Erosion

Photo of an extreme form of erosion.Click to enlarge
Erosion is a gradual process that results in the loss of fertile soil.
Source: S. Marahrens / Umweltbundesamt

When uncovered farmland is exposed to heavy rainfall or high wind, soil particles are set in motion, and can be transported downhill or across open space over long distances. This results in a loss of fertile soil that is essential for life itself. And because farming is becoming ever more intensive and monoculture oriented, serious long term problems can result.

Table of Contents

 

Soil erosion induced by water: an underestimated danger?

Heavy rainfall can mobilize cropland soil particles, causing those located on slopes to be translocated over varying distances, depending on rainfall intensity. This erosion process results in the loss of the fertile, humus soils that are indispensable for crop yields, while at the same time the nutrients and pollutants bound to soil particles are deposited in nearby waterbodies and ecosystems. In some cases, public safety on roads and in residential areas is compromised by mudslides. And while land denuded by severe erosion is an unknown phenomenon in Central Europe, in Germany much cropland suffers from water induced erosion. The resulting barely visible and largely gradual soil loss jeopardizes long term food supply security, as it takes far longer for new soil to form than for existing soil to be lost.

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What are the signs of water induced soil erosion?

Because soil erosion is a rare phenomenon that affects limited areas, it is difficult to observe and measure. Erosion in Germany’s predominantly hilly areas is mainly caused by water, whereas the main erosion factor in northern Germany’s coastal areas is wind.
Depending on cause, farmland water erosion can take the form of areal degradation, vertical erosion, or both. In both cases, soil particles are translocated from upper and middle slopes to lower slopes, or to adjacent waterbodies and nearby fields.

Areal degradation

Areal degradation involves small, minute, and in some cases non-measurable forms of erosion on cropland. This occurs in the case of splash erosion, in which the direct force of raindrops striking a field removes soil particles, primarily resulting in the destruction of smaller and larger clumps of earth and aggregates and in extensive topsoil sludging.

Vertical erosion

Vertical erosion comprises readily visible, measurable and mappable cropland erosion that is differentiated according to depth:

  • Furrows (2 to 10 centimeters)
  • Streams (10 to 40 centimeters)
  • Ditches (41 cm or more)

Erosion usually follows runoff and depth lines, as well as ruts, and tends to encompass over multiple fields. Hence the points at which the erosion originates and the points at which material is deposited are very far away from each other.
Apart from fertile-soil loss, the long term on-site (i.e. cropland) damage in most cases affects crops, while the long term off-site damage affects human settlement and transport infrastructure areas, waterbodies and ecosystems.

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What causes water erosion?

Water erosion in Central Europe is caused by farming. And while some erosion is of natural origin, erosion on this scale only becomes possible through human activity.
The aggregate causes of erosion can be accurately quantified using the Universal Soil Loss Equation (USLE), which encompasses all key factors.

Natural causes of erosion

  • Climate (rainfall intensity)
  • Soil (particle-size erosion proneness)
  • Relief (slopes and topology)

It should be borne in mind that the combination of heavy rainfall and extremely erosion prone soils in sloping areas translates into a high risk of erosion.

Rainfall intensity. Winter and spring erosion is mainly induced by long periods of relatively low intensity rainfall, while in June and late summer heavy rainfall is the main cause.

More than 10 millimeters of rainfall per square meter will normally give rise to erosion.

Particle size composition. Soils containing a high proportion of silt particles, which predominate in loess soils, are highly prone to erosion. A relatively large number of surface rocks and the presence of large amounts of humus protect the soil against raindrops. Activities on the art of earthworms and other soil organisms allow for the creation of myriad vertical soil pores and small tubes, thus expediting rainwater percolation and reducing runoff and erosion.

Relief, and in particular slope gradients, as well as terrain relief, are decisive factors, particularly in cases where soil is tilled in the direction of the slope.

In all cases, erosion can occur in slopes with a gradient of two per cent or more.

Impact of farming on erosion

The following farming related factors – over all of which growers have control – affect erosion:

  • Slope length (farmland configuration, size and geometry)
  • Types of crops (crop diversity and rotation)
  • Tillage practices
  • Tillage direction (tillage orientation relative to cropland slopes)

The effect of slope length is quantified by the length of the barrier and obstacle free terrain that is available for runoff. In other words, the longer the slope, the greater the risk of erosion. Short flow stretches over steep slopes also pose a risk of erosion.

Crop type affects the degree of ground covering at particular times over the course of the year. Crops such as corn and sugar beets provide more than 30 per cent ground over only very late in the growth cycle – the minimum when it comes to effective soil protection.

Tillage practices. Fields are normally tilled using a plow, a process in which the soil is turned and mulch is mixed in with the soil. This process destroys two natural anti-erosion effects. First, mulch that would normally protect the soil against rainfall is removed from the surface. Second, plowing destroys the soil structure in that the soil loses its stability, along with the vertical tubes that allow for percolation. Long term conservation tillage (i.e. without a plow) preserves these various mechanisms, however, by avoiding soil structure destruction. The most effective conservation tillage method is direct sowing, whereby seeds are sowed by the previous crop – and thus the ground is covered at all times.

Tillage direction (i.e. plowing either parallel or perpendicular to the slope) has a major impact on erosion, particularly when it comes to cropland furrows.

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What are the consequences of water erosion?

Erosion results in the loss of fertile soil and is a long term threat to the very foundations of food production.

Apart from the damage done to soil and soil functions, erosion can also degrade and damage public goods such as water quality and human settlement and transport infrastructure elements.

Soil loss of ten tons per hectare and year equates to the loss of one millimeter of soil annually. This in turn means that over the course of an average human lifetime (80 years), around eight centimeters of soil may be lost – the equivalent of 25 to 30 centimeters of fertile topsoil. Tens of centimeters of erosion are not unheard of in some areas in the annals of agriculture.

On-site impact of farmland erosion:

  • Soil fertility loss
  • The soil loses its ability to filter out impurities
  • Soil function loss: The soil loses its water retention capacity
  • Crop loss
  • Mineral-fertilizer loss

 

Off-site impact of erosion:

  • Thoroughfare fouling
  • Ditch and sewer fouling
  • Fouling of residential areas and private property
  • Increased risk of local flooding from surface runoff
  • Crop loss resulting from flooding of neighboring fields
  • Pollutants and mineral fertilizer bound by the soil are deposited in waterbodies and adjacent ecosystems (eutrophication)
Graph illustrating soil loss though water-caused soil erosion.
Soil loss after 80 years of certain annual amounts of erosion.
Source: FG II 2.7 / Umweltbundesamt
 

What is the actual extent of water erosion in Germany?

The extent of water erosion can be best measured by observing and then mapping farmland events involving visible erosion. This method has been in use since 2000 in Lower Saxony and was recently introduced in Baden-Württemberg. More than a decade of investigations in Lower Saxony has yielded the following insights into erosion events:

  • Soil loss associated with specific erosion events can range from less than one to 50 tons per hectare (the equivalent of five millimeters per year, and over the course of a human life, the loss of all fertile topsoil)
  • The mean soil loss for all observed areas ranges from 1.4 to 3.2 tons per year and hectare.
  • The mean soil loss for the 25 per cent of German farmland that is highly prone to erosion amounts to five tons per hectare and year, which equates to 0.5 millimeters of soil loss annually.
  • High levels of erosion are attributable to the following factors: slopes; tillage in the slope direction; slope gradient; tillage method; soil compaction
  • Crops entailing a high risk of erosion: potatoes, corn, sugar beets and winter wheat
  • One sixth of eroded soil is deposited in adjacent waterbodies.
  • More than half of erosion events involve translocation to adjacent areas.

The methodology used for this mapping instrument allows for the quantification of actual vertical erosion. As a rule, soil loss resulting from vertical erosion and areal degradation occurs in a ratio of one to one.

 

Which areas are most prone to water erosion?

Non-farming related erosion risk for Germany is quantified using the Universal Soil Loss Equation (USLE), which projects mean long term soil erosion in tons per year.

In some cases a site-specific variant known as “potential” risk is determined that takes only “natural” factors into account – namely climate, soil, and topography.

Current status is based on rainfall statistics from 1971 to 2000. Crop type distribution information is based on data from 2007 that was compiled for natural areas. Conservation tillage is now being used for around half of Germany’s farmland, although this figure varies from one region to another.

As at 2007, 14 per cent of Germany’s farmland was subject to a medium long term risk of erosion amounting to three tons per hectare and year – the equivalent of high to very high risk. An additional 36 per cent of German farmland is subject to a mean low to medium risk of water erosion, i.e. around half of Germany’s farmland is subject to little or no water ersosion risk. These figures do not take tillage direction into account and are only determinable at the local level or for individual farms.

Water erosion risk exhibits a specific pattern in Germany, whereby the following topographical elements are highly prone to erosion:

Loess clay terrain in the Mittelsächsisches Lösshügelland area

  • Thüringer Becken peripheral areas
  • Eastern and northern Harzvorland
  • Parts of Weserbergland
  • Hellwegbörden
  • Bergisches Land
  • Ville
  • Parts of Saar-Nahe Hügellande
  • Kraichgau and Gäuplatten
  • Markgräfler Hügelland
  • Parts of Schwäbische Alb
  • Donau-Isar-Inn Hügelland
  • Parts of Oberpfälzer Wald and Bayerischer Wald

Comparing a scenario involving widespread conventional tillage with one involving conservation tillage clearly reveals the erosion reduction potential of the latter method.
The findings for Germany as a whole are not applicable to individual farms or parcels.

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What effect does climate change have on erosion?

Climate change is expected to change the nature of soil erosion in Germany, although projections in this regard vary from one region to another. The impact on water erosion will mainly be determined by rainfall and temperature. The regional prevalence of specific crops points the way to targeted adaptation measures.

Climate models allow for statistical projections of rainfall and termperature changes. In one study, the Wettreg climate model from 2006 and the A1b climate scenario were used to project water erosion for 2011 to 2040, 2041 to 2070, and 2071 to 2100.

Areas likely to be threatened by erosion in the coming years

Site-specific erosion risk projections provide insight into how rainfall may change in Germany in the coming years. From 2011 to 2040, rainfall is expected to increase in northeastern and western Germany, and from 2041 to 2070 is expected to shift from northeast to northwest such that mainly northwest Germany will be affected. In the subsequent three decades, a uniform rainfall increase is expected to arise from northwest Germany that will not affect northeastern and southeastern Germany and part of Baden-Württemberg.

Hence the western portion of the Central Upland Range will be the first area to see increased proneness to erosion. This is then expected to encompass Hesse, and in the latter three decades of the 21st century will affect the entirety of the western Central Upland Range. The rates of increase are expected to be low during the first two periods, but are expected to rise substantially from 2071 to 2100.

Areas likely to be threatened by farming related erosion in the coming years

The projected distribution of areas prone to farming related erosion differs considerably from that of areas prone to non-farming related erosion, mainly due to the fact that ground covering patterns are expected to change over time; the scope of this phenomenon will be largely determined by the effect of rainfall and temperature on crops.

The low rates of increase for site specific erosion proneness for 2011 to 2040 and 2041 to 2070 are different when it comes to erosion risk unrelated to farming. From 2011 to 2040, rainfall is expected to increase across the Central Upland Range, particularly in the Thuringian Basin and in central Germany. This trend will abate somewhat between 2041 and 2070, but is expected to increase far more drastically in the subsequent three decades.

Overall water erosion proneness is expected to vary regionally from moderate to medium, while some regions will experience no increase at all. Corn cultivation on set-asides will entail a considerably greater risk of farming related erosion. The various tillage scenarios underscore the fact that the increased risk of erosion can be offset by conservation tillage.

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Which anti-erosion laws are currently on the books?

The following laws contain provisions aimed at fighting water erosion:

  • Bundes-Bodenschutzgesetz – BBodSchG (section 17) and Bundes-Bodenschutz und Altlastenverordnung – BBodSchV (section 8),
  • Direktzahlungen-Verpflichtungengesetz – DirektZahlVerpflG (section 2) and Direktzahlungen-Verpflichtungenverordnung – DirektZahlVerpflV (section 2), which transposes Article 5 of Regulation No 1782/2003 into German law
  • Wasserhaushaltsgesetz (WHG) (sections 78 and 83), which transposes the Water Framework Directive into German law
  • Bundes-Naturschutzgesetz (BnatSchG) (section 5)

Apart from its public-goods protection provisions, the Federal Soil Conservation Act (Bundes-Bodenschutzgesetz) mainly centres around the protection of Germany’s soil. In terms of using land for farming, this is stipulated in section 17 in terms of good agricultural practice. This section states the following concerning soil erosion avoidance: “Soil loss shall be avoided wherever possible, through farming methods that are compatible with local conditions, and in particular by taking into account slope gradients, water and wind conditions, and ground covering.”

The Direktzahlungen-Verpflichtungenverordnung (direct payments regulation) governs grants under the cross-compliance provisions of the Common Agricultural Policy (CAP). The amount of such grants is determined by the erosion risk to which a particular farm is exposed and is subject to the fulfillment of specific anti-erosion requirements. This instrument mainly promotes precautionary measures; its requirements constitute minimum standards, compliance with which is voluntary and unrelated to the Federal Soil Conservation Act (Bundes-Bodenschutzgesetz), whose precautionary and anti-hazard principles are legally binding.

Farmers are free to either conduct an erosion risk assessment and determine the necessary measures on their own initiative, or to seek advice pursuant to section 17 of the Federal Soil Conservation Act. Many of Germany’s regional states have instituted mechanisms to this end, one example being Lower Saxony’s Bodenerosionsschlüssel für Betriebsleiter und Berater (Soil erosion key for site managers and consultants) instrument, which was launched in 2004 and allows those concerned to assess water erosion risk and institute the relevant countermeasures.

In the interest of promoting compliance with the Water Framework Directive and the elaboration of management plans, many regional states have elaborated guidelines and action profiles

Chart showing the determination of erosion hazard.
Exemplary determination of erosion hazard for an arable land.
Source: FG II 2.7 / Umweltbundesamt
 

How can water erosion be prevented?

Various anti-water soil erosion instruments are available in keeping with the section 17 good agricultural practice provisions of the Federal Soil Conservation Act (Bundes-Bodenschutzgesetz).

Farming related measures

  • Extensive crop rotation
  • Catch-cropping (ensuring continous ground covering)
  • Undersowing for crops such as corn and sugar beets
  • Avoiding tillage in the slope direction and tilling perpendicular to the slope instead
  • Soil compaction avoidance
  • Use of organic substances and lime, for soil structure conservation purposes

Tillage methods

The most effective measure is long term conservation tillage and mulching, both of which have been used in Germany for the past two decades. Nationwide assessments show that using these methods can reduce soil loss by up to 50 per cent relative to conventional tillage. Conservation tillage entails use of the following special crop protection measures:

  • Sustained conservation tillage and mulch seeding
  • Introduction of strip tillage
  • Direct sowing (seeds are sowed by the mulch from the previous crop)
  • Earthing up of transverse ridges for potato crops

Ways to avoid linear water runoff and inflow

  • Install barriers, i.e. small terraces and field strips
  • Sustained greening of depressions and channels
  • Avoiding runoff from roadways and the like
  • Installing buffer strips to avoid waterbody pollution
  • Installing water retention facilities and areas

Major deleterious effects on highly susceptible cropland can be avoided by restructuring individual parcels of cropland, as follows:

  • Changing land use and creating permanent set-asides (greening highly susceptible areas)
  • Avoiding the conversion of grassland to arable land
  • Slope and parcel segmentation by sowing crops across slopes rather than parallel to them
  • Catchment area related crop planning (crop distribution optimization)
  • Installing new erosion reducing parcel and tillage structures
Chart showing measure taken to avoid erosion
Measures taken to avoid erosion depend on applicability and acceptance
Source: T. Mosimann