2. Literature review
The aim of the literature review was to determine the total annual costs of soil erosion in Scotland, based on the evidence of:
- The current soil erosion rates in Scotland;
- The on-site and off-site impacts of soil erosion in Scotland;
- The use of mitigation measures that are used to control soil erosion in Scotland.
The full literature review can be found in Appendix 1, but an updated form of the Executive Summary of the Literature Review is presented here.
2.1. Executive Summary of the Literature Review
It is recognised that soils provide a range of ecosystem goods and services that support a broad spectrum of human activities and associated benefits. The continued provision of benefits from soil depends on successfully maintaining its physical, chemical and biological properties. However, evidence suggests that the way soils are currently used degrades the resource (e.g. by soil erosion) resulting in loss of soil quantity and quality, along with the functions that soils support and the ecosystem goods and services delivered by these functions. This can result in significant costs, not only to immediate users of soils but also to society as a whole. Climate change projections for Scotland indicate more heavy rainfall days and an increase in winter rainfall, leading to greater risks of soil erosion in the future, making the status of ‘Soils and agriculture’ in Scotland of ‘high concern’ (Committee on Climate Change, 2019).
Soil erosion is a natural process that is dependent on climate, topography, soil type, vegetation cover, land use and land management. Inappropriate land use and land management can trigger accelerated rates of soil erosion that have both on-site (where the erosion takes place e.g. the field) and off-site (away from the erosion site e.g. road or river) impacts. Soil erosion was identified as one of the main threats to Scotland’s soils in the comprehensive ‘State of Scotland’s Soil Report’ (2011).
The literature review suggests that soil erosion by water is the dominant erosion process in Scotland. Notable soil erosion events are triggered by either high intensity rainfall; prolonged, low intensity rainfall; or rapid snowmelt. Land uses affected include forests and agriculture (especially bare, 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, which can exceed an identified tolerable limit of 1 t ha-1 yr-1 (Verheijen et al., 2009b). Limitations of the current evidence base include a tendency to focus on small areas of severe soil erosion, rather than a systematic approach to monitor and assess more insidious erosion. In general, quantified rates for all forms of soil erosion in Scotland remain sparse.
Regarding the on-site impacts of soil erosion, there is very little quantification of the reductions in crop yields due to soil erosion in Scotland. Single soil erosion events rarely cause significant problems for farmers, but over time may impact on the long-term sustainability of the land and can still result in loss of ecosystem goods and services such as crop production, but also carbon sequestration and water storage. These on-site impacts of soil erosion may take a long time to take effect, especially on deeper soils or where inputs of fertilisers can mask yield declines due to soil loss.
Off-site, soil erosion contributes to increased suspended sediments and turbidity in Scottish watercourses, which can diminish water quality and damage aquatic life, including salmon spawning ground and freshwater pearl mussel beds. In general, the greater the proportion of arable cropping in a catchment (and thus soil erosion risk), the greater the increase in suspended sediment load in waterways. However, the present review has found few quantified studies of the off-site impacts of soil erosion in Scotland.
There is some evidence on the use of erosion mitigation measures in Scotland: most was related to practices on forested land. However, monetary costs (and benefits) associated with measures to combat soil erosion and sediment transport are lacking for Scotland.
The ‘on-site’ and ‘off-site’ costs of soil erosion are incurred in many different ways, affecting a diverse range of ecosystems services and benefits to people, over a range of spatial and temporal scales. This makes estimating the costs of soil erosion particularly challenging and may explain the limited quantified evidence on the costs associated with soil erosion in Scotland.
The literature review demonstrates that evidence on soil erosion rates, impacts, mitigation and costs in Scotland tends to be anecdotal rather than quantitative. These estimates may have some uncertainty, given the paucity of data.
2.1.1. Update on the Literature Review
Extra references on forest haul roads and skid trails in newly planted / felled areas and the likely effect on soil erosion rates in forested areas have been added to the final Literature Review report. Evidence suggests that they are unlikely to contribute much soil erosion, as guidelines for soil protection in place should be properly implemented. A method of calculating the density of forest roads was found, but as this is likely to be very low, forest roads will not be considered as a separate land use in the analysis.