Droughts may have affected larger areas globally than captured by satellites, new analysis suggests

Droughts across the world are significantly more common than the data recorded by Gravity Recovery and Climate Experiment (GRACE) from 2002-2017, a new analysis by scientists from the University of Bonn showed. 

Major droughts are also spread across wider geographies, they found, highlighting limitations of satellite data.

The GRACE twin satellites, launched jointly by the space research organisations of the United States and Germany in 2002, have provided data on all of Earth’s water reservoirs, over land, ice and oceans. While it covered a wide range of reservoirs and thus helped build an exhaustive dataset, it had a limitation, explained Helena Gerdener, one of the analysts from Bonn, in a blog on the university website. “The spatial resolution of the data on the gravitational field is relatively inexact at about 300 to 350 kilometers as a result of the measurement principle applied.”

This means, she added, that reliable statements can only be made for areas around 100,000 square kilometers in size. “To give some idea of scale, this minimum area is still larger than Bavaria, Germany’s largest federal state at ‘only’ 70,000 or so square kilometers.”

By contrast, global hydrological models permit a resolution of 50 kilometers or even less, the article noted. “These use meteorological measurements of precipitation, temperature and radiation as well as maps of land use and soil composition and data on how water is being used by industry, agriculture and other consumers,” according to the authors. 

The researchers who made the new analysis combined the GRACE measurements with the hydrological model WaterGAP for the first time to show, more precisely than ever before, how the total distribution of water over the Earth’s land surfaces has changed in the last two decades. 

The dataset developed by them is called global land water storage data set release 2. 

“The new method allows us to test out model calculations on the future effects of climate change, particularly how rising temperatures and changes in precipitation patterns will impact the water balance in different parts of the world,” Jürgen Kusche from the Institute of Geodesy and Geoinformation at the University of Bonn was quoted as saying in the university blog. 

One of key findings of this reanalysis of global water distribution was that droughts may have ravaged larger areas than captured by the satellites, Kusche noted, adding:

What we’re seeing is that even extensive droughts like the massive one that struck the whole of the Amazon in 2010 are spread across much wider areas than the satellite data indicates on its own. This means that the satellites aren’t picking up many of the more localized droughts.

The new method improved the resolution of the water distribution maps to 50 metres from around 300 km earlier, the authors noted. 

They also tested their findings with actual measurements made by 1,000 stations globally and found that it correlates better with the analysis of the global navigation satellite system than the calculations that had been based purely on either the GRACE satellite data or the hydrological model. 

The new method of studying water distribution will inform understanding of the water balance of the world — why there are widespread droughts in some places and floods in others. It will also help factor in the changes wrought by a warming world with changing rainfall patterns. 

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