For roughly fifteen years, the GRACE satellite mission tracked changes in Earth's freshwater resources. Researchers at the Hydrological Sciences Laboratory at NASA's Goddard Space Flight Center used GRACE observations to identify regional trends in surface and subsurface water. They combined those measurements with information from other satellites, climate models, and precipitation observations to assess changes in freshwater storage around the world. [source]
Gravity-measuring satellites such as GRACE do not collect their principal data by photographing Earth's surface. Instead, they measure changes in Earth's gravitational field over time, including variations associated with changes in the mass of stored water.
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GRACE
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The study discussed in this article used gravity-satellite observations to map freshwater conditions over a fourteen-year period. The mapped trends in terrestrial water storage were derived from GRACE observations from April 2002 through March 2016. Causes of the trends in individual regions were summarized and colour-coded. For visualization, the map was smoothed using a Gaussian filter with a radius of 150 kilometres, while the calculations themselves used the original three-degree data resolution described by the source.
Emerging trends in global freshwater availability [source]
A paper titled “Emerging trends in global freshwater availability”, published in Nature on May 16, 2018, presented newly analysed GRACE observations from researchers associated with Goddard's hydrology group. The results emphasized freshwater scarcity as one of the major challenges of the century and warned that some already-dry regions could face increasingly severe water stress.
Using GRACE, the researchers identified areas where freshwater conditions had deteriorated in association with intensive human water use. Some wet regions had become wetter, while a number of dry regions had become more vulnerable, particularly where groundwater extraction was substantial. The data showed pronounced changes in terrestrial freshwater storage since 2002.
The source identifies Iran and other parts of the Middle East, northern and eastern India, northeastern China, California, southern Russia, and Australia among regions where declining water availability was associated largely with groundwater withdrawals and irrigated agriculture. The article reports the researchers' warning that without governmental action, conditions in some of these areas could deteriorate further.
The study concentrated on regions where freshwater storage had increased or decreased markedly. According to the source's summary, changes associated with water depletion in 14 studied regions were attributed primarily to human activity, while in eight cases the changes were attributed to climatic causes.
Reading the GRACE time series
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GRACE
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The original article also reproduces a time series from the GRACE mission spanning 2002–2016. Areas with freshwater storage above or below the fourteen-year mean are represented in blue and red respectively. Grey circles identify regions where human activity contributed to below-normal conditions, while white and black circles indicate climate-related change and natural variability, respectively, according to the source description.
The Caspian and Aral regions
The article discusses declining freshwater storage around the Caspian Sea. It notes that some earlier interpretations had emphasized natural causes, whereas the study highlighted the contribution of water withdrawals for agriculture and industry. It also refers to the drying of the Aral Sea, once among the world's largest lakes, as an example of severe regional water alteration.
The source contrasts the much larger Caspian with the Aral and states that complete drying of the Caspian would occur on a vastly longer timescale under the scenario being discussed. More immediate concerns identified for neighbouring countries included pollution and changes in the shoreline.
The researchers' broader conclusion, as summarized by the original article, was that drought can affect any region, but vulnerability is greater where people lack access to multiple or resilient water sources.
The measurements, attributions, and projections above are preserved as reported in the original May 2018 article and its discussion of the Nature study.