Planting Trees Across Great Britain Could Worsen Droughts Instead of Reducing Floods

As part of its Net Zero strategy, the UK Government plans to increase the total land area of woodland by approximately 30 000 hectares per year up to 2050. The potential hydrological consequences of such largescale afforestation are unknown at a countrywide scale. An open question is whether afforestation may alleviate floods, accentuate droughts, or have only a minimal impact on catchment water resources.

We have three aims: first, determine whether hydrologically distinct regions respond differently to afforestation; second assess whether realistic afforestation projections significantly alter streamflow, and vegetation and soil water stores and fluxes, relative to climate; third, quantify the alteration of water resources to widespread afforestation up to 2050.

We run a high-resolution land surface model for Great Britain, JULES, with past observed climate (CHESS-met) and our generated afforestation scenarios, to observe shifts in known hydrological events (floods and droughts), as well as independent changes in precipitation, temperature, and CO2. We also use the new CHESS-SCAPE dataset of future climate projections to evaluate terrestrial water resource adjustments to afforestation up to 2050.

Similar sensitivity to afforestation is found across regions of Great Britain: median and low streamflow reductions are related to areal extent, rather than location, of deciduous woodland planted. The direction of change with afforestation for the highest flows depends on catchment soil properties. Afforestation has a minimal impact on hydrological processes relative to changes in precipitation, temperature, and CO2. Spatial differences in future precipitation across Great Britain may lead to variations in the modelled runoff response to afforestation between regions. The proposed scale of afforestation is not found to alter hydrological processes markedly in Great Britain, but is projected to lead to noticeable decreases in low flows whilst also not being sufficient to reduce the risk of flooding.