Modelling the influence of widespread afforestation on UK hydrology

Thesis

With increasing atmospheric CO2, the hydrological system is moving towards more frequent and intense hydroclimatic extremes. With the greater risk this poses to society, we need solutions that reduce atmospheric CO2 and mitigate water-related hazards. Afforestation, mooted to reduce atmospheric CO2 and mitigate flood risk, is being proposed internationally at greater temporal and spatial scales than ever witnessed before. Previous work has assessed the hydrological consequences of woodland planting at relatively small scales (< 10 km2) or at global scales with low process and spatial resolution. There is a clear need for evidence at countrywide scales on whether afforestation will achieve its intended goals.

The work here seeks to determine the influence of widespread afforestation on UK hydrology. The UK plans to annually plant 30 000 hectares of trees to reach its Net Zero goals. This work uses land surface modelling at a higher complexity than is often undertaken when understanding woodland hydrology. Land surface models include a relevant set of Earth system processes, which is critical when drawing conclusions about woodland hydrology. The final research piece uniquely couples a land surface model and convection-permitting atmospheric model to simulate the hydrometeorological consequences of UK widespread afforestation.

In this thesis, afforestation location has a minimal impact on terrestrial hydrology compared to afforestation extent. However, in a land-atmosphere model configuration, woodland along Great Britain’s west coastline increases surface roughness, producing heavier rainfall. Median streamflow reduces by 2.8% ± 1.0 (1 s.d.) for a ten-percentage point increase in catchment broadleaf woodland but there is no consistent reduction of extreme floods. Afforestation minimally impacts hydrological processes compared to changes in precipitation, temperature, and CO2. More arid catchments show greater streamflow sensitivity to woodland expansion potentially increasing the likelihood of drought formation with afforestation.

Work here provides a critical step forward in our understanding of afforestation impact on hydrology and the utility of land surface models in answering policy-relevant questions.

Authors: Buechel, M

• DOI: 10.5287/ora-a4amjyr70
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: land surface model; afforestation; hydrology
• Subjects: Hydrology; Numerical Modelling