Together with the warming climate and the increasing influence of human activities, water reserves in closed water basins are decreasing everywhere. New research has shown that such a decline exacerbates the effects of water scarcity and leads to rising ocean levels.
The study "Recent declines in the world's surface water resources" was carried out by a group of scientists from six countries and its results are published in the journal Nature Geoscience.
“Water resources are extremely limited in the continental hinterland, where runoff does not reach the ocean. Scientifically speaking, these regions are closed drainage basins,”said lead researcher Jida Wang, a geographer at the University of Kansas.
“Over the past decades, we have seen more and more evidence of water imbalance in indoor pools. For example, the disappearing Aral Sea, depletion of the aquifer in Arabia, retreating glaciers in Eurasia. Such evidence gives rise to the question: Are the world's water reserves decreasing in the internal drainage areas that make up one fifth of the entire surface of the continents?"
Using gravity observational data from NASA satellites, the research team has quantified the global water loss in indoor basins at one hundred billion tons annually since the turn of the millennium. This means that in dry closed areas, water disappears annually in a volume equivalent to five Great Lakes.
Surprisingly, these water losses in enclosed basins are twice as large as the simultaneous changes in the amount of water on the rest of the continent, in areas where water enters the ocean. Such waste areas occupy most of the land, and major rivers such as the Nile, Amazon, Yangtze and Mississippi flow through them.
Wang notes that the graph of water stock changes in drainage areas correlates with significant fluctuations in climatic systems, such as the multi-year cycles of the El Niño and La Niña currents. However, water losses in closed areas are less dependent on natural short-term changes. This difference indicates a profound impact on the water balance of inland regions of long-term climatic conditions and human activities, such as altering river channels, building dams and draining water bodies.
According to the researchers, these water losses in arid, closed-flow areas have dual consequences. They not only exacerbate the problem of water scarcity, but also contribute to an important factor of concern on a global scale: rising sea levels. This rise is due to two main reasons: thermal expansion of water as a result of global warming and an increase in the volume of water discharged into the ocean.
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“The volume of water is limited,” says Chunjiao Song of the Nanjing Institute of Geography and Limnology, co-author of the study. - When the water supply in the closed basin falls, the water does not disappear anywhere. It moves, mainly through evaporation, to areas with runoff and gains the ability to influence ocean levels.”
Over 14 years of observations, water losses in closed areas amounted to the equivalent of a 4 mm rise in ocean level. This represents 10 percent of the total sea level rise over this period and corresponds to half of the mountain glacier losses and the total groundwater recovered.
“We are not claiming that the water lost in recent years in closed regions has completely moved into the ocean,” says Yoshihide Wada, deputy director of the International Institute for Applied Systems Analysis in Austria, and co-author of the work. - We showed how significant these losses were. If this continues, excess water flowing into runoff areas will become an important source of ocean level rise.”
By combining satellite observations with the behavior of hydrological models, Wang's group associated global water losses in closed-drainage regions with comparable inputs from the earth's surface - from water bodies and glaciers, as well as from flags contained in soil and from aquifers.
“These comparable losses, however, are the average of regional differences,” Wang says. - For example, in closed Central Asia, about half of the water is lost from the surface, especially in such closed lakes as the Aral and Caspian Seas, Lake Urmia, and melting glaciers in the Himalayas.
The melting of glaciers is a consequence of the rise in temperature, but the loss of water in the closed lakes is the result of the combined effect of droughts and the long-term diversion of the rivers flowing into these lakes.
The cumulative water losses in the closed-flow areas of the Sahara and Arabia, on the other hand, are mainly due to the excessive use of groundwater. In the closed drainage regions of North America, including the Great Basin Highlands, drought-induced soil moisture losses were responsible for most of the total water losses. Losses of water from the surface of reservoirs of the Great Salt Lake and Salton Sea lakes amounted to a significant 300 million tons per year, which was partially facilitated by mining and canal irrigation.
“Water losses in closed basins provide an example of how climate change leads to further dehydration of already desert and semi-desert areas. At the same time, the depletion of groundwater from economic activities significantly accelerates dehydration,”says Jay Famigletti, director of the Water Security Institute in Canada.
Wang spoke about the three main findings of his group.
“Firstly, water reserves in a closed area, although limited, can account for most of the water reserves on the entire earth's surface at time intervals measured in decades. Secondly, water loss in closed areas has recently been less sensitive to natural climate fluctuations, which indicates the possibility of counteracting long-term climatic conditions and human impact on water circulation in nature”.
“Thirdly, water losses in dry closed areas have dual consequences, affecting both local water supply and the level of the world ocean. This underscores the so far underestimated importance of internal drainage basins in water balance, as well as the need for a better understanding of changes in water reserves in the interior of continents.”
Vadim Tarabarko