A new global satellite-based study of freshwater distribution in the Earth has shown that wet areas of the Earth become wetter, while dry areas become drier. The data suggest that this pattern is due to a variety of factors, including human water management practices, man-made climate change and natural climatic cycles.
The NASA-led research team that included Hiroko Beaudoing, a faculty specialist at the Interdisciplinary Center for Earth System Research at the University of Maryland, used 14 years of observations from the GRACE Gravitation Recovery and Climate Experiment (GRACE) Global Trends in Freshwater 34 regions of the world.
The study, published in the May 17, 2018 issue Nature also included precipitation data from the ESSIC-led Global Precipitation Climatology Project; Landsat images from NASA and the US Geological Survey; Irrigation cards; and published reports on human activities related to agriculture, mining and deposits. The study period spans from 2002 to 2016.
"This is the first time that we have studied how freshwater availability changes across the globe using satellite observations," said Matt Rodell, lead author of the newspaper and director of the World Wide Web Hydrological Sciences Laboratory at NASA's Goddard Space Flight Center. "One major objective was to distinguish shifts in terrestrial water storage caused by natural variability – eg wet periods and dry periods related to El Niño and La Niña – due to trends such as climate change or human impacts such as groundwater pumping from an aquifer faster than it is replenished. "
Freshwater occurs in lakes, rivers, soils, snow, groundwater, and glacial ice. The loss of ice sheets at the poles – due to climate change – has an impact on sea level rise. On land it is one of the most important resources of the earth for drinking water and irrigation. While the water supply of some regions is relatively stable, others usually experience increases or decreases. But the current study revealed a new and disturbing pattern.
"What we are experiencing are major hydrological changes," said co-author James Famiglietti of NASA's Jet Propulsion Laboratory. "For the first time, we see a very distinctive pattern of the wet land areas of the world becoming wetter – those are the high latitudes and the tropics – and the arid areas in between are getting drier and drier, embedded in the arid areas, we see several hot spots due to Groundwater deficiency. "
Famiglietti noted that although water loss in some regions is clearly caused by the warming climate, such as the melting ice layers and alpine glaciers, other patterns can be clearly attributed to climate
" The pattern of wet-wetting, drying dryer is predicted by the intergovernmental panel on climate models for the end of the 21st century, but we will need a much longer data set to say definitely that climate change is responsible for the emergence of a similar pattern in the GRACE data, " said Famiglietti. "The current trajectory, however, is worrying."
The twin satellites GRACE launched in 2002 as a joint mission with the German Aerospace Center (DLR) accurately measure the distance between the two satellites to detect changes in the Earth's gravity field caused by mass movements on the planet below. Using this method, GRACE tracked variations in terrestrial water storage on a monthly to annual time scale until its scientific mission ended in October 2017.
However, GRACE's satellite observations did not explain Beaudoing, Rodell, Famiglietti and their colleagues an obvious trend
"We looked at information on precipitation, agriculture and groundwater pumping to find a possible explanation for the trends appreciated by GRACE", said Beaudoing, who also has a date with NASA Goddard.
One of the main causes of groundwater depletion was agriculture, which can be hampered by natural cycles, as seen in California, Famiglietti said. The decline in fresh water due to the severe drought from 2007 to 2015 has been exacerbated by groundwater abstraction to support farms in the Central Valley of the state.
During the same period, southwestern California lost 4 gigatons of freshwater per year. One gigatre of water equals the volume of water in 400,000 Olympic swimming pools. Much of California's fresh water comes in the form of rain and snow, which accumulates in the Sierra Nevada as a blanket of snow and then passes through a series of reservoirs as it melts. When natural cycles led to dry years, reducing snowpack and surface waters, people trusted more groundwater.
The downtrend in freshwater observed in Saudi Arabia also reflects the pressure of agriculture. From 2002 to 2016, the region lost 6.1 gigatonnes of groundwater per year. Images from the Landsat satellite series show the growth of irrigated farmland in the arid landscape from 1987 to today, which explains the increased runoff.
Natural cycles of rainy and dry years can also cause a trend in the 14-year data record, which is unlikely to exist, Rodell said. One example is the western Zambezi Basin and the Okavango Delta, an important wildlife water hole in northern Botswana. In this region, terrestrial water storage increased by an average of 29 gigatons per year between 2002 and 2016. This wet period during the GRACE mission followed at least two decades of drought. Rodell believes that this is a case of natural variability occurring in this region of Africa for decades.
The researchers found that a combination of natural and human pressures in some regions can lead to complex scenarios. In northwest China in Xin Jiang Province, there were undocumented water losses. This region, about the size of Kansas, borders Kazakhstan to the west and the Taklamakan Desert to the south, encompassing the central part of the Tien Shan Mountains.
Rodell and his colleagues had to summarize several factors to explain the disappearance of 5.5 gigatons of terrestrial water storage per year in Xin Jiang Province. Less rainfall was not the culprit. Access to surface water also resulted from the melting of glaciers caused by climate change and the pumping out of groundwater from coal mines. However, these entrances were more than offset by depletion of increased water use for irrigating farmland and evaporating river water from the desert floor.
The successor of GRACE, called GRACE Follow-On, a joint mission with The German Research Center for Geosciences (GFZ) will make final preparations for the launch at Vandenberg Air Force Base in California.
The research work Emerging Trends in Global Freshwater Availability, Matthew Rodell, James Famiglietti, David Wiese, JT Rager, Hiroko Beaudoing, Felix Lander and Min-Hui Lo, was published on May 17, 2018 in the journal Nature published.
GRACE-FO helps to monitor drought
M. Rodell et al. New Trends in Worldwide Availability of Freshwater, Nature (2018). DOI: 10.1038 / s41586-018-0123-1