Page 12 - Perspective Paper

acts as a trigger component that is required for condensation and precipitation to occur (Millán, 2014a).
Evapotranspiration has thus been shown to be the most important mechanism in sustaining rainfall over land
surfaces (Savenije, 1996), due to the cascading effects of moisture moving inland (Schaefli et al., 2012). The
loss of evapotranspiration through land degradation is thus a direct threat towards the establishment of
predictable rainfall in many regions.

Terrestrial moisture recycling is an important concept that refers to the process by which moisture
evaporates from land and returns to land surfaces via precipitation. It has been found that 70% of terrestrial
precipitation from 26 of the world’s largest river basins comes from evaporation over land through
atmospheric moisture recycling (Tuinenburg et al., 2020). On average, a drop of water that evaporates into
the atmosphere from the ocean rains out 2.6 times over land before returning to the ocean via river flow
(van Noordwijk & Ellison, 2019). Land degradation may thus have more profound consequences in the
decrease in terrestrial rainfall than has been previously understood (Savenije, 1995). Studies of regreening
arid or semi-arid areas show that the albedo and roughness mechanisms are dominated by the moisture
recycling feedback mechanism on seasonal and interannual time scales (Yu et al., 2017). The clear link
between terrestrial evaporation and terrestrial rainfall strikes the compelling conclusion that terrestrial
precipitation is a modifiable quantity through land cover change - for better or for worse.

3.3 Afforestation to enhance terrestrial precipitation

In the context of afforestation, increased plant water availability through enhanced precipitation is crucial
for the sustainability of regreening efforts (Huang et al., 2022; Teuling et al., 2019; Wang et al., 2021) and
critical to justify investments and design a credible resource management structure. Several recent studies
have explored the changes in the atmospheric water cycle following the large-scale afforestation of the Loess
Plateau in China completed in 1999 – results show enhanced evapotranspiration (Shao et al., 2019; Tian et
al., 2022; Zhang et al., 2022) as well as enhanced local precipitation (Shao et al., 2019; Tian et al., 2022; Wang
et al., 2021; Zhang et al., 2021, 2022). Although an increase in evaporation could imply a decrease in water
yield (precipitation minus evapotranspiration), studies show that the vegetation rather strengthens the local
moisture recycling (also known as the precipitation recycling ratio) (Shao et al., 2019; Tian et al., 2022; Wang
et al., 2021). Tian et al. (2022) revealed that the increased vegetation cover has led to a 20.25% increase in
precipitation as a result of an overall increase in the humidity of the local atmosphere (+8.49%), heightened
soil moisture-vegetation-precipitation feedback (the spatial pattern of soil-moisture increase matches that
of precipitation), stronger moisture convergence (+28.26%), and more cloud formation (+8.28%) (Tian et al.,
2022).
Although previous afforestation efforts have provided great insight on the role of vegetation in governing
weather and climate in those circumstances, there are still important knowledge gaps with respect to
understanding the local and non-local effects of afforestation and how this links to meso- and synoptic-scale
phenomena (Bosveld et al., 2020). Predicting the wider effects of afforestation, and land cover change in
general, on the global terrestrial water cycle requires an understanding of the (non-)biological mechanisms
of moisture transport, a widely discussed topic amongst scientists today. Although there is general consensus
that vegetation cover plays an important role in the global water cycle (Sheil, 2018), the physical principles
behind atmospheric motion and moisture transport, including the role of aerosols, the formation of clouds
and the (bio)physical drivers of wind have, however, proven to be less trivial (Marotzke et al., 2017). The
biotic pump theory, first introduced by Makarieva & Gorshkov (2007) suggests there is an “evaporative force”
driving surface winds - induced by changes in atmospheric pressure caused by condensation above forests.
The theory contradicts the longstanding “traditional” theory in meteorology: that the differential heating of
the atmosphere is the principle driving force of wind (Bunyard, 2014).

The physical principles of the biotic pump theory have been a topic of debate amongst scientists (Bunyard,
2014; Jaramillo et al., 2018; Meesters et al., 2009; Pearce, 2020), and the role of vegetation in generating

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