In addition to the warming climate and intensified human activities, water storage in global inland basins has taken a long way to go. A recent study shows that this decline has worsened local water stress and caused possible sea level rise.
The study "Recent Global Decline in Endorheic Cistern Water Storage" was conducted by a team of scientists from six countries and appears in the current issue of Nature Geoscience .
"Water resources are extremely limited in the continental hinterland, where the current does not reach the ocean, and these areas are scientifically referred to as endorheic basins," said Jida Wang, a geographer at Kansas State University and principal author of the study.
"Over the last few decades, we have seen increasing signs of endothermic water disturbances," said Wang, an assistant professor of geography. "These include, for example, the dehydrating Aral Sea, the depleted Arabian aquifers, and the retreating Eurasian glaciers, which leads us to the question: is the total water reservoir in the global endorheic system, which traverses about one-fifth of the continental surface?" A net loss? "
Using gravitational observations from the Gravity Recovery and Climate Experiment (NASA) of the German Aerospace Center (GRACE), Wang and his colleagues quantified a net water loss in global endorheic basins of approximately 100 billion tons of water per year since the beginning of the current millennium. This means that a body of water equivalent to five Great Salt Lakes or three Lake Meads disappears each year from the arid endorheic regions.
Surprisingly, this endorheic water loss is twice as high as the simultaneous water change in the remaining landmass except Greenland and the Antarctic, Wang said. In contrast to endorheic basins, the remaining regions are exorheic, which means that the river flows from these basins into the sea. Exorheic basins cover most of the continental surface and host many of the world's largest rivers, including the Nile, Amazon, Yangtze, and Mississippi.
Wang noted that the signature of reservoirs in exorheic basins resembles some conspicuous oscillations of the climate system such as El Niño and La Niña in perennial cycles. However, water loss in endorheic basins seems less responsive to such short-term natural fluctuations. This contrast could point to a profound influence of longer-term climate conditions and direct water management such as river diversion, dam control and groundwater abstraction on the water balance in the dry hinterland.
This endorheic water loss has two different effects on the researchers. This not only worsens water pollution in dry endorheic regions, but can also contribute to a significant factor in the global environmental concern: sea-level rise. The rise in sea level is due to two main causes: the thermal expansion of seawater due to the increased global temperature and the additional body of water in the ocean.
"The hydrosphere is mass-preserving," said the researcher of the Chunqiao Song Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences and co-author of the study. "If water storage in endorheic basins is in deficit, the reduced mass of water does not disappear, it has been attributed to the exorheic system mainly due to steam flow and if this water is no longer inland it may affect the sea level budget"
Despite an observation period of 14 years, the endorheic loss of water equates to an additional sea-level rise of 4 millimeters, according to the study. The researchers said that these effects are not trivial. It accounts for about 10 percent of the observed sea level rise over the same period; compared to nearly half of the simultaneous losses in mountain glaciers, except Greenland and Antarctica; and corresponds to the total contribution of global groundwater consumption.
"We are not saying that the endorheic water loss has recently completely landed in the ocean," said Yoshihide Wada, deputy director of the Water Program of the International Institute for Applied Systems Analysis in Austria and a co-author of the study. "Instead, we are showing a perspective on the recent significant endorheic water loss, and if the excess of water added to the exorheic system continues beyond the decade of decadence, for example, this could be an important source of sea-level rise."  Through the synergy of multiple satellite observations with multiple missions and hydrological modeling, Wang and his colleagues attributed this global endorphic water loss to comparable surface contributions – such as lakes, reservoirs, and glaciers – as well as soil moisture and aquifers.
"However, such comparable losses are an accumulation of varying regional variations," Wang said. "For example, in endorheic central Eurasia, about half of the water loss came from the surface, especially from large estuaries such as the Aral Sea, the Caspian Sea and Lake Urmia, as well as the retreat of the glaciers in the high mountains of Asia."
While the glacier retreat was a response to the warming temperature, the water losses in the watersheds were a combination of meteorological droughts and long-term water diversions from the rivers of food.
The net loss of water in endorheic Sahara and Arabia, on the other hand, was characterized by unsustainable groundwater abstraction, according to the researchers. In endorheic North America, including the Great Basin of the US, dryness-induced soil moisture loss was likely responsible for most of the regional water loss. Despite a lesser extent, surface water loss in the Great Salt Lake and the Salton Sea was significant at 300 million tonnes per year, partly due to mineral depletion and diversion-induced irrigation.
"Water Losses The world's endorheic ponds are another example of how climate change is further drying up the already arid and semi-arid regions of the world, and human activities such as groundwater mining are significantly accelerating this drying process," said Jay Famiglietti. Director of The Global Institute of Water Security, Canada 150 Research Chair of Hydrology and Remote Sensing at the University of Saskatchewan, Canada, and co-author of the study.
Wang said the team wanted to send three takeaway messages from their research.  "First, the endorheic water reservoir, though limited in its total mass, can dominate the water storage trend across the entire surface of the land during at least the decade-long timescales," Wang said. Second, the recent endorheic loss of water is less susceptible to the natural variability of the climate system, suggesting a possible response to longer-term climatic conditions and the management of human water.
"Third, such endorheic water loss has a dual effect on the regional sustainability of water and global sea level rise," he said. These messages underline the undervalued importance of endorhegic catchment areas in the water cycle and the need for a better understanding of changes in water storage in the global hinterland. "
Discrepancies between satellite and global model estimates of land-based storage
Jida Wang et al. Recent Global Decline in Endorheic Pool Water Storage, Nature Geoscience (2018). DOI: 10.1038 / s41561-018-0265-7