Factors that influence erosion and runoff - EverySiteadlib.everysite.co.uk/resources/000/263/234/chapter2.pdf · Environment Agency thinksoils factors that influence erosion and runoff

Environment Agency thinksoils factors that influence erosion and runoff

Factors that influence erosion and runoff

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thinksoilsFactors that influenceerosion and runoff



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Soil● texture● wetness● structure● soil surface roughness

Weather● rainfall intensity● climate and soils

Landscape● steep slopes● field size and valley features● proximity of watercourses● field tracks and roads

Land use● risks associated with crops and livestock

Weather

Soil Landscape

Land use

Environment Agency thinksoils factors that influence erosion and runoff / soil texture


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Soil texture

soil texture

Soil texture

Soil with high sand content is not cohesive and has a high risk of erosion

Soil texture refers to the relative proportion ofclay, silt and sand.

The risk of runoff and erosion is affected by smalldifferences in texture. This is because textureinfluences the degree of percolation of waterthrough the soil, and also the stability of soil.

Soils containing large proportions of sand haverelatively large pores through which water candrain freely. These soils are at less risk ofproducing runoff. As the proportion of clayincreases, the size of the pore space decreases.

This restricts movement of water through the soiland increases the risk of runoff.

Soils with low clay content are less cohesive andare inherently more unstable. These soils are atgreater risk of erosion by water and wind.

Environment Agency thinksoils factors that influence erosion and runoff / soil wetness


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Soil wetness

soil wetness

Soil wetness

Naturally waterlogged soils are known as gleysoils. Soils affected by a high water table aregroundwater gley soils, and those that arewaterlogged due to slow percolation of water areknown as surface water gley soils. Wet soils havegreater risk of runoff.

After the summer and in well structured soils,without deep fissures or cracks, rain wets the soilprogressively from the surface. This creates awetting front that moves down the soil profile.Compacted layers within the soil will affect thiswetting front and it may cause areas of surfaceponding across a field.

Mottles indicate periods of waterlogging

Grey and bluish-grey colours develop insaturated soils due to a lack of air and thereduction of iron compounds. A patchwork ofbluish-grey colours occur together with orange,yellow or rusty colours (mottles) in the zonewhere there is waterlogging for part of the year.Orange / yellow colours occur where iron has re-oxidised.

Environment Agency thinksoils factors that influence erosion and runoff / soil structure


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Soil structure

soil structure

Soil structure

Soil structure refers to the arrangement of soilparticles in the soil. Clay content, organic matter(and in some soils calcium and iron compounds)help to bind the soil together into structuralunits, aggregates or peds.

Well structured soil allows the free movement ofair and water through fissures (or cracks)between the structural units. Pores within theunits also allow the movement of air and water. Asoil with poor soil structure has a high risk ofgenerating runoff. The risk of runoff is greatestwhen poor soil structure is near the soil surface.

Soil structure deteriorates when structural unitsare deformed producing a dense single mass ofsoil (or large soil units). This occurs whenpressure is applied to a wet and soft soil.

Poor soil structure

Good soil structure

Pressure squeezes the soil units together andreduces pore space within the units. A dry soilcan withstand pressure without deforming soilstructure.

Some soils are unstable when clay, calcium ororganic matter content is low. Unstableaggregates disperse when wet, forming a solidmass as the soil dries. Where this occurs at theimmediate soil surface, the soil may form a capor crust.

Soils can restructure due to natural fracturingprocesses when clay shrinks and swells, and bycultivation. Biological activity also restructuressoil.

Environment Agency thinksoils factors that influence erosion and runoff / soil surface roughness


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Soil surface roughness

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Ploughed land providessurface storage of rainwater

soil surface roughness

Soil surface roughness

Rough surfaces (e.g. in ploughed land, coarseseedbeds, or pressed land with indentations)help to slow down runoff. Roughness providesstorage of rainwater, allowing water to collectbefore it soaks into the soil. For some fields,extra storage can be created if the ploughed landis worked across a slope and not up and down aslope i.e. the ridge and furrows now act as littledams and storage areas.

Rough surfaces also help to reduce wind speedat the immediate soil surface, preventing winderosion.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups


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Generic soils groups

generic soil groupsGeneric soil groups

This approach enables soils to be groupedaccording to risks associated with:

● clay content (affecting porosity, risk of runoff,stability and erodibility),

● shallow soils on chalk and limestone (that arevery stable, freely draining with low risk ofrunoff),

● peaty soils (with high risk of runoff and erosionin the uplands, and high risk of wind erosion inthe lowlands).

Soil texture classification for mineral soils, andfor soils with high organic matter

Peat

Loamy peat (LP)or

Sandy peat (SP)

Peaty loam (PL)or

Peaty sand (PS)

Organic mineral soil

Mineral soil

100

50

35

25

20

106

10

50

35

25

10

0 50 100

% clay in the mineral fraction

Less than 50% sand in the mineral fraction50% sand or more in the mineral fraction

1

2

1

2

1

2

% o

rgan

ic m

atte

r

There are a number of methods for grouping soilsaccording to their risk of erosion and runoff. Theapproach used in this document has been togroup soils into 5 generic soil groups:

● sandy and light silty soils● medium soils● heavy soils● chalk and limestone soils ● peaty soils

Heavy soils

Medium soils

Sandy and light silty soils

100 0

2080

5550

45

30

1815

10

0100 85 80 70 50 20 0

100

82

65

% silt

% sand

% c

lay

clay

sandyclay

sandy clayloam

clay loam silty clayloam

siltyclay

sandy loam

loamy sandsand

sandy siltloam

siltloam

Peaty soils

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / sandy and light silty soils


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Generic soils groups / sandy and light silty soils

generic soil groups3

Sandy and light silty soils

These soils have low clay content (<18%) in thetopsoil, and include sand, loamy sand, sandyloam, sandy silt loam and silt loam textures.

Due to the low clay and organic matter contentthese soils have low aggregate stability. Soilsreadily disperse (slake) in water causing internalslumping and capping at the surface.

Where these soils are freely draining, and wellstructured, they have low risk of runoff.

However where the drainage is impeded by highwater table or a slowly permeable subsoil, theyare at high risk to structural damage and runoff.

Where runoff occurs these soils have a high riskof erosion.

Fine sandy soils have a high risk of wind erosion.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / medium soils


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Generic soils groups / m

edium soils

generic soil groups

Medium soils

Medium soils include sandy clay loam, clay loamand silty clay loam textures. They have a claycontent between 19 and 35% in the topsoil. Thehigher clay content produces a greater aggregatestability than lighter soils. However, mediumsoils with a high content of silt or fine sand arenot as stable, and are prone to capping,particularly where the organic matter content islow.

Clay content in the subsoil (and depth to thewater table) affects the drainage of medium soils.Where clay content is low in the subsoil thesesoils can be freely draining with low risk tostructural damage.

Conversely, where the clay content is high, theyare prone to waterlogging and structural damage.

Structural damage, or poor drainage, in mediumsoils can lead to runoff and soil erosion,particularly in areas of high rainfall and onslopes.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / heavy soils


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Generic soils groups / heavy soils

generic soil groups3

Heavy soils

These soils have a clay content >35% and includesandy clay, clay and silty clay textures. They arenaturally slow draining and lie wet for longperiods.

Some clay soils have a naturally well developedsoil structure which lessens the incidence ofwaterlogging.

The stability of clay and porosity is dependent onthe type of clay and the calcium content. Lessstable acidic clays have lower porosity andhigher risk of runoff than calcareous clays.

Heavy, slowly draining soils have a high risk ofstructural damage and generating runoff, buthave a low risk of erosion.

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / chalk and limestone soils


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Generic soils groups / chalk and lim

estone soils

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Shallow chalk and limestone soils

These thin soils are less than 30cm deep. Theyare highly calcareous, often with a mediumtextured topsoil. The soils have stable aggregatesand form a strongly developed soil structure.Topsoil can be lost on these soils, exposing thebedrock, due to soil creeping down slopes.

The soils are naturally well drained and acceptmost winter rainfall with low risk of runoff.

generic soil groups

Environment Agency thinksoils factors that influence erosion and runoff / generic soil groups / peaty soils


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Generic soils groups / peaty soils

Peaty soils

This group includes peaty soils where the organiccontent of the topsoil is more than 20% organicmatter (or 12% organic carbon). They includepeaty sand, peaty loam, loamy sand, loamy peatand peat textures.

Peaty soils are widespread in the uplands andare also found in lowland bogs and river valleys.The Fens of eastern England and the LancashireMosses are very productive agricultural soils.These flat, artificially drained peaty soils have alow risk of runoff.

Peaty and organic soils that lie wet in the uplandshave a small capacity to accept winter rainfall,with a consequent high risk of generating runoffand soil erosion.

In the lowlands and drier parts of the country,peaty soils are prone to wind erosion because oftheir low density and loose soil structure. Whendrained they are vulnerable to oxidation, causingpeat ‘wastage’.

generic soil groups

Environment Agency thinksoils factors that influence erosion and runoff / weather


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weather / rainfall intensity

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1

weather

Rainfall intensity

Runoff occurs when rainfall intensity exceedsinfiltration rate and the soil becomes saturated atthe surface.

During winter, soils are often described as beingat field capacity. This is the maximum watercontent held in the soil under free drainage. Atfield capacity, air is held in macropores and thesoil can absorb rainfall until it becomes saturated.

Naturally well drained soils rarely becomesaturated and readily absorb most rainfall. Wherethe surface loses its porosity, runoff can occur onwell drained soils when rainfall is as low as1mm/hr.

Raindrops can detach anddisperse soil particles, washingthem into pores, causingsealing of the soil surface

Environment Agency thinksoils factors that influence erosion and runoff / weather


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weather / clim

ate and soils

Climate and soils

Soil structure is at risk of being damaged whenpressure is applied to a wet soil with a putty-likebehaviour (i.e. in a moist plastic state). Trafficking,trampling and working of soils when they are toowet is a major cause of deterioration in soilstructure.

Eastern England is much drier than the west andthere is much greater opportunity for timelylandwork without damaging soil structure.

The risk of wind erosion is greatest in the driereastern areas of England.

Applying pressureto moist soil

Well structured soil

Applying pressureto moist soil

Structurelessmass of soil

weather

Environment Agency thinksoils factors that influence erosion and runoff / landscape


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landscape

landscapeSteep slopes

Steep fields can causewater to run off at a rapidrate. This is particularlythe case where waterpercolation into the soil isslow (on naturally slowlydraining soil or wherethere is poor soil structure,or both). Highest riskfields are those greaterthan 7O.

Fields with gentle slopesless than 3O are at lowerrisk to rapid runoff anderosion. But water can stillrun and gather momentumon gentle slopes,particularly where theslope is long andinfiltration rate is slow.

Where the ground is level,water will tend to pond.

Field size and valleyfeatures

Large fields with longslopes can accumulatelarge volumes of water.Valley floors canconcentrate water flowcausing channel erosion.Wheelings and cultivationmarks can also influencethe direction of watermovement.

Wind erosion tends tooccur on unsheltered landexposed to strong winds,and in areas where windis funnelled.

Environment Agency thinksoils factors that influence erosion and runoff / landscape


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landscape

landscape

Proximity of watercourses

Fields adjacent towatercourses are athigher risk of causingwater pollutionassociated with runoffthan those where there isno connectivity towatercourses.

Field tracks and roads

Field tracks and roadsprovide a route-way forrunoff, soil sediment andassociated pollutants toenter watercourses.Roads and field trackscan link fields withwatercourses that arekilometres apart.

Runoff from roads andadjacent land can alsowash onto fields causingfield runoff and erosion.

Environment Agency thinksoils factors that influence erosion and runoff / land use / cereals


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land use / cereals

land use / Late sowing in theautumn

Winter cereals sownduring late October andNovember can put theland at risk of runoff anderosion on sandy andlight silty soils due to lackof crop cover over thewinter and because of thehigh risk of the soilsurface becomingcapped.

Fine smooth seedbeds

Land with a fine andsmooth seedbed provideslittle surface storagecapacity, and on sandyand light silty soil is atrisk of becoming cappedcausing runoff anderosion.

Fine, dry sandy tilths arevulnerable to winderosion.

Environment Agency thinksoils factors that influence erosion and runoff / land use / cereals


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land use / cereals

Compacted seedbeds

Land under winter cerealsis at risk to generatingrunoff where the soil iscompacted (e.g. whensowing is carried out onwet soil, or where soil hasbecome compactedduring previous land workin the rotation).

Crops established byshallow cultivation are atrisk of runoff if there ispoor soil structure nearthe soil surface.

Compacted cereal stubble

Where cereals areharvested in wetconditions, there is a riskof causing soilcompaction and runoff.

Compacted tramlines andwheelings are at most riskof runoff, especially whenaligned up and down aslope.

land use /

Environment Agency thinksoils factors that influence erosion and runoff / land use / livestock


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land use / livestock

land use / Grassland management

Grassland is at risk topoaching and compaction due to grazing when thesoil is too wet. Risks areparticularly high duringautumn and spring.

Farm vehicles can alsocause compaction (e.g.transporting heavy trailerloads of silage in wetspring conditions).

Soils can restructure ingrassland due to highearthworm activity, thedense network of roots,and drying of the soilcausing shrinkage andfissuring of clay.

Out-wintering of stock

Out-wintering of stock hasa high risk of causing soilcompaction and runoff.Soils are trampled,particularly in areasaround ring feeders.Trafficking to feed stockcan also cause soilcompaction.

Cultivation is oftennecessary to improve soilstructure in badlydamaged fields followingout-wintering of stock.

Runoff from compacted soilcaused by out-wintering of sheep

Wheel ruts

Poached soil

Environment Agency thinksoils factors that influence erosion and runoff / land use / livestock


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land use / livestock

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3 Harvesting forage crops

Maize and other foragecrops are often harvestedwhen soils are wet andvulnerable to compactionduring autumn (andwinter). Compacted maizestubble is a commoncause of field runoffduring winter.

These crops are also atrisk to runoff during heavy rainfall in early summerwhen there is a lack ofcrop cover and the soilbecomes capped (or dueto runoff alongcompacted tramlines).

Spreading slurry andmanure during winter

Soil structure is at risk ofbeing damaged whenslurry and heavy loads ofmanure are spread in thewinter onto wet soft soils.

Where soils arecompacted slurry can runoff the soil surface duringrainfall.

Wheel ruts caused duringharvesting of maize

Slurry spreading on wet soil isat high risk of being washed off

land use /

Environment Agency thinksoils factors that influence erosion and runoff / land use / root crops and vegetables


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land use / root crops and vegetables

land use /

Spring land work

Growing root crops and vegetables often involves deep cultivation, stone removal and clodseparation, bed forming, and use of plastic orfleece. These operations are carried out in springwhen subsoils can still be too wet and vulnerableto compaction by heavy machinery.

In steep fields, rows and beds are formed up and down the slope because harvesting equipment can not operate across the slope. Rows and bedschannel water downhill increasing the risk ofrapid water runoff.

Fine tilths (e.g. for carrots, onions and sugarbeet) on sandy and peaty soils are alsovulnerable to wind erosion

Runoff from potatoes grownon long steep slopes

Environment Agency thinksoils factors that influence erosion and runoff / land use / root crops and vegetables


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land use / root crops and vegetables

Headlands, tramlines and tracks

Potatoes and vegetablesinvolve many vehiclemovements to spraycrops and for irrigation.

Headlands, tramlines andtracks are most at risk ofbecoming compacted andcausing runoff.

Compacted headlandsand wheelings causingrunoff and soil erosion

Autumn harvesting

Where root crops andvegetables are harvestedin autumn and winter(when soils are wet andsoft) there is a high risk ofcausing compaction andrunoff.

Transporting heavy loadsof root crops is a majorhazard to the soil in wetconditions where wheelruts result in deepcompaction andsubsequent channellingof runoff.

Lifting and transportingheavy loads of potatoescausing soil compaction

land use /

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Factors that influence erosion and runoff - EverySiteadlib.everysite.co.uk/resources/000/263/234/chapter2.pdf · Environment Agency thinksoils factors that influence erosion and runoff