Academician Park Shilong's team wrote an article in Nature Earth Science showing that greening vegetation is beneficial to global surface water resources through cross-regional water vapor transport

Over the past 20 years, global change, especially the implementation of afforestation projects, has led to a significant increase in terrestrial vegetation activity (i.
e.
the "greening" of the planet).

This trend is profoundly altering the terrestrial water cycle and affecting regional water security
.
Previous studies have suggested that greening of vegetation promotes evapotranspiration on the surface, while the dissipated water vapor continues to participate in the local scale water vapor recirculation to compensate for the loss
of surface available water in the form of precipitation.
In addition to this local effect, transpiration water vapour from vegetation can be transported hundreds to thousands of kilometers through atmospheric circulation, forming precipitation in downwind areas, thereby affecting surface water resources
across regions.
This suggests that current site- or local-scale studies may generally underestimate the intensity
of feedback from vegetation change to terrestrial precipitation as well as available water.
How to use observation data to quantify the cross-regional hydrological effects of vegetation change is one of the difficult problems that have long plagued academia, and it is also the key to solving the hot scientific question of "whether ecological engineering such as afforestation is beneficial or disadvantageous to global surface water resources
".

Based on the global vegetation data observed by remote sensing and combined with the theory of atmospheric water vapor transport, the team of Academician Park Shilong of the School of Urban and Environmental Sciences of Peking University established a hydrological weighted vegetation change index
that considers the contribution of both local and upwind water vapor source areas by assigning water vapor contribution weight factors to the local and upwind water vapor source areas.
This index breaks through the methodological limitations of traditional observational studies that cannot consider the cross-regional hydrological effects of vegetation, and supports the diagnosis of the feedback effect
of vegetation changes on local precipitation in water vapor source areas through empirical models.

The study found that the greening of global vegetation in the past 20 years has increased the surface water availability at a rate of 0.
26 mm per year (the change in precipitation evapotranspiration caused by greening), which has offset the decrease in surface water availability by about 15%
in the same period.
The area where vegetation changes increases the availability of water accounts for about 53% of the world's land area, mainly distributed in eastern China, Europe, western Siberia, and western and southern Africa
.
In these areas, the evapotranspiration water loss caused by greening of vegetation is less than that of greening of vegetation in the upwind water vapor source area to promote local precipitation, resulting in an increase
in surface water availability.
However, the reduction in surface water availability due to local vegetation changes is very significant
.
For example, widespread deforestation in the tropics disrupts the positive feedback mechanism of vegetation to regional precipitation, drastically reducing water availability locally and downwind
.

Figure 1.
Pattern of integrated hydrological effects of global vegetation (+ indicates significantly advantageous, O indicates insignificant, in the legend on the left, ?) indicates significant disadvantage; The left and right symbols indicate the impact on local/regional and downwind surface water availability, respectively)

The research team further focused on the cross-regional impact of the current large-scale afforestation projects around the world, and developed the first global comprehensive hydrological pattern of afforestation (Figure 1).

Encouragingly, the study found that in 45% of the world's land mass (green dot in Figure 1), afforestation benefits both local and downwind water
availability.
This suggests that afforestation in these areas not only promotes carbon sinks, but also yields considerable water benefits
.
However, in water-restricted areas and high altitudes, increased vegetation leads to reduced
water availability locally and/or downwind.

This study is the first to estimate the cross-regional hydrological effects of global vegetation change by observation, quantitatively confirming the important role of vegetation change on the surface water availability in the downwind direction through long-distance water vapor transport, and providing key scientific support
for comprehensively assessing the impact of ecological projects such as afforestation on regional climate and surface water resources in China.

The study was published in the journal Nature Geoscience (doi:10.
1038/s41561-022-01061-7) under the title "Global water availability boosted by vegetation-driven changes in atmospheric moisture transport
" 。 The Nature Asia website reported the findings in "Research highlights" under the title "Earth science: Plants boost global water availability
".
Cui Jiangpeng, a distinguished associate researcher at the School of Urban and Environmental Sciences of Peking University, is the first author, and Lian Xu, a doctoral student (who has obtained a degree), is the corresponding author
.