6.1.1.3 Flying Rivers and Watersheds of the Sky
The mechanisms driving the water cycle were confirmed using heavy oxygen isotopes (oxygen-18), and it was found that half of the rainfall over the Amazon came
from the transpiration of the forest itself (Salati et al. 1979).
Additionally, van der Ent et al. (2010) computed that, on average, 40% of
terrestrial precipitation originates from land evaporation and that 57% of all terrestrial evaporation returns as precipitation over land. Moisture evaporated from the
Eurasian continent is responsible for 80% of China’s water resources. In South
America, the Río de la Plata basin depends on evaporation from the Amazon forest
for 70% of its water resources. The main source of rainfall in the Congo basin is
moisture evaporated over East Africa, particularly the Great Lakes region. The
Congo basin, in turn, is a major source of moisture for rainfall in the Sahel.
Furthermore, it has been demonstrated that due to the local orography, local moisture
recycling is a key process near the Andes and the Tibetan Plateau.
The biotic pump theory, proposed by Makarieva and Gorshkov (2010), pertains to
the importance of forests in the water cycle in determining the levels of rainfall. The
biotic pump theory suggests that intense condensation, associated with high evaporation from natural forest cover, maintains regions of low atmospheric pressure on
land, causing moist air to flow from ocean to land, which compensates for river
runoff. The biotic pump governs continental moisture supplies through horizontal
pressure gradients. High evaporation rates and extensive natural forests guarantee
stable and high-throughput hydrological cycles, protecting continents against devastating floods, droughts, hurricanes, and tornadoes; therefore, sustaining natural
forests is a sound strategy for promoting water security and stabilizing the climate.
Makarieva and Gorshkov (2010) described how the water vapor produced by
trees drives wind patterns: (1) winds cross the continent, taking moist air from
Europe, through Siberia, and into Mongolia and China; (2) winds deliver rains that
keep the very large rivers of eastern Siberia flowing; and (3) winds deliver the
precipitation that waters China’s northern plain, the breadbasket of the most populous nation on earth (Fig. 6.5). As Pearce (2020) reported, Makarieva stated that
“Forests are complex, self-sustaining rainmaking systems and the major driver of
atmospheric circulation on earth.” Forests recycle vast amounts of moisture into the
air and, in the process, also generate winds that pump that water around the world.
Additionally, Pearce (2020) notes that many meteorology textbooks still teach a
caricature of the water cycle, with ocean evaporation responsible for most of the
atmospheric moisture that condenses in clouds and falls as rain, ignoring the role of
vegetation and, in particular, trees, which act as water fountains. Their roots capture
water from the soil for photosynthesis, and microscopic pores in leaves release
unused water as vapor into the air, a process that is known as transpiration. Through
transpiration, a single mature tree can release hundreds of liters of water a day.
The precipitation cycle in an area can be considered a watershed of the sky, which
identifies the origin of the precipitation falling in a given region. By applying a
model similar to that by van der Ent, Keys et al. (2018) found that 19 of 29 megacities
204
M. Osaki et al.
The mechanisms driving the water cycle were confirmed using heavy oxygen isotopes (oxygen-18), and it was found that half of the rainfall over the Amazon came
from the transpiration of the forest itself (Salati et al. 1979).
Additionally, van der Ent et al. (2010) computed that, on average, 40% of
terrestrial precipitation originates from land evaporation and that 57% of all terrestrial evaporation returns as precipitation over land. Moisture evaporated from the
Eurasian continent is responsible for 80% of China’s water resources. In South
America, the Río de la Plata basin depends on evaporation from the Amazon forest
for 70% of its water resources. The main source of rainfall in the Congo basin is
moisture evaporated over East Africa, particularly the Great Lakes region. The
Congo basin, in turn, is a major source of moisture for rainfall in the Sahel.
Furthermore, it has been demonstrated that due to the local orography, local moisture
recycling is a key process near the Andes and the Tibetan Plateau.
The biotic pump theory, proposed by Makarieva and Gorshkov (2010), pertains to
the importance of forests in the water cycle in determining the levels of rainfall. The
biotic pump theory suggests that intense condensation, associated with high evaporation from natural forest cover, maintains regions of low atmospheric pressure on
land, causing moist air to flow from ocean to land, which compensates for river
runoff. The biotic pump governs continental moisture supplies through horizontal
pressure gradients. High evaporation rates and extensive natural forests guarantee
stable and high-throughput hydrological cycles, protecting continents against devastating floods, droughts, hurricanes, and tornadoes; therefore, sustaining natural
forests is a sound strategy for promoting water security and stabilizing the climate.
Makarieva and Gorshkov (2010) described how the water vapor produced by
trees drives wind patterns: (1) winds cross the continent, taking moist air from
Europe, through Siberia, and into Mongolia and China; (2) winds deliver rains that
keep the very large rivers of eastern Siberia flowing; and (3) winds deliver the
precipitation that waters China’s northern plain, the breadbasket of the most populous nation on earth (Fig. 6.5). As Pearce (2020) reported, Makarieva stated that
“Forests are complex, self-sustaining rainmaking systems and the major driver of
atmospheric circulation on earth.” Forests recycle vast amounts of moisture into the
air and, in the process, also generate winds that pump that water around the world.
Additionally, Pearce (2020) notes that many meteorology textbooks still teach a
caricature of the water cycle, with ocean evaporation responsible for most of the
atmospheric moisture that condenses in clouds and falls as rain, ignoring the role of
vegetation and, in particular, trees, which act as water fountains. Their roots capture
water from the soil for photosynthesis, and microscopic pores in leaves release
unused water as vapor into the air, a process that is known as transpiration. Through
transpiration, a single mature tree can release hundreds of liters of water a day.
The precipitation cycle in an area can be considered a watershed of the sky, which
identifies the origin of the precipitation falling in a given region. By applying a
model similar to that by van der Ent, Keys et al. (2018) found that 19 of 29 megacities
204
M. Osaki et al.