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

8. Discussion of results

8.1. Catchment 1: The Ugie

For total annual on-site costs of soil erosion in the Ugie, 60% and 22% are associated with losses of soil N and soil P respectively through loss of soil via erosion, with only 15% due to losses of crop productivity brought about by reduced rooting depth (water store of plant available water etc.) due to soil erosion. For total annual off-site costs, the highest costs come from having to remove eroded soil (sediment) from waterbodies used for drinking water supplies (48% of total off-site costs), followed by the costs associated with greenhouse gas emissions arising from carbon stored in the soil being exposed to the atmosphere via soil erosion processes (19%). Off site costs are substantially higher than on-site costs (£278,634 (including drinking water costs) versus £60,281 respectively), demonstrating the importance of preventing erosion at source, rather than treating it after it has occurred. Annual costs of soil erosion per hectare of catchment area (including treatment for drinking water) are the highest (£11.34 ha^-1 ) for the Ugie catchment, compared with the other case study catchments.

8.2. Catchment 2: The Pow

Of all on-site costs, 53% comes from losses of soil N. Losses of soil P make up 26% of all on-site costs. Crop yield losses due to soil erosion make up 18% of all on-site costs. Most off-site costs (55%) are associated with sediment removal for drinking water (so may be an overestimate as no drinking water abstraction currently takes place). The next largest contribution to off-site costs is removal of sediment from rivers and canals (16% of all off-site costs). The costs of greenhouse gas emissions due to soil erosion contribute 12% of the total off-site costs.

8.3. Catchment 3: The Girvan

15% of the Girvan catchment is considered to be at risk of soil erosion. Most of the annual on-site costs of erosion are related to losses of soil N (£22,182 = 63% of all on-site costs). Annual losses of soil P due to erosion are also important (21% of all on-site costs). Crop yield losses due to erosion cost £4,236 per annum (= 12% of total on-site costs). Half of the total annual off-site costs are associated with removal of sediment from watercourses where drinking water may be abstracted. Costs of N and P in watercourses are relatively low. Costs of greenhouse gas emissions make up 18% of the total off-site costs of soil erosion in the catchment.

8.4. Catchment 4: The Esk

As with the other catchments, the majority of annual on-site costs are associated with soil N losses (64% = £47,302) and P losses (£12,600 = 17%). This demonstrates the importance of controlling erosion to keep nutrients on the land / in the soil, rather than washing off during erosion events. If catchment waters are needed for drinking water abstraction (although not the case currently), then this will incur costs of £164,511 for removal of sediment and £14,332 for N removal. This represents 46% and 4% of total offsite costs associated with soil erosion respectively. This is in addition to the annual dredging costs of removing sediment from rivers and canals (estimated to costs £47,069 = 13% of total off-site costs). Almost a quarter of off-site costs are associated with greenhouse gas emissions from soil erosion exposing buried soil carbon (£83,831).

8.5. Catchment 5: The Tweed

As the largest of the case study catchments (351,190 ha), total annual soil erosion costs are highest for the Tweed (£1.773 million (without drinking water treatment costs); £3.071 million with drinking water treatment costs). Most on-site costs came from losses in soil nutrients, N (£351,722 = 64% of total on-site costs) and P (£108,668 = 20% of total onsite costs). Reduced yields contributed 12% of the total on-site costs. Off-site costs were incurred when drinking water was treated to remove sediment (47% of total off-site costs). Greenhouse gas emissions were also important costs associated with soil erosion (20% of total off-site costs).

8.6. The national scale

The total area of the unsealed land use/soil type categories was 7.108 million ha, of which approximately 18% or 1.271 ha was estimated to erode each year at the rates defined for the different land use categories identified for Scotland. This resulted in estimated erosion of 0.9 Mt yr^-1, at a mean erosion rate of 0.13 t ha^-1 yr^-1, and implied a mean soil depth loss of 0.23 mm yr^-1. Overall, the estimated annual on-site costs of £9.695 million were substantially less than the estimated annual off-site costs of £39.803 million (with drinking water treatment costs) or £21.277 million (without drinking water treatment costs). The annual costs associated with lost productivity were relatively minor (£1.244 million = 13% of all on-site costs) and it is primarily the replacement value of the stock of nutrients (especially N) that are removed in the eroded material that comprise the majority of the on-site costs. The annual off-site costs are primarily associated with a) the cost of removing sediment from drinking water (£16.541 = 42% of all off-site costs million), b) carbon losses through greenhouse gas emissions (£10.880 million = 27% of all off-site costs) and c) sediment removal from water courses (£4.732 million = 12% of all off-site costs).

The calculations of annual costs for arable and improved grassland on organo-mineral soils and peats may be overestimated due to mismatches in scale between the soil spatial data and the land use data and as soil attributes and probabilities of erosion were averaged for both organo-mineral and peat soils. This means that small areas of arable and improved grasslands are predicted to occur on organo-mineral soils and peats which are then estimated to have high erosion rates, higher probability of erosion and greater concentrations of carbon and nitrogen compared to the mineral soils. In reality, these areas are likely to have carbon concentrations close to the threshold between mineral and organo-mineral soils (i.e. around 12% carbon). However, the assigned weighted averages of carbon and nitrogen concentrations are around an order of magnitude greater than mineral soils. This is further compounded by assigning a single probability of erosion risk of 21% to both organo-mineral and peat soils whereas, the organo-mineral soils had a calculated probability of only 12.1% compared with 30% for peats.

For example, while these mismatches may be small in extent (<3% of the land area of Scotland), they are responsible for 12% of the costs of GHG emissions from eroded soils (Table 24) and 9% of the annual cost of removing N from rivers and canals (Table 26).

8.7. National estimates: comparison with England and Wales (Graves et al., 2011)

The current estimates of annual soil erosion costs in Scotland were based on a methodology similar to that used by Graves et al. (2011; 2015) for England and Wales. Meaningful comparison of results is not possible because of the revisions made to the approach used in the present project. Also, market prices used to value soil services and costs of production will have changed since 2011, making the values for England and Wales out of date.

For reference, the on-sites of soil erosion in England and Wales were estimated at £39.874 million (on-site) and £108.153 million (off-site) per annum (Graves et al., 2015), making total annual costs being £148.027 million for England and Wales (2011 prices are used in Graves et al. (2015)).