Hydrological impact of widespread afforestation in Great Britain using a large ensemble of modelled scenarios

Summary

This modelling study assessed the hydrological impacts of widespread afforestation across twelve Great Britain catchments using an ensemble of land-surface model scenarios. Afforestation consistently reduced median and low streamflow, with a 2.8% reduction in median flow per ten-percentage-point increase in broadleaf woodland cover, but did not produce nationally-consistent reductions in extreme flood flows. The findings suggest that whilst afforestation may offer carbon and environmental benefits, large-scale planting risks reducing water availability—particularly in drier regions—whilst providing only modest flood mitigation benefit.

Regional applicability

This study is directly relevant to United Kingdom policy and practice, as it evaluates afforestation scenarios across diverse Great Britain catchments and informs current proposals for large-scale tree planting. The findings have direct applicability to English, Scottish, and Welsh water resource and flood risk management planning, suggesting that afforestation trade-offs between carbon storage, water availability, and flood mitigation require careful catchment-specific consideration.

Key measures

Median streamflow reduction (percentage and mm yr⁻¹), low flow response, extreme flood magnitude, catchment broadleaf woodland extent (percentage), catchment size, planting location effects

Outcomes reported

The study modelled streamflow responses to afforestation across twelve diverse catchments using a high-resolution land-surface model with multiple afforestation scenarios. It quantified changes in median flow, low flow, and extreme flood response across different spatial scales and planting configurations.

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