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Valuing flood regulation services for natural capital accounts

Home Research Forestry and Natural Flood Management Valuing flood regulation services for natural capital accounts

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

We have adopted a relatively simple approach for valuing the flood regulation service provided by existing forest cover in Great Britain (GB). The method is heavily caveated but improves on previous estimates (e.g. Ricardo, 2016). Forest cover is based on the FC GB National Forest Inventory, which maps the extent of all woodland >0.5 ha. Small woodlands, trees and hedges are therefore excluded and no consideration is given to forest age. Managed grassland is selected as the counterfactual land cover.

The study uses the Joint UK Land Environment Simulator (JULES) model and expert judgement from floodplain modelling to estimate the additional volume of flood water potentially lost by woodland water use or retained by hydraulic roughness of floodplain woodland for existing GB woodland, compared to an alternative grass cover. The assessment was limited to ‘Flood Risk Catchments (FRC)’ defined as areas draining to downstream communities impacted by flooding.

Calculated volumes are expressed in m3/ha and considered to be equivalent to effective flood water storage that would have to be provided if the woodland cover was absent and replaced by managed grassland.

The value of this woodland flood water storage was estimated based on the average cost per m3 for providing the same volume by constructing and operating a flood storage reservoir. A central estimate of £12.7/m3 at 2018 prices was obtained from seven reservoir storage schemes. These replacement costs were annualised assuming a 100 year life span for the constructed reservoir storage and gave a central estimate of £0.42/m3/yr.

Expressing the flood regulation service as an annualised central estimate gave values of £218.5 million/yr as an annual average. These numbers are heavily caveated by a range of limitations of the approach. It is thought likely that the assessment underestimates the contribution of woodland to flood risk management, especially for conifer woodland. Nevertheless, the method is considered to improve on previous estimates and to provide conservative lower bound values for the flood regulation service provided by existing GB woodland.