Earth science: Plants boost global water availability

Increases in vegetation cover over the last two decades have enhanced global water availability, but the hydrological impacts are beyond the regions undergoing vegetation changes, according to a paper published this week in Nature Geoscience.

Vegetation cover across much of the world has changed substantially in recent years due to a combination of factors including afforestation (tree planting), deforestation, and the fertilization effect of rising atmospheric carbon dioxide. Plants play an important role in the water cycle through their influence on evapotranspiration and subsequent precipitation.

Jiangpeng Cui, Xu Lian, and colleagues show how vegetation changes between 2001 and 2018 have influenced surface water availability by combining atmospheric moisture tracking data with observations of precipitation and leaf area index — a measure of the amount of foliage in a given area. Globally, the authors find that recent changes in vegetation have increased water availability by around 0.26 mm per year, equivalent to 15% of the overall recent decline in global freshwater availability. However, on a regional scale, the response is variable as the recycled water vapour is transported elsewhere. The authors produce a map quantifying how an increase in plant cover at any given location would impact water availability. More vegetation increases both local and downwind water availability for about 45% of the¬¬ global land surface, including central and eastern North America, Europe, and eastern China. For 34% of global land area, mainly in water-limited regions, increased vegetation reduces regional water availability but has a positive effect downwind. Increased vegetation reduces both regional and downwind water availability for 8% of the land surface, in regions such as western Canada, the southern Tibetan Plateau, and the northern Congo basin.

With widespread afforestation expected to play a key role in climate mitigation strategies, the authors argue it is important to understand how vegetation changes can impact the water cycle particularly in regions downwind, and inform future afforestation strategies.