14. Erosion

Question 1

80% of global soil degradation is

Answer 1

erosion

Question 2

3 stage process erosion

Answer 2

1. detachment
2. transport
3. deposition (when energy runs out)

Question 3

Erodibility

Answer 3

soil properties reflecting vulnerability to dislodgement (opp of natural resistance to dislodgement)
–> high = more easily detached/transported
–>low = more resistant to erosive forces

Question 4

energy needed to dislodge individual particles

Answer 4

energy available in enviro. to drive erosion

kinetic energy (1/2 mv^2)
v= wind or water velocity

–> raindrops/surface flow = energy that can have dislodging effects (litter = protection)

Question 5

erosion

Answer 5

the movement of soil material by wind, water, or tillage implements

process that transforms soil into sediment

damages the site on which it occurs and also has undesirable effects off- site in the larger environment.

Question 6

Land degradation may be defined as a

Answer 6

reduction in the capacity of land to provide ecosystem goods and perform functions and services that support society and nature

Question 7

desertification + 3 causes

Answer 7

spreading of desertlike conditions that disrupt semiarid and arid ecosystems (including agroecosystems)

poorly managed grazing by cattle, sheep, and goats,
indiscriminate felling of rain forest trees
inappropriate agricultural practices

Question 8

soil erosion by water

Answer 8

Exposure of bare soils by tillage agriculture, deforestation, improper grazing on sloping lands in humid to semiarid regions.

Question 9

soil erosion by wind

Answer 9

Semiarid to arid regions. Disturbance of soil, vegetation, or bio-crusts by agricultural tillage and improper grazing or recreational trafficking

Question 10

soil erosion by tillage

Answer 10

Summits and slopes in hilly cultivated landscapes, especially with tillage up and downslope.

Question 11

Geological erosion

Answer 11

–>takes place naturally, without the influence of human activities, natural leveling process. It inexorably wears down hills and mountains, and through subsequent deposition of the eroded sediments, it fills in valleys, lakes, and bays.

= landforms (canyons, buttes, rounded mountain remnants, river valleys, deltas, plains, and pediments)

wears down the land slowly enough that new soil forms from the underlying rock or regolith faster than the old soil is lost from the surface.

–>soil profiles

Question 12

human-accelerated erosion

Answer 12

humankind = preeminent force on the landscape, now moving nearly twice as much soil per year as global geologic processes, and two-thirds of that unintentionally through erosion, mainly associated with agricultural activities

The amount of soil moved by human beings.

Intentional soil movement: construction and excavation activities.

Unintentional soil movement: soil loss due to agricultural activities (land clearing, tillage, overgrazing, and long periods without vegetative cover).

Humans now move more soil material than all natural processes combined.

Accelerated erosion occurs when people disturb the soil or the natural vegetation by overgrazing livestock, cutting forests for agricultural use, plowing hillsides or tearing up land for construction of roads and buildings. Accelerated erosion is often 10 to 1,000 times as destructive as geological erosion. Accelerated erosion often makes the soils in a landscape more heterogeneous

soil is commonly washed, blown, or scraped away faster than new soil can form by weathering or deposition. = reduced suitable soil depth suitable for plant roots

Question 13

Erosion and deposition occur … across a landscape

Answer 13

simultaneously

Question 14

The off-site costs of erosion relate to the …

The damage done to the soil is greater than the amount of soil lost would suggest because the soil material eroded away is almost always …

Answer 14

effects of excess water, sediment, dust, and associated chemicals on downhill, downwind, and downstream environments.

more valuable than that left behind.

Question 15

1. impaired topsoil quality:

Erosion by wind and water selectively removes…, while leaving behind mainly relatively less active, coarser fractions.

The soil left behind usually has …. (4)

Answer 15

organic matter and fine mineral particles

lower water- holding
lower cation exchange capacities, less biological activity
reduced essential nutrients+capacity to supply nutrients for plant growth.

Question 16

4 on-site damages

Answer 16

1. Impaired topsoil quality
2. spreading of plant diseases
3. deterioration of soil structure
4. Gullies

Question 17

3. deterioration of soil structure

Answer 17

= dense surface crust
= lower water infiltration
= higher water runoff

= seeds washed away
=trees uprooted
= small plants sediment buried or windblowned

Question 18

4. gullies

Answer 18

field equipment cant be used
instability of land (cant build)

Question 19

3 off-site damages

Answer 19

1. water pollution from nutrients (eutrophication)
2. water pollution from sediments (toxic metals, organic compounds, pesticides)
3. damages from sediments

Question 20

3. damages from sediments (4)

Answer 20

A. makes the water cloudy or turbid. High turbidity prevents sunlight from penetrating the water and thus reduces photosynthesis and survival of the submerged aquatic vegetation (SAV). = degrades the fish habitat and upsets the aquatic food chain.

B. The buildup of bottom sediments = raise the level of the river= flooding becomes more frequent and more severe.

=disastrous effect on many freshwater fish spawning-sites (burying the pebbles and rocks among which they normally spawn)

C. filled reservoirs (lowered capacity to storage water for irrigation/municipal water systems + hydroelectric generation)

D. shipping channels impassible = $$$$$$

Question 21

Rainfall erosivity

Answer 21

humid tropics= annual rainfall high and intense storms = high value

Northern Europe =rain mostly falls gently over long periods = lower

Question 22

3 off-site damages of wind erosion

Answer 22

A.bury roads and fill in drainage ditches, necessitating expensive maintenance.

B. sandblasting effect of wind-borne soil particles may damage the fruits and foliage of crops in neighboring fields

C. health hazards
Finer windblown dust with clay-size particles causes the most expensive and far-reaching damages, (inhale very fine windblown particles,
fine fugitive dust = health hazard)

windblown dust is a global problem. wind erosion in the Sahara desert in Africa and Gobi desert in China has been implicated in respiratory diseases in North America.

Question 23

Although extreme soil erosion can eventually reduce soil productivity to almost zero (as when only exposed rock remains), in most cases the effect is too ….Where farmers can afford to do so, they compensate for the loss of nutrients by ….

The losses of … (3) are much more difficult to overcome, though irrigation may partially do so.

Answer 23

subtle to notice between one year and the next.

increasing the use of fertilizer.

organic matter, rooting depth, and water-holding capacity

Question 24

Ultimately, the rate of decline of soil productivity = the cost of maintaining constant food production levels, is determined by such soil properties as (…2)

Answer 24

depth to a root-restricting layer + permeability (or chemical favorability) of the subsoil.

–> shallow, low-permeability soil = +rapid productivity decline

–> deep, permeable, well-drained/mgmt, not as much decline

Question 25

most erosion is initiated by the

Answer 25

impact of raindrops, rather than the flow of running water.

conservation efforts :
protecting the soil surface from the impact of raindrops»»controlling the more visible flow of water across the land,

Question 26

Raindrop impact exerts 3 important detrimental effects:

Answer 26

(1) it detaches soil;

(2) it destroys granulation;

(3) its splash, can cause an appreciable transportation of soil.

Question 27

If the rate of rainfall exceeds the soil’s infiltration capacity, water will …

Sheet flow: flowing smoothly in a thin layer (sheet flow), =…. power to detach soil.

irregularities in the soil surface = channelized flow = increase in both velocity and turbulence:

Answer 27

running downslope.

little
–>flowing water carry detached soil particles from raindrops down the slope.

—> carries soil splashed by raindrops + detach particles as it cuts into the soil mass.
–> accelerating process, for as a channel is cut deeper, it fills with greater and greater volumes of flowing water

Question 28

3 types water erosion

All three types may be serious, but …, although less noticeable than …. are responsible for most of the soil moved.

Answer 28

Sheet = pedastals–> raindrop impacts
Rill–> overland flow
Gully–> more water, deeper, more energy available to erode soil

sheet and rill erosion

gully erosion

Question 29

Land managers and policymakers need to predict the extent of soil erosion in order to plan the best …, evaluate consequences of ….practices on a farm, determine compliance with environmental regulations, develop sediment-control plans for construction projects, and estimate the years it will take to silt-in a reservoir.

Answer 29

management of soil resources

alternative tillage

Question 30

The detachment, transport, and deposition processes of soil erosion can be predicted mathematically by ….. These are equations—or sets of linked equations—that interrelate information about the ..(4). of a site with the amount of soil likely to be lost by erosion.

Answer 30

soil erosion models

rainfall, soil, topography, vegetation, and management

Question 31

3 models to predict water-induced soil erosion

Answer 31

Process-based:

1. WEPP
   water erosion prediction project
2. SWAT
   soil and water assessment tool

Factor based:
3. USLE + revised USLE

Question 32

what cant be predicted by USLE

Answer 32

erosion from a specific year or storm

extent of gully erosion and sediment delivery downslope or to streams

not for channel, snowmelt, wind

poor in wildland situations

–> applicable to sheet + rill erosion of CULTIVATED land

Question 33

WEPP

Answer 33

process-based model for water erosion

distributed model (not lumped)

daily time steps, changing factors (erodibility, plant cover/ height/biomass…)

applies to all types of water erosion (hillslope, gully, snowmelt)

Question 34

USLE Equation + factors

Answer 34

A = RK(LS)C*P

A= soil loss (erosion) in (mass/unit area)

R= rainfall erosivity (energy available)

K= soil erodibility (vulnerability to erosion)
–> result of soil aggregation
–> determined by texture (p.s.d), clay%, o.m. content, soil structure

L= slope length (ratio factor)
–> of uninterrupted slope being drained

S= slope gradient

C= cover factor (ratio of erosion reduction)

P= erosion control structure (reduction ratio)
–> contour ridges, strip cropping

Question 35

RUSLE

Answer 35

VM (vegetation management) instead of C + P

incorporate C+P in 3 sub-factors:
a. canopy cover
b. effects of low-growing vegetation
c.bare ground with fine root network

+

Unlike the USLE, the RUSLE takes into account interactions between support practices and subfactors such as slope and soil water infiltration.

Question 36

The rainfall erosivity factor, R, represents :

Answer 36

driving force for sheet and rill erosion

considers total annual rainfall + rain intensity +seasonal distribution

index of the kinetic energy of each storm is calculated from data about intensity + amount of rainfall.

All indices added up for all storms in 1 year = annual index
–> average for many years used as R-value in USLE

–> little variation in arid parts of the western United States, North Africa, and Central Asia

–>humid tropics = high variations:
rainfall tends to be more intense and more erosive in subtropical and tropical regions than in temperate regions.

Question 37

The soil erodibility factor, K, indicates:

Answer 37

soil’s inherent susceptibility to erosion.
–>for particular type of soil

indicates the amount of soil lost per unit of erosive energy in the rainfall

Erodibility influence factors:
(1) infiltration capacity (2) structural stability.

High infiltration = less water available for runoff=surface is less likely to be ponded= can more susceptible to splashing.

Stable soil aggregates resist the beating action of rain + detachment
–> tropical clay soils high in hydrous oxides of iron and aluminum =highly stable aggregates
–> swelling-type clays = NOT

Question 38

The topographic factor, LS, reflects :

Answer 38

influence of length and steepness of slope on soil erosion.

unitless ratio:
soil loss from 1 area / soil loss of standard plot

longer the slope, the greater the opportunity for accumulation and concentration of the runoff water.

Question 39

….(2) are markedly affected by different types of vegetative cover or cropping systems.

….(2) provide the best soil protection and are about equal in their effectiveness.

Even in semiarid regions unable to support dense vegetation, increasing the density of trees and shrubs to cover more than … of the soil surface can dramatically reduce the loss of both soil and water in runoff.

On agricultural land, ….. are most effective in staunching erosion because of their relatively dense cover.

Answer 39

Erosion and runoff

Undisturbed forests and dense grass

one-third to one-half

forage crops (both legumes and grasses)

Question 40

The C factor in the USLE or RUSLE

Answer 40

ratio of soil loss under the conditions in question to that which would occur under continuously bare soil.

indicates ratios of soil eroded from a particular vegetation system to that expected if the soil were kept completely bare.

The C values are situation specific and must be calculated from local information on plant growth habits, climate, canopy cover, tillage, crop rotation…etc.

little soil cover:
ratio C will approach 1.0 (e.g., a bare seedbed in the spring or freshly graded bare soil on a construction site).

lots of plant residues left/dense perennial vegetation:
low (e.g., <0.10)

Question 41

support practices:

P factor:

Answer 41

construction of physical structures or other steps aimed at guiding and slowing the flow of runoff water.

they determine the value of the P factor in the USLE.

the ratio of soil loss with a given support practice to the corresponding loss if row crops were planted up and down the slope.

no support practices:
P factor = 1.0.

Many of the erosion-control practices or management techniques discussed with regard to the C and P factors and in later sections of this chapter are considered to be best management practices (BMPs).

Question 42

The support practices include :….

all of which will tend to reduce the P factor.

Answer 42

1. tillage on the contour (contour cultivation)
2. contour strip-cropping
3. terrace systems
4. grassed waterways,

Question 43

conservation tillage are systems that leave at least… % of crop residues on soil surface

Well-managed continuous no-till systems in humid regions include …during the winter and …. in the rotation. Such systems keep the soil nearly … covered at all times and build up organic surface layers somewhat like those found in forested soils.

Answer 43

30%

cover crops

high-residue- producing crops

100%

Question 44

Benefits of conservation tillage systems on soil properties (5)

Answer 44

1. improves o.m.

2.improves aggregate stability
–> increased macroporosity and aggregation as o.m builds up
–> no-till = more biopore developments promoting more rapid infiltration, –> could = more rapid leaching of nitrates

3. improved Ks
4. improved water-holding capacity
5. improved microbe populations
   –> increased abundance, diversity, activity of soil organisms

all of which = less erosion, less surface runoff (= lower C values)

Question 45

vegetative barries

Answer 45

Vegetative barriers create natural terraces

Narrow rows of permanent vegetation (usually grasses or shrubs) planted on the contour can be used to slow down runoff, trap sediment, and eventually build up “natural” or “living” terraces.

deep-rooted grass plants have dense, stiff stems that tend to filter out soil particles from muddy runoff and catch soil thrown downslope by tillage. This sediment and soil accumulates on the upslope side of the grass barrier and, in time, actually creates a terrace that may be more than 1 m above the soil surface on the downslope side of the plants.
Narrow grass hedges, buffer strips, winter cover crops = effectively reduced runoff = can improve crop production in the short term as well as protect soil and water quality in the longer term.

=
reduced erosion and increased crop yields

Question 46

mass wasting

Answer 46

downhill movement of large masses of unstable soil

moves a much thicker layer of soil and regolith materials all at once.
(NOT = erosion of the soil surface)

–> commonly occurs on very steep slopes (usually greater than 50% slope).
–> most common on nonagricultural land

Gradual process:
a. Soil creep: slow deformation (without shear failure) of the soil profile as the upper layers move imperceptibly downhill.
–> common sign of soil creep= curved trunks (trees attempting to right themselves)

b. Mud flows: partial liquefaction and moderately rapid flow of saturated soil due to loss of cohesion between particles

Sudden process:
c. Landslides: sudden shear failure, usually under very wet conditions, causes the rapid downhill movement of a mass of soil

sometimes triggered by human activities that undermine natural stabilizing forces or cause the soil to become water saturated as a result of concentrated water flow

Question 47

Erosion in rangelands

Answer 47

lose large amounts of soil under natural conditions. greater risk of erosion in drier landscapes

–>humans = accelerated erosion =even greater losses

Overgrazing by cattle, =deterioration of the vegetative cover on rangelands
–> increases exposed ground, disturbance, compaction + chelling of water

Grass cover, vegetation mgmt (seeding, cover, terraces retaining water on site)

Because of the prevalence of dry conditions, wind erosion = major role in the deterioration of rangeland soils.

Question 48

erosion in forested lands

Answer 48

healthy, undisturbed forests loses very small amounts of soil.

–>accelerated erosion due to:
higher rates of soil loss + higher amount of land involved

Disturbances: roads + harvesting

forest floor (NOT tree canopy /roots) protects the soil from erosion
–> leaf mulch on the forest floor provides most of the protection against erosion
–> leafless canopy wont intercept rain as much

forest floor disturbed+mineral soil exposed= splash erosion
–>larger drops dripping from taller trees, quicker, more energy, bigger impact on ground (than direct rain from even intense storms)

water is concentrated (by unplanned footpaths or poorly designed roads) = gully erosion

Question 49

Roads = destabilization, exposed surfaces, drains

how mitigate?

Harvesting mitigation?

Answer 49

minimize road length/use

avoid unstable areas

erosion control measures

apply USLE principles; cover, LS, erosivity
in
path design/use + repair/rehabilitation

Question 50

The main sources of eroded soil from timber production are (3)

Strategies to control erosion should include consideration of (4)

Answer 50

logging roads (built to provide access to the area by trucks)

skid trails (the paths along which logs are dragged),

yarding areas (the places where collected logs are sized and loaded onto trucks).

(1) intensity of timber harvest

(2) methods used to remove logs

(3) scheduling of timber harvests

(4) design and management of roads and trails. Soil disturbance in preparation for tree regeneration = should be limited to sites with low susceptibility to erosion.

Question 51

When forests are harvested, …. as wide as 1.5 times the height of the tallest trees should generally be left untouched along all streams

Answer 51

buffer strips

–> high capacity to remove sediment and nutrients from runoff water.

protect the stream from excessive logging debris.

shade the water, protecting it from the undesirable heating that would result from exposure to direct sunlight.

Question 52

The goals of erosion control on construction sites are (2)

Answer 52

(1) to avoid on-site damage, such as undercutting of foundations or finished grades and loss of topsoil needed for eventual landscaping

(2) to retain eroded sediment on-site so as to avoid environmental damages (and liabilities)

Question 53

Soils freshly disturbed by excavation or grading operations are characterized by very high ….. This is especially true for low-organic-matter subsoil materials. Potential erosion can be extremely high unless the C value is made very low by providing good ….. This is best accomplished by

Answer 53

erodibility (K values)

soil cover

allowing the natural vegetation to remain undisturbed for as long as possible, rather than clearing and grading the entire project area

Question 54

erosion blankets

Answer 54

Seeded areas should be covered with mulch or specially manufactured erosion mat or blankets : made of various biodegradable or nonbiodegradable materials, provide instant soil cover, protect the seed from being washed away, and are highly effective at reducing soil erosion

Question 55

A commonly used technology to protect steep slopes and areas difficult to access, such as road cuts, is the hydroseeder :

Answer 55

that sprays out a mixture of seed, fertilizer, lime, mulching material, and sticky polymers.

Question 56

wind erosion

Answer 56

wind erosion is most serious in dry regions and water erosion in wet regions (lacks vegetation, large bare areas)

Overgrazing (arid regions) =destroys the protective biological soil crusts + native vegetation.

Tillage for dryland crop production also dries out and lays bare soil, = much more vulnerable to the wind than when the native vegetation was undisturbed

when strong winds blow across soils with relatively dry surface layers.

–> silty and fine sandy soils generally are most quick to “blow.” carried forever

–>Sand particles =pile up in dunes,

=
-widespread damage to the vegetation and soils of the eroding site, but also to

-anything that can be damaged by the abrasiveness of soil-laden wind, and finally to the

-off-site area where the eroded soil material settles back to earth

dust storms: drastic off-site effects on air quality and dust deposition

existing soil and plants are covered by deposits that are quite unproductive because the soil structure has been destroyed

Question 57

…is the main variable driving the vulnerabilities to water or wind erosion, with additional influences of …(3)

Answer 57

Climate

topography, soil properties, and vegetation.

Question 58

conservation management practices are those that improve soil health in more ways than just by protecting the soil from erosion. Improved soil health, in turn, enhances the soil’s capacity to support plants, resist erosion, prevent environmental contamination, and conserve water. Conversation management therefore can lead to the upward spiral of soil and environmental improvements.

examples (6)

Answer 58

Soil properties enhanced (soil quality) with:

minimizing tillage,

maximizing residue

cover of the soil surface,

providing for diversity of plant types,

keeping soil under grass sod vegetation for at least part of the time,

adding organic amendments where practical,

maintaining balanced soil fertility.

Question 59

The deposited material has little value as productive soil because transport by wind and water destroys … segregates out the various particle sizes, and often removes organic matter and nutrients.

In contrast, the material moved downslope by tillage is still productive soil when it …. That is, the soil eroded by tillage is not mere sediment, but generally retains most of the desirable physical, chemical, and biological properties of a productive soil. Therefore, where movement by tillage is the main erosion process, the potential may exist to remediate the erosion damage using mechanized land scrapers that can efficiently move many megagrams per hectare of eroded soil back upslope to restore the thickness of productive soil on the hilltops

Answer 59

soil structure,

accumulates in the low-lying, concave parts of the landscape

Question 60

Barriers or windbreaks: reducing wind velocities for short distances and for trapping drifting soil.

Answer 60

tree shelter belts
Tree windbreaks
rows of tenacious shrubs
Rows of grasses → microwindbreaks.
Narrow rows of perennial grasses
picket fences/burlap screens,= less efficient, but can be moved from place to place as crops and cropping practices are varied.

Question 61

wind-erosion control methods (3) + 1 general

Answer 61

1. tillage
2. barriers
3. soil cover

–> increased soil moisture = increased soil cohesiveness so need higher winder speed to be able to detach soil particles

Question 62

Although the wind itself has some direct influence in picking up fine soil, the …is probably more important.

Answer 62

impact of wind-carried particles as they strike the soil (saltation)

Question 63

Wet soils do not blow because of the ….. Dry winds generally lower the …. to below the wilting point before wind erosion takes place.

Answer 63

adhesion between water and soil particles

soil moisture content

Question 64

wind erosion works in conjunction with water driven erosion ex:

Answer 64

post-glacial redistribution of meltwater deposits/runoff

glaciofluvial deposits

Question 65

3 step process of wind erosion

Answer 65

1. detachment (wind + other particles)
2. transport:
   a. saltation; jumping of medium particles along surface

b. soil creep

c. suspension; carried horiz + upwards

–> wind erosion limited to <1 mm particles (silt, fine sands)

3. deposition

Question 66

wind erosion equation

Answer 66

E = f(I KCLV)

I; erodibility factor (particle mass + cohesion)

K; roughness

C; climatic factor

L; unsheltered median travel distance

V; vegetative cover factor

Question 67

4 factors affecting wind erosion

Answer 67

1. wind velocity + turbulence
2. surface roughness
3. soil properties
   a. soil moisture
   b.stability aggregates
   c. stability crust
   d. bulk density
   e. size of erodible fraction

–> soil with enough clay, o.m. (good aggregation) will be less available to wind erosion

–> crust = broken aggregates, pores blocking by detached soil particles

4. vegetation

–> slope NOT a factor, but fetch (uninterrupted length) is (wind uninterrupted so build momentum, speed)

–> combining factors (bare, no vegetation, sediment accumulation, dry = more vulnerable)

Question 68

4 wind + water erosion ways to measure

Answer 68

1. reservoir surveys in streams (of deposition)
2. measurements of suspended solids, bedloads in streams
3. plots; walled plots lead to collector for eroded material
4. erosion stakes/pins, pedestals, erosion scars, lichen on rocks
   –>at smaller scale

Question 69

erosion control + conservation planning (2 steps)

Answer 69

1. determine acceptable level of erosion (Te)
   –> range t/ha/yr
   –> not exceed natural soil forming rates
   –> integrate vulnerability to irreversible damages
2. Determine degree of protection needed to keep losses within acceptable limits
   –> CP= Te/RKLS

overall: anything to keep water and increase infiltration of water into soil

Question 70

general erosion prevention (3)

Answer 70

1. prevent inappropriate use of risk areas
   –> risk areas = slopping ground, low permeability, vegetation prone to denudation
2. maintain vegetative cover (soil cover)
3. consider factors in USLE (reduce energy)
   –> use those factors to build risk profile for particular location + to make erosion control plan
   –> understand how those factors change with disturbance/land-use + how change erosion

Question 71

Cropping management factor (C)

Answer 71

integrates
vegetation cover
plant litter
soil surface
land mgmt

reflects exposure of site to erosive forces (lack of protective canopy and ground cover)

correlates with mass of dry o.m/unit area or % ground cover

Question 72

ways to manage water on slopes

Answer 72

1. drainage
   -> landscape scale
   –> medium scale (cross drains)
2. back country roads

3.hydromulch/hydroseeding