Q
About 4 billion Mg of soil is moved annually by erosion in the United States
A
- 2/3 by water, 1/3 by wind
- Somewhat more than ½ occurs on cropland
- The remainder from rangeland, timber harvest, construction sites
5
Q
Erosion damage: On-site
Loss of soil itself
A
- Lose topsoil
- Selective removal of organic matter, fine particles, nutrients Enrichment ratio. Eroded sediments are enriched in N, P, K, and OM relative to soil from which they eroded.
- Remaining soil has lower infiltration rates and water holding capacity
- Gullies make equipment operation difficult, undercut roads, buildings
6
Q
Erosion damage: Off-site
A
- Nutrient and pesticide transport degrades water quality
- Sediments
- May smother crops, other low-lying vegetation
- Fill in drainage ways, cause flooding, reduce reservoir capacity, require dredging
- Degrade stream beds
- Cause turbidity
7
Q
Economic costs of erosion
A
- In the USA
- Annual on-site erosion costs in USA are estimated to be between $8 and $40 billion
- Annual off-site costs due to erosion estimated at $10 to $30 billion
- Costs (both human and economic) are much higher in developing countries with fewer resources to devote to conserving soil resources and restoring losses.
8
Q
Tolerable soil losses
A
T value – the maximum amount of soil that can be lost annually by erosion on a particular soil without degrading its long-term productivity.
- T values for most agricultural soils in the USA are 5 – 11 Mg/ha
- 11 Mg/ha is ~0.9 mm of soil/yr (about 225 years to lose an entire Ap horizon)
- 33% of cropland in the USA is eroding at rates greater than its assigned T value
9
Q
Mechanics of water erosion
A
- Detachment
- Transport
- Deposition
-Raindrop splash
10
Q
Detachment
Mechanics of water erosion
A
of soil particles from the soil mass
11
Q
Transport
Mechanics of water erosion
A
of detached particles downhill
12
Q
Deposition
Mechanics of water erosion
A
of transported soil particles as sediment
13
Q
Raindrop splash effects
Mechanics of water erosion
A
- Raindrops kinetic energy
- Detaches soil particles
- Destroys granulation
- Transports soil
- Suspends soil particles in water, contributing to further transport
14
Q
Role of running water
A
- When precipitation rate exceeds infiltration rate water begins to flow on the soil surface
- Particles transported by raindrop splash fall in running water which carries them further downslope
- Sheet flow of water has little ability to loosen soil particles and initiate erosion
- Channelized water flow increases velocity, turbulence, and energy to detach more soil particles, digging deeper channels …
15
Q
Types of water erosion
A
- Sheet erosion
- Rill erosion
- Interrill erosion
- Gully erosion
16
Q
Sheet erosion
Types of water erosion
A
-More or less uniform movement of soil from the entire surface
17
Q
Rill erosion
Types of water erosion
A
-Sheet flow begins to concentrate in small channels or rills
18
Q
Interrill
Types of water erosion
A
-Sheet erosion between regularly spaced rills
19
Q
Gully erosion
Types of water erosion
A
-Concentration of rills into deep, large channels that cannot be easily removed by tillage
20
Q
Deposition of eroded soil
A
Eroded sediments may travel a few meters or thousands of kilometers
21
Q
Delivery Ratio
A
- The amount of soil delivered to a stream divided by the amount of soil eroded
- Delivery ratio is large where valley slopes are steep (up to 60%)
- Delivery ratio is small on gently sloping landscapes
- Delivery ratio tends to be smaller in larger watersheds because there is more opportunity for deposition before a stream is reached
- In North America it is estimated that about 5 – 10% of eroded soils are transported to oceans, the remainder is deposited in rivers, streams, flood plains, lakes, and reservoirs.
22
Q
Water Erosion Prediction Models
A
Universal Soil Loss Equation (USLE) A = R • K • LS • C • P A = predicted soil loss R = rainfall erosivity K = soil erodibility L = slope length S = slope gradient or steepness C = cover and management P = erosion control practices
Predicts the amount of sheet and rill erosion lost from a specific site in an average year
Cannot predict erosion from a specific year or storm
Cannot predict gully erosion
23
Q
History
A
- In 1954, W.H. Wischmer established the USLE
- Goal was to develop an erosion prediction equation compatible with data from U.S.
- Incorporates several factors
- USLE database started in the 1930s contains information that can be used to calculate USLE
- Maintained by the USDA
24
Q
R – rainfall erosivity
A
- The driving force for sheet and rill erosion
- Based on total rainfall, intensity, and seasonal distribution
- Rainfall intensity important because
- More intense rain has larger drop size, greater kinetic energy
- Higher rate of rainfall leads to more runoff and more transport of detached soil particles
- Actual storm erosivity can vary widely from long-term averages
25
K – soil erodibility
- Indicates the inherent susceptibility to erosion of a soil kept bare by tillage
- The amount of soil lost per unit of erosive energy in rainfall
- Two soil characteristics strongly influence K
- Infiltration capacity
- Structural stability
- Soils high in silt and very fine sand or expansive clays tend to have high K values
- Soils high in OM, non-expansive clays and with strong granular structure have low K values
26
Centre County Soils
Series Texture K-factor
Hagerstown Silt loam 0.31
Leetonia Loamy sand 0.17
Morrison Sandy loam 0.20
27
LS – the topographic factor
L – slope length
S – slope steepness
-Unitless ratio of soil loss from area in question to loss from standard plot (9% slope, 22 m long)
28
C – cover and management factor
-Extent and type of vegetative cover and plant residue cover
-C is a unitless ratio of soil loss under the conditions in question to soil loss from a continuously bare soil
1 on bare soil
<0.1 in perennial vegetation or significant residue cover
29
Residue cover and erosion losses
- Even 25% residue cover makes a significant impact on protecting against erosion
- 100% residue cover, erosion is basically 0
30
Effect of residue cover on water infiltration rate
- Covered soil infiltration rate stays relatively stable as cumulative rain fall increases
- As cumulative rainfall increases, infiltration rate decreases for uncovered soil
31
Residue management and tillage
| Conventional tillage
- Primary tillage (plowing) to invert soil and bury weeds and crop residue
- Main implement for primary tillage has been the moldboard plow.
- Secondary tillage (disking or harrowing) to break up soil clods and prepare a suitable seed bed. May make multiple passes with these implements.
- Tertiary tillage. A field cultivator may be used to further work and smooth the soil. Cultivators may also be used after planting the crop remove weeds between crop rows.
32
P – erosion control practices
- A unitless ratio of soil loss with a given erosion control practice to soil if row crops were planted up and down the slope.
- With no control practices P = 1
- Practices include
- Contour tillage
- Contour strip cropping
- Terrace systems
- Grassed waterways
33
Example of USLE
Hagerstown silty clay loam in State College, PA: T = 9 Mg/ha (4 T/acre)
R = 2128 (divide by 17.02 to convert to English units)
K = 0.041 (multiply by 7.6 to convert to English units)
-Slope gradient 4%, slope length 50 m
LS = 0.62
-Maize-oats-hay-hay rotation, conventional tillage (spring before planting)
C = 0.05
-Contour strip cropping
Contour P factor = 0.5, Strip P factor = 0.25
P = (0.5 x 0.25) = 0.125
```
A = R x K x LS x C x P
A = 2128 x 0.041 x 0.62 x 0.05 x 0.125
A = 0.34 Mg/ha y (0.15 T/acre)
```
Corn soybean rotation with fall plowing and no strips or contours
C increases to 0.53 and P increases to 1
A increases to 29 Mg/ha (12.8 T/ha)
34
Erosion on Forestlands
- Low erosion in forest soils is due to the O horizon and litter layer (not canopy cover or roots)
- Any management that exposes soil to raindrop impact will increase erosion
- Main accelerators of erosion are
- Logging roads and skid trails
- Intensity of timber harvest
- Erosion control measures
- Well designed and maintained roads
- Timing/intensity of harvest
- Timber removal methods
35
Wind Erosion
- Wind is the erosive agent
- Moving air detaches some particles
- Abrasive power of moving air increases greatly as it becomes more laden with soil particles
- Mechanics of wind erosion
- Saltation. Movement of particles in series of short bounces along the ground (0.1-0.5 mm particles)
- Soil creep. Larger particles or aggregates rolling and sliding along the surface. (up to 1mm)
- Suspension. Fine particles carried upward (meters to kilometers) into the atmosphere and transported hundreds of kilmeters (<0.1mm)
