Soil Erosion: Risk Assessment Field Guide for Farmers & Consultants (PB4092)

Soil Erosion: Field guide for an erosion risk assessment for farmers and consultants

PB4092
1999
rev. 2005

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Site characteristics

This chapter provides the basis for preparation of an erosion risk map for the farm.

The criteria of importance at this stage are:

  • Soil Texture
  • Slope
  • Flooding frequency

Subsequently you will need to consider cropping and soil structural condition.

The risk of runoff or soil wash and erosion depends on the physical features of the farm and upon soil management. Actual events are determined by rainfall. Very high intensity storms or repeated storms can cause serious erosion in many situations and the following assessment procedure does not necessarily cover such events.

In making a risk assessment, each field should be examined. Runoff and erosion risk in any part of a field will depend on the soil texture and steepness of slope. The uniformity of slope above and below a particular area, are also important in determining the likelihood of rill or gully formation.

For assessment purposes large fields might be sub-divided if slope, soils or topography differ significantly, but for whole field assessment the worst scenario should generally be mapped. Field entrances should be marked on the map where they may influence erosion by channelling water movements into or out of a field. An example of a typical farm map is illustrated at the end of this section.

If required, soil textures can be obtained from a laboratory analysis of particle size distribution. The diagrams at the end of this Chapter show:

  • The percentages of sand, silt and clay within each textural class.
  • A hand texture assessment which can be carried out in the field and will be adequate for most situations.

It is helpful to assess slope angles as accurately as possible however slopes are frequently uneven and variable and it is more important to determine the relative overall risk of an area of land than to worry about precise angles of slope.

Typical situations which would fall into different risk categories are outlined in the tables below. The criteria given are guidelines and professional judgment should be used to upgrade or downgrade a site, taking into account additional factors such as:

  • Soil structure
  • Organic matter content
  • Valley features which tend to concentrate runoff water
  • Long unbroken slopes
  • Land restored following opencast mining or landfill operations
  • Very steep slopes (i.e. greater than 110)

Very light soils with low organic matter on gentle slopes, even in low rainfall areas, can erode more seriously than indicated in the following risk assessment, sometimes by as much as two risk classes. Therefore, in addition to a field assessment, local knowledge is also useful in estimating risk, as previous erosion occurrences are often well remembered.

The following assessment procedure estimates the risk of runoff from fields carrying nutrients and soil down slopes. Runoff pathways, slope patterns and valley features will influence the likelihood of this runoff causing further erosion or having deposition impacts beyond the field. Areas where this could happen should also be indicated on the plan. You should also consider if your land receives runoff from elsewhere that will increase erosion problems on your land.

The following tables provide a guide to field classification for runoff and erosion. They assume moderately good soil conditions. If the land is currently in grass you should still apply this risk assessment. It will act as a guide to what might happen if you decide to reseed or introduce arable cropping in future.

Water erosion

This part of the risk assessment refers to the movement of sediment within the field and possible transfer to watercourses or other places such as neighbouring properties or on to roads.

Soil Textures

Steep slopes >7 Moderate slopes 3 - 7 Gentle slopes 2 - 3 Level ground    <2

Sandy and light silty soils

Very high

High

Moderate

Lower

Medium and calcareous soils

High

Moderate

Lower

Lower

Heavy soils

Lower

Lower

Lower

Lower

Signs of erosion that may be associated with each of the risk classes are described below. Such observations should override an assessment derived solely from the table.

Very High Risk Areas Rills are likely to form in most years and gullies may develop in very wet periods.

High Risk Areas Rills are likely to develop in most seasons during wet periods.

Moderate Risk Areas Sediment may be seen running to roads, ditches or watercourses and rills may develop in some seasons during very wet periods.

Lower Risk Areas Sediment rarely seen to move but polluting runoff may enter ditches or watercourses.

Runoff or soil wash

This part of the risk assessment refers to runoff which is usually but not always discoloured. This runoff may carry very fine soil particles, soluble pollutants such as plant nutrients and pesticides or manures to watercourses.

Soil Textures

Steep slopes >7 Moderate slopes 3 - 7 Gentle slopes 2 - 3 Level ground    <2

All soils

High

Moderate

Lower

Lower

Signs of runoff that may be associated with each of the risk classes are described below. Such observations should override an assessment derived solely from the table.

High Risk Areas Runoff seen in most years during wet periods

Moderate Risk Areas Runoff seen in some years during wet periods and in most years during very wet periods

Lower Risk Areas Runoff seen in some years during very wet periods

Remember that: The accumulated runoff from a catchment with a large proportion of only lower risk fields can still cause serious damage to watercourses and may require action to be taken.

Flood risk

Land that floods is susceptible to erosion and runoff, particularly when under cultivation. Land that floods regularly (at least 1 year in 3) must be regarded as highly vulnerable and should be indicated on your map.

The map overleaf showing the erosion risk categories outlined above should serve as a basis for planning crop rotations and management to reduce run-off risks and soil loss.

Farm and crop planning

The risk map shows which fields or parts of fields are most at risk when exposed to heavy or prolonged rain or flooding. At this stage, it might become clear that new hedge plantings could usefully reduce erosion risks or relocation of field entrances could reduce deposition of sediment onto roads or into watercourses.

The next step is to plan crop rotations and land use to minimise exposure of bare, vulnerable land to the erosive effects of rainfall.

The susceptibility of soil to erosion is dependent upon the land cover or livestock enterprise using the land, and can be considered in three broad categories. Some examples of land management practices within each category are listed below.

Highly susceptible land use

On Very High Risk and High Risk sites, avoid these land uses unless precautions are taken as outlined in Chapters 4 and 5. If these precautions do not control the problem then discontinue the land use. Some of these precautions may be necessary on all sites.

Late sown winter cereals
Potatoes
Sugar beet
Field vegetables
Outdoor pigs
Grass re-seeds
Forage maize
Outwintering stock
Grazing forage crops in autumn or winter

Moderately susceptible land use

On Very High Risk and High Risk sites these moderately susceptible land uses can be carried out with care.

Early sown winter cereals
Oilseed rape winter and spring sown
Spring sown cereals
Spring sown linseed
Short rotation coppice/Miscanthus

Less susceptible land use

Consider the following land uses on Very High Risk and High Risk sites as a means of reducing the overall erosion risk.

Long grass leys
Permanent grass
Woodland (excluding short term coppice)

By altering rotations and changing land use, for example, switching from late sown autumn to spring sown crops on higher risk sites, the likelihood of erosion can be reduced significantly.

Soil texture

Texture classes for mineral soils are defined by the relative proportions of sand, silt and clay sized particles.

Particle sizes

  Particle Diameter mm
Sand 20.06
Silt 0.060.002
Clay less than 0.002

Triangular diagram

Limiting percentages for the 11 main texture classes are defined within the triangular diagram below.

Sand, loamy sand, sandy loam, sandy silt loam and sandy clay loam classes may be subdivided according to the sand size.

Fine more than two thirds of sand less than 0.2mm
Coarse more than one third of sand greater than 0.6mm

Assessment of soil texture

Accurate measurements of soil texture requires laboratory analysis, but for practical purposes, texture can be assessed by hand, using the following method:

Take about a dessert spoonful of soil. If dry, wet up gradually, kneading thoroughly between finger and thumb until soil crumbs are broken down. Enough moisture is needed to hold the soil together and to show its maximum stickiness. Follow the paths in the diagram to get the texture class:


DEFRA PB4092, 1999, revised 2005
(C) Crown Copyright. Reproduced for ADLib under Licence.

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