14. Erosion Flashcards
80% of global soil degradation is
erosion
3 stage process erosion
- detachment
- transport
- deposition (when energy runs out)
Erodibility
soil properties reflecting vulnerability to dislodgement (opp of natural resistance to dislodgement)
–> high = more easily detached/transported
–>low = more resistant to erosive forces
energy needed to dislodge individual particles
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)
erosion
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.
Land degradation may be defined as a
reduction in the capacity of land to provide ecosystem goods and perform functions and services that support society and nature
desertification + 3 causes
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
soil erosion by water
Exposure of bare soils by tillage agriculture, deforestation, improper grazing on sloping lands in humid to semiarid regions.
soil erosion by wind
Semiarid to arid regions. Disturbance of soil, vegetation, or bio-crusts by agricultural tillage and improper grazing or recreational trafficking
soil erosion by tillage
Summits and slopes in hilly cultivated landscapes, especially with tillage up and downslope.
Geological erosion
–>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
human-accelerated erosion
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
Erosion and deposition occur … across a landscape
simultaneously
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 …
effects of excess water, sediment, dust, and associated chemicals on downhill, downwind, and downstream environments.
more valuable than that left behind.
- 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)
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.
4 on-site damages
- Impaired topsoil quality
- spreading of plant diseases
- deterioration of soil structure
- Gullies
- deterioration of soil structure
= dense surface crust
= lower water infiltration
= higher water runoff
= seeds washed away
=trees uprooted
= small plants sediment buried or windblowned
- gullies
field equipment cant be used
instability of land (cant build)
3 off-site damages
- water pollution from nutrients (eutrophication)
- water pollution from sediments (toxic metals, organic compounds, pesticides)
- damages from sediments
- damages from sediments (4)
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 = $$$$$$
Rainfall erosivity
humid tropics= annual rainfall high and intense storms = high value
Northern Europe =rain mostly falls gently over long periods = lower
3 off-site damages of wind erosion
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.
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.
subtle to notice between one year and the next.
increasing the use of fertilizer.
organic matter, rooting depth, and water-holding capacity
Ultimately, the rate of decline of soil productivity = the cost of maintaining constant food production levels, is determined by such soil properties as (…2)
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
most erosion is initiated by the
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,
Raindrop impact exerts 3 important detrimental effects:
(1) it detaches soil;
(2) it destroys granulation;
(3) its splash, can cause an appreciable transportation of soil.
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:
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
3 types water erosion
All three types may be serious, but …, although less noticeable than …. are responsible for most of the soil moved.
Sheet = pedastals–> raindrop impacts
Rill–> overland flow
Gully–> more water, deeper, more energy available to erode soil
sheet and rill erosion
gully erosion
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.
management of soil resources
alternative tillage
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.
soil erosion models
rainfall, soil, topography, vegetation, and management
3 models to predict water-induced soil erosion
Process-based:
- WEPP
water erosion prediction project - SWAT
soil and water assessment tool
Factor based:
3. USLE + revised USLE
what cant be predicted by USLE
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
WEPP
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)
USLE Equation + factors
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
RUSLE
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.
The rainfall erosivity factor, R, represents :
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.
The soil erodibility factor, K, indicates:
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
The topographic factor, LS, reflects :
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.
….(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.
Erosion and runoff
Undisturbed forests and dense grass
one-third to one-half
forage crops (both legumes and grasses)
The C factor in the USLE or RUSLE
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)
support practices:
P factor:
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).
The support practices include :….
all of which will tend to reduce the P factor.
- tillage on the contour (contour cultivation)
- contour strip-cropping
- terrace systems
- grassed waterways,
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.
30%
cover crops
high-residue- producing crops
100%
Benefits of conservation tillage systems on soil properties (5)
- 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
- improved Ks
- improved water-holding capacity
- improved microbe populations
–> increased abundance, diversity, activity of soil organisms
all of which = less erosion, less surface runoff (= lower C values)
vegetative barries
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
mass wasting
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
Erosion in rangelands
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.
erosion in forested lands
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
Roads = destabilization, exposed surfaces, drains
how mitigate?
Harvesting mitigation?
minimize road length/use
avoid unstable areas
erosion control measures
apply USLE principles; cover, LS, erosivity
in
path design/use + repair/rehabilitation
The main sources of eroded soil from timber production are (3)
Strategies to control erosion should include consideration of (4)
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.
When forests are harvested, …. as wide as 1.5 times the height of the tallest trees should generally be left untouched along all streams
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.
The goals of erosion control on construction sites are (2)
(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)
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
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
erosion blankets
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
A commonly used technology to protect steep slopes and areas difficult to access, such as road cuts, is the hydroseeder :
that sprays out a mixture of seed, fertilizer, lime, mulching material, and sticky polymers.
wind erosion
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
…is the main variable driving the vulnerabilities to water or wind erosion, with additional influences of …(3)
Climate
topography, soil properties, and vegetation.
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)
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.
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
soil structure,
accumulates in the low-lying, concave parts of the landscape
Barriers or windbreaks: reducing wind velocities for short distances and for trapping drifting soil.
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.
wind-erosion control methods (3) + 1 general
- tillage
- barriers
- soil cover
–> increased soil moisture = increased soil cohesiveness so need higher winder speed to be able to detach soil particles
Although the wind itself has some direct influence in picking up fine soil, the …is probably more important.
impact of wind-carried particles as they strike the soil (saltation)
Wet soils do not blow because of the ….. Dry winds generally lower the …. to below the wilting point before wind erosion takes place.
adhesion between water and soil particles
soil moisture content
wind erosion works in conjunction with water driven erosion ex:
post-glacial redistribution of meltwater deposits/runoff
glaciofluvial deposits
3 step process of wind erosion
- detachment (wind + other particles)
- 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)
- deposition
wind erosion equation
E = f(I KCLV)
I; erodibility factor (particle mass + cohesion)
K; roughness
C; climatic factor
L; unsheltered median travel distance
V; vegetative cover factor
4 factors affecting wind erosion
- wind velocity + turbulence
- surface roughness
- 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
- 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)
4 wind + water erosion ways to measure
- reservoir surveys in streams (of deposition)
- measurements of suspended solids, bedloads in streams
- plots; walled plots lead to collector for eroded material
- erosion stakes/pins, pedestals, erosion scars, lichen on rocks
–>at smaller scale
erosion control + conservation planning (2 steps)
- determine acceptable level of erosion (Te)
–> range t/ha/yr
–> not exceed natural soil forming rates
–> integrate vulnerability to irreversible damages - 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
general erosion prevention (3)
- prevent inappropriate use of risk areas
–> risk areas = slopping ground, low permeability, vegetation prone to denudation - maintain vegetative cover (soil cover)
- 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
Cropping management factor (C)
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
ways to manage water on slopes
- drainage
-> landscape scale
–> medium scale (cross drains) - back country roads
3.hydromulch/hydroseeding
