Forests have long been associated with an ability to reduce flood flows, however valuing the contribution that forest cover makes to downstream flood alleviation is very difficult given the multiple factors and associated uncertainties involved.
Despite a strong understanding of the processes by which woods and trees affect the generation and conveyance of flood waters, there remains a lack of measurements to fully quantify effects at the catchment scale, particularly on large floods and within large river basins. This means that we continue to largely rely on modelling studies to estimate impacts.
Findings and Recommendations
A previous valuation by Broadmeadow et al. (2018) was heavily caveated and thought likely to underestimate the contribution of woodland to flood risk management. Consequently, the work has been updated to provide improved estimates of the flood regulation service provided by GB woodland. A companion report (Fitch et al., 2022) is available that applies the approach to other natural capital types, including woodland, although values are not directly comparable due to methodological differences.
As before, the study uses the Joint UK Land Environment Simulator (JULES) model and expert judgement to estimate the additional volume of flood water potentially lost by woodland water use or retained by hydraulic roughness of floodplain woodland. The parameterisation of the model was improved following an expert workshop, and unlike before, separate values were generated for conifer and broadleaf woodland. Trees outside woodland were included but young trees and recently felled woodland excluded. The flood regulation service was compared with two alternative land covers, short grass and bare soil. It was assumed that an equivalent level of flood water storage would have to be provided if the woodland cover was absent, with the “replacement” cost based on the average cost per m3 of constructing reservoir flood storage.
The natural capital value (over 100 years) of the flood regulation service provided by woodland across Britain, including trees outside woodlands, was estimated at £25.1 billion (£7,974/ha) compared to bare soil and £12.5 billion (£3,970/ha) compared to grass. Expressing the flood regulation service as an annualised central estimate gave values of £843 million/yr (£268/ha/yr) and £420 million/yr (£133/ha/yr) compared to bare soil and grass, respectively. These values are significantly greater than those generated by the previous assessment, reflecting the improved modelling of woodland water use and the broader range of woodland types in the analysis. The results are informing natural capital accounts.
Research Report: Broadmeadow, S., Nisbet, T., Valatin, G., Blyth, E., Robinson, E., Fitch, A., & Jones, L. 2023. Revised valuation of flood regulation services of existing forest cover to inform natural capital accounts. Forest Research Report to Forestry Commission, Scottish Forestry, Defra and Welsh Government. Forest Research, Alice Holt Lodge, Farnham, Surrey, 38pp.
