The team was led by Matroder of the Goddard Space Flight Center and used 14-year observations of the GRACE spacecraft. The GRACE satellite was launched in 2002 jointly with NASA and the German Aerospace Center (DLR). With two satellites, the distance between two precisely-measured spacecraft can detect changes in the gravitational field caused by mass movement on Earth.
NASA also used precipitation data from the Global Precipitation Climatology Project, Landsat Images, Irrigation Maps, and published reports of human activities related to mining and reservoir operations. Fresh water can be found in lakes, rivers, soil, snow, groundwater and ice. Researchers say they are witnessing a major hydrological change, including a wetland that is getting wetter and drier and the dry area getting drier. In particular, it has been found that there are multiple hot spots of groundwater depletion in dry areas.
Some of the water loss in some areas, such as the melting of ice sheets and high mountain glaciers, is driven by climate warming. The study found that the southwestern California lost 40 million tons of freshwater each year during the study, enough to fill 400,000 Olympic swimming pools. In Saudi Arabia, the declining trend of freshwater also reflects agricultural demand. Between 2002 and 2016, the region lost 610 million tons of groundwater each year. The study also found that the amount of freshwater resources in the Zambezi Basin and the Okavango Delta in western Africa has increased by 2.9 billion tons annually.