Economic costs of soil erosion

F.J.P.M. Kwaad,
physical geographer


Other websites by the author:
- Images of soil erosion
- Ephemeral gully erosion
- Soil erosion control in Europe

The website is a means of rapidly locating information on the economic costs of soil erosion.

Soil erosion is a form of soil degradation on agricultural land. It is the relatively rapid removal of topsoil by rain or wind. Under natural conditions - in climates where agriculture can be practiced - the soil is protected by vegetation and the rate of soil loss by erosion is very low. On cultivated land the soil is exposed to rain and wind during part of the year and can easily be washed or blown away. The rate of erosion can increase to more than a thousandfold compared to forest (e.g. tables 1, 2 and 3 in Panagos et al, 2015). The dark coloured upper layer of soil, rich in humus (termed A1-horizon), in temperate climates has favourable properties for plant growth, such as good soil structure and aeration, and timely providing the plant with water and nutrients. Topsoil removal, therefore, leads to a decline in soil productivity. This is an irreversible process, as topsoil forms very slowly. For tropical soils see: Hartemink, 2002 , Amazon Soils and Labrière et al., 2015.

The soil material that is lost through erosion, is deposited elsewhere, in places where it is not wanted like roads, streets, people's houses, drainage ways, rivers, dams and reservoirs.
It can lead to pollution of watercourses and reservoirs with agrochemicals (herbicides, pesticides) adsorbed on soil particles. Heavy rains can cause muddy flooding. The effects of soil erosion on farmers' fields are called onsite effects, the effects outside agriculture offsite effects. The most important onsite effect, the loss of soil productivity, is a long term process seen from the farmers' point of view. The mud that is deposited in built-up areas during heavy rainfall must be cleared away directly after the event, and is therefore a short term effect.

Govers (1994) drew attention to the net downslope displacement of soil material on sloping cultivated land as a direct effect of tillage. This he called 'tillage erosion'. Tillage erosion does not cause offsite damage. The soil that is displaced by tillage remains on the tilled field. Another cause of downslope movement of soil and earth material is 'mass wasting' which takes place under the influence of gravity alone (soil creep, frost creep, solifluction, mudflow, slumping, landslides). The rapid forms of mass wasting (mudflows, landslides, rockfalls) can cause extensive damage with loss of lives. The processes of mass wasting will not be discussed in this website. For more info go to: mass wasting 1 and mass wasting 2.   

Soil erosion by rain occurs when more rain falls than the soil can absorb, either by exceeding the infiltration capacity or the storage capacity of the soil. The excess water runs downslope as overland flow. The erosion can take on several forms, such as splash erosion, rill erosion, interrill erosion, ephemeral gully erosion or gully erosion. Sheet erosion, though often mentioned, is questionable. Rills and ephemeral gullies can be smoothed (filled) by normal tillage. However, soil loss by rill erosion and ephemeral gully erosion is not made undone by tillage. The loss is merely evenly divided (averaged) across the field by tillage and contributes to a gradual lowering of topsoil depth. Real gullies are too deep to be removed by normal tillage.    

Soil erosion control
Can soil erosion be stopped, or at least reduced to an acceptable level (soil loss tolerance or T-value)? Already in the 1930's several methods, techniques and practices were devised for soil erosion control, mostly in the USA, such as: crop rotation, contour farming, strip cropping, vegetated waterways, cover crops, green manuring, crop residue mulch, subsoiling and terracing. Later, starting around 1970, various forms of conservation tillage were introduced and increasingly used (minimum tillage, zero tillage, no till, direct drill, reduced tillage, strip till, mulch till, non-inverting tillage). For descriptions of the erosion control practices and how and when to use them see e.g. Erosion Control and Conservation Planning. The effectiveness of the control practices is dealt with by Maetens et al. (2012) in their article 'How effective are soil conservation techniques in reducing plot runoff and soil loss in Europe and the Mediterranean?' They considered 353 runoff plots (corresponding to 2093 plot-years of data) for 103 plot-measuring stations throughout Europe and the Mediterranean. Some of their findings are:

- A key finding of this study is that SWCTs
(soil and water conservation techniques) are generally much more effective in reducing soil loss than in reducing runoff. Both for soil loss and runoff, the measured effectiveness for a given SWCT varies widely between different studies.
- Crop and vegetation management techniques (i.e. buffer strips, mulching and cover crops) and mechanical techniques (i.e. geotextiles, contour bunds and terraces) are generally more effective than soil management techniques (i.e. no-tillage, reduced tillage and contour tillage). Despite being generally less effective, no-tillage, reduced tillage and contour tillage have received substantially more attention in the literature than the other SWCTs .
- For no-tillage and, to a lesser extent, reduced tillage and contour tillage, clear indications were found that these techniques become less effective in reducing runoff after consecutive years of application.

Because of the offsite damage to public and private property a strong impetus to take measures to control soil erosion comes from outside agriculture. This entails a conflict of interests, because in order to safeguard property outside agriculture from flooding and mud deposition, measures must be taken by the farmer. Farmers themselves in general are not inclined to apply anti-erosion measures on their fields (see below). They do not consider soil erosion an important problem as it has no economic consequences for them on the short term. Therefore, a financial incentive is needed to ensure the voluntary cooperation of farmers. If this is not successful, regulations or legislation are indicated. In the USA (USDA-NRCS) several "2014 - Farm Bill Financial Assistance Programs" are offered to eligible landowners and agricultural producers in the form of voluntary programs to provide financial and technical assistance to help manage natural resources in a sustainable manner.

How successful have we been in soil erosion control over the past decades? Not very successful. In a straightforward and critical essay, well-informed Ted Napier, recipient of the Hugh Hammond Bennett medal for soil conservation, analysed the conservation situation in the USA (Guest Editorial Newsletter ESSC 1/2012, pp. 3-10):

p. 6:
If the goal of conservation efforts is to permanently reduce the severity of soil and water conservation problems within the US, I seriously doubt that continued use of existing conservation policy and program approaches will ever achieve that objective. I firmly believe that conservation subsidies have become nothing more than another form of transfer payments to the agricultural sector. I also believe that most program participants are involved in conservation programs because they perceive program participation as profitable. This is particularly true for government set-aside programs. Enrolment of agricultural land in
set-aside programs results in government representatives becoming renters. Land-owner decisions to enrol land in set-aside programs have little association with the conservation orientations of land-managers. The decision to participate in set-aside programs is a function of rent value. In my opinion, enrolment in set-aside programs is a business decision by the land-manager and nothing more.

The primary lesson learned from decades of US public soil and water conservation efforts is the importance of economic incentives for motivating land-managers to become involved in conservation efforts. The only policy instrument that has been shown to consistently motivate farmers to adopt and to continue using soil and water conservation production systems is the use of economic inducements. It has been repeatedly demonstrated that land-managers can be bribed to adopt and to continue using soil and water conservation, if economic incentives are sufficiently high to make adoption profitable in the short-term. Land-managers are most willing to engage in conservation actions if they are not required to internalize any of the associated costs.

p. 9: The thesis of this essay is that the major components of US soil and water conservation policy and programs are no longer relevant to contemporary conservation issues due to the following:
(1) Economic resources will no longer be available at the level required to bribe farmers to adopt and use conservation production systems.
(2) Farmers need little conservation information or environmental education due to the formal educational training they have achieved.
(3) Technical assistance is seldom needed, due to the extensive technical training that most US farmers have acquired.
It is also argued in this essay that farmers will not participate in any program perceived to generate economic loss to the farm enterprise. If participation in conservation programs that are not profitable at the farm level in the short-term and perhaps not in the long-term is required to achieve the conservation goals of society, then some form of coercion will be required to achieve that objective.

Unfortunately, Napier did not give any data to substantiate his views.

In the light of this, what about scientific soil erosion and conservation research? It is the nature of scientists, when confronted with a problem that is not solved with existing knowledge, to say that more research is needed. This seems not to be true for soil conservation. Soil conservation research started a hundred years ago in the USA. The first experimental soil erosion plots were installed in the USA in 1917 (Laflen and Flanagan, 2013). Measurement results and erosion control methods were described in great detail by H.H. Bennett in his book 'Soil Conservation' (1939, 993 pages). Thousands of scientific articles followed after that. We now know a lot about soil erosion and how to control it, but the application of the acquired knowledge by farmers lags behind.
In 'Soil Conservation in Europe: Wish or Reality' (Panagos et al., 2016) it is stated that: 'The main reason for soil loss is not the lack of knowledge on how to protect soils, but a lack in governance into policy as a priority.'

One would expect data to be widely available concerning the total surface area or extent of land that is effectively under some form of soil erosion control. However, this not the case. This type of data is hard to find (e.g. Magleby et al., 1995; Conservation effects Great Lakes Region, 2011; Kassam et al. 2014). In 2004 a first draft of a world map on soil and water conservation achievements was presented by WOCAT, with the following explanation of this initiative:

WOCAT has launched this new initiative for a global SWC map in order to show in which areas water, soil and vegetation are used sustainably. Existing maps show degradation at various scales for various land use and degradation types, but there are hardly any maps focusing on the positive aspect of the countless conservation efforts undertaken all over the world. Along with the demand for such maps at a regional,national, or sub-national level, there is also a need for a global-scale overview of achievements inpreventing and combating land degradation. While there are many global maps of negative human impacts on the natural environment, almost none show any positive efforts and achievements made against those impacts.

A follow-up by WOCAT was the 'Questionnaire for Mapping Land Degradation and Sustainable Land Management (MapQuest QM)' (2008). The goal of the questionnaire was:

The ultimate goal of this exercise is to obtain a picture of the distribution and characteristics of land degradation and conservation / SLM activities for a district, a province, a country, a region, or ultimately world-wide. The final output will be maps of land degradation status, causes and impacts, and conversely the conservation status and impacts for major land use systems in the area.

For some results see: WOCAT - Mapping Results and WOCAT - Where the land is greener (2007).  Methodological difficulties encountered in mapping sustainable land management systems at different spatial scales were analysed by Schwilch et al. (2011).

In Europe, member states of the European Union are invited by the Union to develop and adopt regulations or legislation on soil protection under the condition of cross-compliance with the GAEC's of the Common Agriculture Policy (CAP) of the EU. Belgium is most advanced in this.
Görlach et al. (2004) give a summary of data on the costs of erosion in the UK and France. See also the final report of the SoCo-project (2009) on 'Sustainable Agriculture and Soil Conservation' in the EU. 

At the moment, soil is not subject to a coherent set of rules in the European Union: the proposal for a Soil Framework Directive has been withdrawn in May 2014 after it ran into a blocking minority for eight years.
The UK, Germany, France, the Netherlands and Austria had remained firmly against the proposals since they formed a blocking minority in 2007.

Existing EU policies in other areas are not sufficient to ensure an adequate level of protection for all soils in Europe.
In the recent report Ecologic Institute, Berlin, 2017. Udated Inventory and Assessment of Soil Protection Policy Instruments in EU Member States (462 pp.) the conclusion was reached (p. 179):

When looking at the weaknesses of EU level policy instruments in protecting Europe’s soils
the lack of a coherent, strategic policy framework was highlighted across all policy clusters. This lack of a common and integrated strategic policy frame is an important gap, one that had been intended to  be filled by the withdrawn Soil Framework Directive  proposal. Therefore, a
strategic policy framework is missing that would, in an integrated manner: conceptualise soil issues  (including  common definitions on good status); set out priorities and targets;  define monitoring parameters and desired end points; and define the role of different policy instruments in delivering good soil status. In the absence of a common policy framework, soils are addressed in many policy instruments but there is no EU level political or legislative driver for establishing integration and coherence towards an agreed strategic aim and objectives. Not only does this mean the EU policy frame is limited for soils, it means that existing strengths and opportunities that have been identified cannot be fully explored and exploited.

Therefore, in January 2016 "People4Soil" launched a petition to put pressure on European institutions to adopt specific legislation on soil protection, fixing principles and rules to be complied by the Member States. "People4Soil" is a free and open network of European NGOs, research institutes, farmers associations and environmental groups.

How much do we know, in fact, of the economic effects of soil erosion? Thousands of scientific publications -
Google Scholar gives 742,000 hits on 'soil erosion' excluding citations (6th April 2016) - have been written that deal with
the extent, rate, on- and offsite effects and environmental and ecological impact of soil erosion, as well as with the causes, conditions, factors and processes of soil erosion and with quantitative predictive models of the rate of soil loss in tons/ha/year and various methods to control soil erosion. Large sums of money have been spent in carrying out the necessary research and collection of data.

But, according to Google Scholar, a surprisingly low number of only 1840 (or about 0.25%) of these publications deals with the 'economic costs of soil erosion'. The forty five selected references at the end of this website give an impression of the limited available literature on the economic costs of the damage caused by soil erosion. You will notice that most of these references are from the last twenty years.  This is because there aren't many publications on the costs of soil erosion available older than that.

What would it cost to prevent, reduce or mitigate the on- and offsite effects of soil erosion? There are hardly publications available that give a precise answer to this question or an estimate or indication or allow a calculation of the costs involved in preventive action, such as the application of soil erosion control measures by individual farmers. An exception is Région Nord-Pas de Calais (North of France), where costs are specified of nine different soil erosion control measures. Another exception is Conservation practices for landlords (Iowa). Some data is given by public authorities on subsidies payed to farmers for erosion control (Hoag, 2004) and of the costs spent in preventing off-site damage, e.g. by installing and managing retention or buffer basins for the temporary storage of muddy floodwater.

Farmers' perception and attitude concerning soil erosion
An interesting early publication is 'Farmers' attitudes concerning soil erosion and its control' by Gardner and Seitz (1977) based on a survey with questionnaires sent to 135 farmers in 11 counties in Illinois.

Robinson (1999) presents the results of questionnaire surveys of two samples of farmers from erosion-prone areas of southeast England:
The majority of farmers in the two surveys consider erosion to be a minor annoyance to their farming operations which has minimal impact on their past, present or future agricultural land-use policy.

A thorough analysis is given by Erenstein (1999, 302 pp.):
p. viii. The study first assesses the economics of soil conservation in general - with special emphasis on the relationships between technology, economic analysis and policy implications. The quantification and valuation of soil erosion and soil conservation are highly controversial and present considerable analytical challenges that have been tackled in varying ways. By implication, government intervention is controversial too - and has typically been unsuccessful. This has direct implications for both the development of conservation technology and the implementation of conservation interventions.

pp. 61-62.
The analysis is confounded by different interpretations of conservation - namely
absolute, standards-based, efficient and optimal. The different interpretations imply different degrees of erosion control with significant implications for the analysis. Absolute and optimal conservation remain largely hypothetical in view of the costs and complexities involved respectively. In practice, the implementation of soil conservation tends to be standards-based in view of the prevailing uncertainty, whereas economic analysis tends to assess the efficiency implications. The assessment of soil conservation poses considerable analytical challenges that have been tackled in varying ways. The different analytical approaches can be broadly grouped under two main schools of economic analysis: (i) the evaluation school and (ii) the adoption school. Each school focuses on different aspects of soil conservation and to a certain degree is complementary to the other.
The adoption school tries to explain and predict the divergences in soil conservation behaviour between economic agents. However, the extreme socio-economic site-specificity of soil conservation both warrants and entangles this line of research. The numerous factors that potentially influence the adoption decision of the farm household include resources, preferences, technology, and institutions. The availability of land, labour and capital resources constrains the farm household's feasible set of activities. Implementing soil conservation may thereby imply substantial opportunity costs. The household's objectives and attitudes determine whether these costs are compatible with utility maximisation. Compatibility is more likely when implementing conservation implies short-term returns and reduces risk. In part this will depend on technology factors: neither conservation or production technology is homogeneous. This implies significant differences exist between the different conservation technologies themselves; as well as their technological compatibility with current production practices. The compatibility of conservation with utility maximisation will also depend on institutional factors, which encompass a highly influential set of external conditioning variables. Of particular relevance for institutional compatibility are the market imperfections and the corresponding institutional adaptations. The security of rights can be influential in this respect.
The different factors have a profound influence on the incentives for, and capacity of, farm households to invest in soil conservation. The relative importance of each factor is likely to vary over time and space. Understanding such divergences between farm households is of crucial importance in devising effective and efficient government policy and conservation technology - an issue elaborated in Chapter 4.

In the Report of the COST 634 workshop (2006, p. 16) the following is said on the attitude of farmers:
Erosion is generally not the farmer’s problem: What to do if soil erosion does not have any negative impact for the farmer? (Example: the olive orchards in Spain where soil erosion does not effect production.) If the farmer is forced to prevent soil erosion, whereas it is not for his own benefit, society should pay for the environmental services the farmer provides. Added comments by Helge Lundekvam: Even so, farmers often want to be on good terms with society, and goodwill may be a reason to do something, especially since EU agriculture is heavily subsidised. There is now an opportunity, as subsidies are now more linked with environmental impacts of practices. Thus, a more holistic view of production, environmental problems and the farmers’ situation should be used on EU-level and country level down to catchment / community level.     

Lambert et al. (2007) present and discuss "Profiles of US farm households adopting conservation-compatible practices".

Evans (2010) explains in his paper 'Runoff and soil erosion in arable Britain: changes in perception and policy since 1945' why farmers are reluctant to adopt erosion control practices:
For sound economic reasons until 2005 farmers did not think runoff and soil erosion were factors they needed to take into account because they were so little affected (Table 2) by them (Evans, 1996). Over the short term, erosion in Britain rarely strips soil from the land at a rate which affects crop productivity, although that is not true over the long term (Evans, 1996). Rarely is erosion so severe that a gully impedes working the land and spraying and harvesting the crop. Rarely, too, is it the land owners’ property that is affected by muddy floods. The land owner is just one of many who may have to pay the water company more money for treating water to make it drinkable, and that will be a small amount. Hence, the erosional impacts of managing the land have not been apparent to the land manager.

Ingram, Fry and Mathieu, 2010 are "Revealing different forms of knowledge held by agricultural scientists and farmers in the context of soil protection and management":
This paper aims to analyse results from similar studies in England, Switzerland and France which investigated farmer knowledge and understanding of soil, and compared it with the scientists’ or technical experts’ perspective. This analysis aims to highlight similarities between the results and discuss their relevance for implementation of soil protection measures.

Wauters et al. (2010) are identifying factors that explain the adoption of erosion control practices by farmers in Belgium in 'An examination of the theory of planned behaviour in the agri-environmental domain':
Our research shows that in Belgium, the success of many policy instruments will be limited unless we succeed in breeding a more positive attitude towards soil conservation practices into the minds of farmers and landowners. For instance, efforts to improve the ease of use of reduced tillage are likely to have little effect when farmers’ attitudes remain as negative as they are. A lot of extension work in Belgium is currently dealing with technical difficulties when implementing a best management practice. Although this is undoubtedly very useful, to affect farmers’ intentions to adopt and adoption rate itself, one should also try to improve farmers’ attitude towards this practice. Such a challenge might require much more than technical assistance.

Images of soil erosion
Below, a few pictures of on- and offsite effects of soil erosion are shown. For twelve extensive series of photographs of manifestations of soil erosion see Images of soil erosion.

Soil erosion South Limbourg

Rill erosion and colluvial mud deposit, early spring, South-Limbourg, The Netherlands (photo F. Kwaad).

Rill erosion South Limbourg

Details of rill erosion of the picture above, South-Limbourg (photo F. Kwaad).

Rill erosion

Rill erosion grading into gully erosion, April, South-Limbourg (photo F. Kwaad).

Ephemeral gully South-Limbourg The Netherlands

Ephemeral gully ending in a colluvial mud fan along a road in South-Limourg, after a soil erosion event (photo F. Kwaad).

muddy flood South-Limbourg Netherlands

Muddy flood deposit on bare arable field, South-Limbourg, The Netherlands, location same as picture above (photo F. Kwaad).

 Mud on road

Mud on a road in South-Limbourg after a soil erosion event, same location as above (photo F. Kwaad).


Mud in street Valkenburg

Mud in a street in the town of Valkenburg, South-Limbourg, after a soil erosion event (photo F. Kwaad).

Mud on street and in cellar

Mud in a street of the town of Valkenburg and pumping of a cellar after a soil erosion event (photo F. Kwaad).

Soil loss plot

Soil erosion research on experimental plots, using a rainfall simulator, South-Limbourg (photo F. Kwaad).

Morocco rill erosion

Rill erosion going on during a rainstorm on bare arable land (barley and almond trees) in the Rif Mountains, Morocco (photo F. Kwaad).

Oued Morocco

Sediment laden water during a heavy rainstorm. The oued was dry before the storm, Rif Mountains, Morocco (photo F. Kwaad).

Morocco road blocked by soil erosion

Road crossing a dry oued, blocked by debris after a soil erosion event, just downstream of the picture above, Rif Mountains, Morocco (photo F. Kwaad).

Morocco Deposit

Road along the main oued covered with mud after the same soil erosion event as above, Rif Mountains, Morocco (photo F. Kwaad).

Gully Morocco

Over 2 m deep gully, Rif Mountains, Morocco (photo F. Kwaad)

Morocco eroded land

Hills eroded to bedrock and gullies incised in valley-infill material, Rif Mountains, Morocco (photo F. Kwaad).

Economic costs of soil erosion
The following questions can be asked related to the economic effects of soil erosion:
      1. How high is the decrease in crop yield per cm loss of topsoil depth? Does, in fact, soil productivity decrease much further, once the original humus rich A1-horizon of the natural soil profile is gone, such as is the case in large parts of Europe with its centuries old cultural landscapes (provided that sufficient rooting depth remains)?
      2. What is the extent of the offsite damage caused by soil erosion around the world?  How widespread is it?
3. How high are the economic costs of soil erosion around the world today? Is there a trend in economic costs visible over the past hundred years?
4. How much does it cost farmers to apply soil erosion control measures on their farms?
      5. On how many acres of cultivated land around the world soil erosion control measures are practiced today, and has this led to a reduction of the onsite and offsite effects and related economic costs of soil erosion?

Quantitative data to answer these questions are scarce.

A preliminary question is: How well do we really know the rate of soil erosion? On the measurement of soil erosion Lal (2001) remarks:

The measurement of soil erosion is still more of an art than science, and a wide range of techniques are used to monitor soil erosion. There is a strong need to standardize methods of measurement of soil erosion rates at field, hillside and watershed scales.

Fourteen years later Garcia-Ruiz et al. (2015) in their paper "A meta-analysis of soil erosion rates across the world" still warn that the data obtained have not been independent of the method used. See also the review paper on "Natural and anthropogenic rates of soil erosion" by Nearing et al (2017).

Contrary views on the importance of soil erosion have been expressed by Pimentel (1995, 2006, 2013) and Crosson (1995, 2000).

Trimble and Crosson (2000): 
Soil erosion in the United States has been a matter of public concern since the 1930s. Conditions were improved by the 1960s, although no one knew just how much. Starting in the 1970s, however, several studies concluded that erosion was high. Although a few studies have been skeptical of these high rates, most have suggested that soil erosion is an extremely serious environmental problem, if not a crisis. Quantification of the problem has been elusive, and average annual U.S. cropland soil erosion losses have been given as 2 billion, 4.0 billion, 4.5 billion, 4.8 billion, 5 billion, or 6.8 billion tons.

The remarkable feature of all this discussion and attempted rectification is that it was based mostly on models. Little physical, field-based evidence (other than anecdotal statements) has been offered to verify the high estimates. It is questionable whether there has ever been another perceived public problem for which so much time, effort, and money were spent in light of so little scientific evidence.

Pimentel and Burgess (2013):
Since humans worldwide obtain more than 99.7% of their food (calories) from the land and less than 0.3% from the oceans and aquatic ecosystems, preserving cropland and maintaining soil fertility should be of the highest importance to human welfare. Soil erosion is one of the most serious threats facing world food production.   Each year about 10 million ha of cropland are lost due to soil erosion, thus reducing the cropland available for world food production. The loss of cropland is a serious problem because the World Health Organization and the Food and Agricultural Organization report that two-thirds of the world population is malnourished. Overall, soil is being lost from agricultural areas 10 to 40 times faster than the rate of soil formation imperiling humanity’s food security.

Regarding the first of the five questions above Bakker et al. (2005) write as follows:
Although the problem has received much attention recently, hardly any quantitative information on the effect of erosion on agricultural productivity exists. The quantitative information derived at the plot scale is scattered and incoherent, and no quantitative information at the regional or national level (i.e. the level relevant for food production) exists. Inferences made from the synchronicity of soil erosion events and societal changes are therefore not based on quantitative assessments of the impact of soil erosion on agricultural productivity, nor are analogies between the collapse of an-cient societies and the risks facing modern society. For this reason, the extent to which soil erosion is indeed a significant threat to the agricultural productivity of modern societies is an important subject for debate. 

The research presented here reports of statistical analysis of both plot and regional scale with respect to the erosion-productivity relationship. A meta-analysis of plot scale experiments shows that the different methodologies used for the erosion-productivity assessments bear part of the responsibility for the incoherence of the outcomes.At the plot scale, the effect of soil erosion on crop growth has been assessed in numerous experiments where erosion was either simulated by artificial desurfacing, or where productivity losses in strongly eroded areas were compared with losses from less eroded areas. A systematic overestimation of the effects may apply to the first category of experiments, which make up a large part of the research results. Correcting for this overestimation reveals that under intensive, mechanized agriculture yield reductions at the field scale are of the order of only 4% for each 0.1 m of soil loss. Given the fact that the removal of 0.1 m of soil required either long time-spans, or very high erosion rates, this number makes it highly unlikely that erosion may pose a serious threat to food production in modern societies within the coming centuries. An empirical analysis of the relationship between erosion and productivity for modern agriculture at the regional scale, also shows no agreement with previous assumptions concerning the importance of the impact of erosion on agricultural productivity either. The results of this analysis converge with the corrected plot-scale findings of approx-imately 4% per 0.1 m of soil loss. (underscored by FK)

The most comprehensive study of the impact of soil erosion on crop productivity is Den Biggelaar et al. (Parts 1 and 2, 2004).
They start by remarking:
Despite millions of dollars invested in erosion research, it is difficult to state precisely what effect the loss of a unit of soil has on crop yield (Lal, 1987a). This is due in part, as Perrens and Trustum (1984) and Erenstein (1999) observed, to the fact that there is no direct, clear-cut relationship between erosion and productivity, making the assessment of the impact of erosion on productivity difficult. Productivity decline may not relate directly to the amount of soil loss (expressed in Mg or cm ha 21 yr 21 ), but may be a result of erosion-induced changes in the physical, chemical, and biological qualities of soil that influence production (e.g., water holding capacity, soil organic matter (SOM) and nutrient contents, and bulk density). Moreover, soil is only one of the factors affecting productivity, as crop yield is a function of many variables.

One of the conclusions of Part 1 (p. 36) is:
The results of the present analysis show that average crop yields and effects of past erosion on yields (measured in Mg yield decline per cm of erosion) differ greatly by crop, continent and soil order. However, aggregated across soils on the continental level, differences in productivity declines per Mg of soil erosion are fairly small. The absolute yield loss ranged between -0.49 and 1.44 kg/ha/Mg of soil erosion for grain and leguminous crops, and 0.69 and 127.0 kg/ha/Mg for root crops. However, due to differences in mean yields, the relative yield losses per Mg of soil erosion vary more, even though losses were generally small (<< 0.1%/Mg of soil erosion). The exceptions to this general rule were studies on potatoes in North America, in which yields declined by 0.42%/Mg.

In Part 2 (pp. 91-92) Den Biggelaar et al. conclude:
Three main conclusions can be drawn from our analyses:
     First, estimated annual losses at a global scale for the crops and continents considered in our analyses are small relative to the total agricultural production and value of the selected crops. The losses are likely to be masked over the short term by market fluctuations, weather, and other environmental perturbations, diminishing incentives for farmers to adopt conservation practices. Moreover, erosion’s impacts are cumulative and may cause more serious losses if it continues unabated over a long period of time.
     Second, our estimated global annual losses in crop yields and production are at the lower end of the range of previously published estimates of erosion-induced productivity losses (Lal and Stewart. 1990; Janargin and Smith, 1993; Crosson, 1997; Lal, 1998;Young, 1999). Of more interest, especially for soil conservation policy is the finding that losses vary widely between crops, soil orders and regions, and in selected situations can be quite substantial. In general, though, little is known about these losses for many important crops in many developing countries.
     Third, estimated losses in productivity are probably small in relation to offsite impacts (such as sedimentation). These findings underscore the importance of continued policy measures to encourage soil conservation. They also underscore the importance of improved understanding of erosion and its impacts for these crops, soils, and regions where its impacts are most severe or least understood. Finally, more precise estimation of actual losses due to erosion (as opposed to the potential losses estimated here) depends on improved understanding of farmers’ optimal response in the face of changing physical, market, and policy environments.

Inman (2006, p. 16) on the economics of soil erosion in England and Wales:
The costs borne by society as a consequence of soil erosion are known as externalities by economists because they are costs which are not taken into account either by producers or consumers of agricultural goods and services. When such externalities are not included in prices, they distort the market by encouraging activities that are costly to society even if individual (private) benefits are large.
In the agricultural sector, externalities are regarded as having five characteristics or features: 1) their costs are often neglected; 2) they often occur with a time lag; 3) they often damage groups whose interests are not represented; 4) the identity of the producer of the externality is not always known and; 5) they result in sub-optimal economic and policy solutions. Whilst it is relatively straight forward to identify generic externality categories, quantifying many of these categories in monetary terms is extremely difficult. Firstly, it is necessary to know the value of nature’s goods and services, and what happens to these when they are impacted. Secondly, many externalities are associated with non-market goods. For example, how do we value soil-water chemical interactions which produce clean water. 

Based on the available data, it is very difficult to assess how much soil erosion is costing society in England and Wales. We believe the absence of such analysis severely limits the ability to develop appropriately funded policy options to deal with soil erosion effectively. This position is succinctly stated in a June 2003 report of an OECD expert meeting on agricultural soil erosion and soil biodiversity indicators: Off-site costs of erosion and sediment redistribution are probably at least an order of magnitude greater than on-site (private) costs. It should be noted that there is considerable ambiguity in quantification of off-site costs, and especially in how to quantify the impact of agriculture on soil and other natural resources (air and water). This ambiguity needs to be addressed. It was also noted that our capacity to model and characterise off-site impacts remains rudimentary.’

The following Table (from Telles, 2011, p. 293) gives estimates of soil erosion costs in dollars per year. The onsite costs were estimated on the basis of the loss of soil, nutrients, organic matter, productivity and yield. Offsite costs were estimated in various ways. However, the main offsite effects are linked to sedimentation. Depending on the methods used to estimate on- and offsite soil erosion costs, the results can be extremely variable. The majority of studies estimate onsite costs, and these studies show an even wider fluctuation in the estimated figures. Moreover, in terms of the breakdown of total erosion costs, offsite costs are higher than onsite costs.

Telles, 2011, Table Costs of Erosion

Soil erosion costs in dollars/year, data assembled from literature by Telles, 2011.

The following figure gives an impression of the economic costs of soil erosion today:

FAO Costs of soil erosion

Costs of soil erosion by water worldwide (from: FAO, 2014. Food Wastage Footprint, p. 76).

Evrard et al. (2007) give the following overview of the costs of muddy floods in Belgium:
After a flood, the fire brigade and municipal workers clean up public infrastructure and private property (Table 7). Fire brigade interventions cost between 2250 € and 25,000 € per event, while cleaning operations lead to an estimated cost that ranges between 500 € for a single road segment and 11,000 € for a whole village. Several additional repairs to infrastructure may be required, such as unclogging of sewers, local replacement of tarmac or pavements. These works are very costly, ranging between 14,000 € and 300,000 € per event and per municipality. In total, damage to public infrastructure and cleaning induce a global cost of 12.5–122 million €/yr for the entire Belgian loess belt. It must be underlined that the highest costs are only reached after widespread extreme thunderstorms (e.g. August 26–28, 2002, with rainfall depths of more than 100 mm in 24 h in some areas). Damage to houses is also very important, affecting gardens, garages or even the ground floor of the houses. According to the analysis of the Disaster Fund database, mean damage costs reach 4436 € ± 3406 per house. The number of flooded sites with affected houses obtained from the analysis of the regional database (4.2 floods per 100 km2/yr) can be extrapolated to the entire Belgian loess belt (8867 km2 ). Assuming 1 and 10 affected houses per flooded site, damage to private property varies between 1.6 million and 16.5 million €/yr, respectively.

In a follow-up paper (Evrard et al., 2008) it is added:
The Flemish authorities calculated that the construction of all the control measures proposed in the municipal erosion mitigation schemes that were approved by their administration would cost between 7.7– 9.6 million €/yr during the period 2006 – 2025, which is not disproportionate compared with the total damage cost associated with  muddy floods in the Flemish municipalities of the Belgian loess belt (between 8 – 86 million €/yr; Evrard et al., 2007a)

Here is another example of the costs and benefits of soil erosion control measures in Belgium:

Table III Boardman-VanDaele 2015

Table III from: Boardman, J. and Vandaele, K., 2015.  Effect of the spatial organization of land use on muddy flooding from cultivated catchments and recommendations for the adoption of control measures. Earth Surface Processes and Landforms.

The cleaning of mud from watercourses in Belgium amounted to an average volume of 144,440 m3/year in the periode 2012-2016 at an average cost of 24 Euro/m3, total 17.3 million Euro in 5 years (Vlaamse Milieumaatschappij, VVM).

Evans (1995) published data on on- and offsite costs of soil erosion in England and Wales:

TABLE 7. Comparison of some present day on- and off-farm costs of erosion in England and Wales.
Source: based on Evans, 1994 and 1995, 1994 reference refers to 'Report to Friends of the Earth', London, pp.145; published in 1996

On-farm                                                  £ million per year
        Loss in agricultural production               700.0
        Loss in value of land                                  0.04
        Loss of agricultural inputs and outputs        4.8
        Land value of eroded floodplain                 3.8
        Roads and property                                   3.4
        Footpaths                                                  0.98
        Stream channels                                         7.0
        Water pollution*                                     260.0

*Much nitrate probably leaches through the soil rather than being carried off in runoff, and so removal of nitrate from water supplies has been discounted in this estimate of costs

Hein (2007) studied the costs of land degradation in the Puentes Catchment in Southeast Spain and concluded:

Erosion, especially in the form of gulleys and badlands, is highly visible in the landscape and erosion is the main form of land degradation in the area. However, the local costs of erosion are limited. The most important impact of erosion is on agriculture in the upland parts of the catchment. On annual crops, the costs erosion of erosion vary from less than 4 Euro/ha/y on slopes <10% to 35 Euro/ha/y on slopes exceeding 30%. In almond orchards, erosion costs range form at most 7 Euro/ha/y on slopes <10% to 52 Euro on slopes exceeding 30%. However, all irrigated and most dryland agriculture in the catchment is conducted on slopes of less than 20%. These results explain the lack of interest among farmers in applying erosion control measures. Currently, the only measures applied by farmers are contour ploughing, gulley plugging and maintenance of the terraces. These are all low cost measures, of which the benefits are in line with their costs. The use of more expensive erosion control measures, such as terracing, would not be financially viable from a farmers’ perspective. The study shows that visual impressions of highly eroded landscapes or erosion measurements in selected areas may lead to overestimating the economic impacts of erosion, and that analysing the actual costs of erosion is crucial to understand farmers’ responses to land degradation.

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