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

Marcus Buechel,Louise Slater,Simon Dadson

doi:10.1038/s43247-021-00334-0

Abstract

Ambitious afforestation proposals in the last decade target potential flood mitigation and carbon storage benefits but without a systematic, large-scale (>1000 km2) quantitative evaluation of their impacts on streamflow. Here, we assess the impact of afforestation on streamflow across twelve diverse catchments (c.500-10,000 km2) using a high-resolution land-surface model with a large ensemble of afforestation scenarios. Afforestation consistently decreases median and low streamflow. Median modelled flow is reduced by 2.8% ± 1.0 (1 s.d.), or 10 mm yr−1 ± 2.1 (1 s.d.), for a ten-percentage point increase in catchment broadleaf woodland. We find no nationally-consistent reduction of extreme floods. In larger catchments, planting extent is a stronger control on streamflow than location. Our results suggest that despite its potential environmental and societal benefits, widespread afforestation may inadvertently reduce water availability, particularly in drier areas, whilst only providing a modest reduction in extreme flood flows.

Highlights

Ambitious afforestation proposals in the last decade target potential flood mitigation and carbon storage benefits but without a systematic, large-scale (>1000 km2) quantitative evaluation of their impacts on streamflow
Few systematic evaluations exist of how afforestation location and extent may influence catchment hydrology across a wide range of climatic and physiographic conditions[19,20]
Our work is the first, to our knowledge, to employ a high-resolution 1 km[2] physics-based landsurface model to quantify the impact of afforestation on streamflow across multiple catchments using a large ensemble of land-cover scenarios. This approach allows us to isolate drivers influencing catchment sensitivity, determine individual hydrological processes altered by widespread afforestation, elucidate the uncertainty generated by afforestation location and most importantly find the impact of afforestation over large spatial extents on streamflow

Summary

Introduction

Ambitious afforestation proposals in the last decade target potential flood mitigation and carbon storage benefits but without a systematic, large-scale (>1000 km2) quantitative evaluation of their impacts on streamflow. Numerous studies have investigated the influence of afforestation on hydrology, including plot-scale studies to understand infiltration rates and groundwater levels, and small catchment studies evaluating the impact of land-cover change on streamflow and water quality[13,14,15] These studies are often smaller than 1000 km[2] in size, and it is unknown whether the impacts of afforestation may scale up over larger catchments, given its complex influence on streamflow[16,17,18]. Our work is the first, to our knowledge, to employ a high-resolution 1 km[2] physics-based landsurface model to quantify the impact of afforestation on streamflow across multiple catchments using a large ensemble of land-cover scenarios This approach allows us to isolate drivers influencing catchment sensitivity, determine individual hydrological processes altered by widespread afforestation, elucidate the uncertainty generated by afforestation location and most importantly find the impact of afforestation over large spatial extents on streamflow. Beyond the British Isles, our results describe the influence of afforestation on temperate catchment hydrology

Methods

Results

Conclusion