The final exam

Mon, 8p WGND 115 Comprehensive “Essentials of CSS 305” on BB Don’t forget calculator! Review Exams 1 and 2 and labs

and homework assignments (especially practice problems)

Erosion:• A process that transforms soil into sediment

– Natural = geologic erosion: weathering means soil formation usually > soil loss

– Human-induced • over-grazing – 1/3 of all land degradation, • forest harvest – in rain forests, bad practices

responsible for 0.5 b ha of land degradation…so far

• Tied with damage to plant communities (increasing susceptibility to erosion)

• 85% of degradation of soils is due to the destructive action of wind & water (2/3 of that is by the action of water)

Estimated cost in US $9-$44 billion annually!!!

Geologic erosion - Geologic erosion - greatest in greatest in semi-aridsemi-arid environments environments

Soil degradationSoil degradation •Physical properties: Physical properties: compaction, crustingcompaction, crusting

•Chemical properties: Chemical properties: acidification, salt acidification, salt accumulationaccumulation

•Erosion (loss): wind & Erosion (loss): wind & waterwater

Downward spiral of land degradation

Degraded land poor crops human poverty reduced protection of soil resources increased erosion desperate people clear, cultivate, & degrade more land

Volcanic PM, equatorial climate… Easter Island -

1st settlers – 400ad – rich in plant and animal resourcesBuilding of moai (statues)by 1400 – the two native trees (one palm one woody) were extinctRapid and catastrophic loss of forest nutrients and soil erosionConsumed the birds & fisheries till they ran outAte rats to survive – cannibalism emerges – societal breakdownAbandon villages – moved to caves The most total environmental and social breakdown in recorded history

Factors affecting rates of erosion

• Topography

• Land use practices

• Vegetation type

• Rainfall amount, frequency, and intensity

• Soil chemical properties (high CEC = more plant cover = less erosion)

Factoid:Approximately 53% of all wood consumed globally is used for home heating and cooking. (Bowyer et. al., Forest Products and Wood Science: An Introduction. Iowa State Press 2003. p.ix)

Effect of agriculture

Fig 17.5, Brady & Weil

health hazard too!

Erosion mechanics

Fig. 13.8, p. 333

Most erosion is initiated by the Most erosion is initiated by the impact of raindrops, impact of raindrops, NOTNOT by by the flow of running waterthe flow of running water

1. Detachment2. Transport3. Deposition

Factoid: large raindrops fall at 30 km/hr !

USLE - Universal Soil Loss Equation

• R= rainfall erosivity

• K= soil erodibility

• L = slope length

• S = slope steepness

• C = cover and management

• P = erosion-control practices

A = RKLSCP

In 1990s In 1990s RRevised (evised (RRUSLEUSLE) computer-based, more ) computer-based, more data, improved more sophisticated modelingdata, improved more sophisticated modeling

See Table 17.3 of Brady & Weil for detailsSee Table 17.3 of Brady & Weil for details

Soil-related factors

Land management factors

K: soil erodibility factor

• Infiltration capacity

• Structural stability (next slide)

A = RKLSCP

Time

Infilt

rati

on r

ate

Ksat

Rain intensity

Excess water(runoff)(c

m/h

r)

K: soil erodibility factor

• Infiltration capacity (previous slide)

• Structural stability

– Particle cohesion• frictional resistance (block vs. sphere)

• cementation by OM, clays

– Particle mass (2.0 g/cm3 for bauxite, 5.3 for hematite, 7.6 for galena)

A = RKLSCP

• High organic matter content

• Non-expansive clays

• Strong granular structure

• “stoniness” & macropores

Soil properties resulting in low K values (less erosion)

Practices that minimize erosion

1. Mulching - C

2. Contour cultivation – C, L & S

3. Grass contour hedges - L

A = RKLSCP

L = slope lengthS = slope steepnessC = cover and management

1. Mulch as a means to reduce erosion

C =cover and management

A = RKLSCP

Effects of mulching, contour cultivation, and grass hedges

Practice Reduction in soil erosion

(%)

Increase in crop yield (%)

Mulching 88 98

Contour cultivation

46 36

Grass contour hedges

55 56

A few practices to reduce soil loss caused by timber production

• Tree removal: cable not skidder

• Scheduling: when dry or frozen. snow great.

• Road design: 99% of soil loss avoided by gravel, planting grasses on road cuts

• Buffer strips: 1.5 times the height of the tallest trees

Wind erosion

• 40% of eroded soil is transported by wind erosion in USA

• In six of the Great Plains states, wind erosion exceeds water erosion

• Fine particles (aeolian dust) can even be transported to other continents (Saharan sands found in Hawaii)

Wind erosion mechanicsWind erosion mechanics 1.1. DetachmentDetachment2.2. TransportTransport3.3. DepositionDeposition

Surface creep, saltation, suspension.

Dust Bowl – 1930’s United States, great plains

Factors affecting wind erosion• Wind velocity

> 25 km/h to get started – After which soil movement is proportional to the cube of the wind velocity

• Wind turbulence – though it’s the airborne particles that do most of the abrasion/erosion

• Surface roughness - decreases wind erosion• Vegetation - decreases wind erosion• Soil properties – Aggregate & surface crust

stability, bulk densityNaturally, there is Naturally, there is a wind erosion a wind erosion prediction prediction equation . . .equation . . .

E = ICKLVI – soil erodibilityC – climateK - soil roughnessL – field widthV – veg cover

Controls of wind erosion• Add moisture: often not practical• Conservation tillage

– stubble mulch – strip-cropping perpendicular to the wind – if tilling, do so when soils moist, not dry

• Windbreaks: trees best, but even cereal crops can help!

Coming full circle . . .

Soil is:

A natural, evolutionary body that is the product of the

environment under which it develops…

The real voyage of discovery consists not in seeking new landscapesbut in having new eyes. ~ Marcel Proust