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

Abstract Ambitious afforestation proposals in the last decade target potential flood mitigation and carbon storage benefits but without a systematic, large-scale (>1000 km 2 ) quantitative evaluation of their impacts on streamflow. Here, we assess impact streamflow across twelve diverse catchments (c.500-10,000 using high-resolution land-surface model with large ensemble scenarios. Afforestation consistently decreases median low Median modelled flow is reduced by 2.8% ± 1.0 (1 s.d.), or 10 mm yr −1 2.1 for ten-percentage point increase catchment broadleaf woodland. We find no nationally-consistent reduction extreme floods. In larger catchments, planting extent stronger control than location. Our results suggest that despite its environmental societal benefits, widespread may inadvertently reduce water availability, particularly drier areas, whilst only providing modest flows.