Developing a method to estimate the costs of soil erosion in high-risk Scottish catchments: final report

Executive summary

The aim of this project is to estimate the financial costs of soil erosion in Scotland. These costs are incurred by land-based businesses where soil erosion occurs, and by society where soil is deposited ‘off-field’ or in-stream. Understanding the impacts and costs of soil erosion will inform policies designed to value the soil resource in delivering a range of ecosystem goods and services that support a broad spectrum of human activities and associated benefits.

Reviewing the current evidence base suggests that soil erosion by water is the dominant erosion process in Scotland. Land uses affected include forests and recently ploughed / seeded arable fields in winter cereals. Observed erosion rates in arable areas of Scotland range from 0.01 t ha^-1 yr^-1 to 23.0 t ha^-1 yr^-1, compared to a tolerable limit of 1 t ha^-1 yr^-1 (Verheijen et al., 2009b). Off-site, soil erosion can diminish water quality and damage aquatic life, including salmon spawning ground and freshwater pearl mussel beds. In general, evidence of erosion rates, their impacts and mitigation in Scotland remains sparse, and tends to be anecdotal rather than quantitative.

To estimate the total on-site and off-site costs of soil erosion per annum in Scotland, an updated version of the ecosystem service framework approach used by Graves et al. (2011; 2015) was used. Here, the identified and quantifiable annual costs of soil erosion include: a) on-site costs (e.g. decline in agricultural and forestry yields caused by the reduction in soil depth, and the cost of replacing losses in soil-based C, N, P and K); and b) offsite costs (e.g. declines in water quality and greenhouse gas emissions).

A number of representative catchments were selected for the analysis: the Ugie; the Pow; the Girvan; the Esk and the lower Tweed catchment. Once the methodology was applied successfully at the catchment scale, it was then extrapolated to the national scale. Land use was simplified into 10 categories: Urban; Horticulture; Arable intensive; Arable extensive; Grassland improved; Grassland unimproved; Rough grassland; Forestry; Woodland; and Wildscape. The catchments were also classified into Soil Erosion Risk Classes: High, Moderate and Low for mineral soils, and one class for organo-mineral and peat soils.

The first step was to assess the degree and severity of soil erosion in the catchments. Given the lack of quantified erosion rates for Scotland, it was decided to use the most comprehensive database of soil erosion observations (the “National Erosion Survey”; Evans (2005), with over 1,680 field observations. This showed that soil erosion rates were largely driven by land use. The probability of erosion occurring was more related to Soil Erosion Risk Class.

The results are presented on a ‘per annum’ basis and can be used to justify the need for and expenditure on soil protection interventions that control soil erosion. Currently, there is no policy related specifically to soil protection in Scotland (McKee, 2018). Most policy instruments are not explicitly designed to target soil, rather they view soil as an asset that underpins agricultural production, water quality or climate change mitigation (Prager et al., in press). Post-CAP, soil protection should be incorporated into national policies, although the efficacy of measures to prevent soil erosion should be reviewed. The question arises as to who should pay for implementation of mitigation measures, if deemed necessary (i.e. to reduce the costs of soil erosion), which has policy implications.

This assessment of the annual costs of soil degradation largely confirms the difficulties of deriving complete and reliable estimates of the benefits provided by soils and how these change according to soil condition. There are three aspects to this challenge (i) ‘identifying’ biophysical relationships between soil properties, soil functions and ‘performance’ of soils in particular applications (ii) ‘valuing’ the diverse range of market and non-market benefits and costs attributable to soils in different applications and (iii) assessing the ‘dynamics’ of soil properties, especially under conditions of climate change, as these affect changes in the supply and value of services.

The present study estimated that the total annual costs of soil erosion in Scotland are £49,498,461 (including drinking water treatment costs) or £30,972,155 (not including water treatment costs). Off-site costs of soil erosion always exceeded those associated with on-site costs for all the 5 study catchments. The same was true at the national scale.

Understanding the impacts and costs of soil erosion will inform national policies and local practices designed to value the soil resource in delivering a range of ecosystem goods and services that support a broad spectrum of human activities and associated benefits.