Dr Chris Huntingford from the UK Centre for Ecology & Hydrology collaborated on a new study which explains how global water availability is affected by changes in vegetation cover...
Our new paper in Nature Geoscience quantifies the impact of regional and major upstream changes in vegetation cover on local rainfall and freshwater resources. The analysis, led by Peking University, reveals how vegetation changes such as tree planting programmes or deforestation have potentially strong effects on rainfall levels and water resources beyond the areas undergoing those changes.
The study finds that for nearly half of the planet’s land surface (45 per cent), an increase in upstream vegetation cover will raise rainfall levels, promoting increased water availability. For these locations, which include central and eastern North America, Europe, and eastern China, avoiding upstream deforestation allows the forests to continue to recycle water into the atmosphere by evapotranspiration, making more water available again.
This suggests that with careful geographical selection, increasing vegetation lushness (for example, reforestation and afforestation) as part of efforts to achieve Net Zero, has the potential co-benefit of improving water security.
In a smaller number of places (34 per cent of the globe), increased vegetation cover upstream actually lowers rainfall locally by adjusting atmospheric circulation patterns. However, when considering all locations downstream in these regions, rainfall changes increase overall water resources. In only a small number of places (8 per cent of the globe) afforestation reduces water availability both locally and downstream.
Developing a single statistic
Central to the detection of vegetation-rainfall teleconnections is the use of known mean wind fields and their ability to carry vegetation-sourced moisture that returns to land as rainfall (P). Changes in vegetation cover are defined by a summary quantity of “Leaf Area Index” (LAI). The analysis uses historical fluctuations in upstream LAI and its known contribution to local P through atmospheric moisture transport (called LAI_w) to develop a single statistic. This easy-to-understand statistic, technically expressed as '∂P/∂LAI_w', is the ratio of change in rainfall per unit of change in major upstream LAI.
This new ratio is calculated for all land points and shows substantial geographical variation. However, it is the simplicity of the ratio that means it provides a “one stop” number to broadly assess where substantially changed local and upstream land cover is likely to be most helpful towards securing on-going water availability.
To support future decisions on land modification and management requires simple metrics that quantify any implications downstream, whether they are welcome or otherwise. We hope our new metric will prove highly useful in that context.
Read the full paper: Cui, J, Lian, X, Huntingford, C. et al. Global water availability boosted by vegetation-driven changes in atmospheric moisture transport. Nat. Geosci. (2022). https://doi.org/10.1038/s41561-022-01061-7