From oceans and rivers to lakes and wetlands, 71 percent of the Earth’s surface is covered in water. As well as this surface water, ice and snow hold water in the form of glaciers, ice caps, snow cover and permafrost. There’s moisture in soil, while vegetation also stores water. Terrestrial Water Storage (TWS) refers to all of this water combined—i.e. the summation of all water on Earth. According to the Global Climate Observing System, TWS is an essential climate variable; monitoring its changes can show the impact of climate change on our planet.
Floods and droughts are increasing while groundwater levels decline
In the 1990s, an average of 86 flood disasters were recorded per year. By 2023, this figure had risen to 170. 2023 also saw 32 million people impacted by flooding, from injuries to displacement, and 29.4 million affected, injured or left homeless by droughts.
Meanwhile, groundwater levels are declining. Groundwater is the water underneath the Earth’s surface in the cracks between soil, sand and rocks. It’s a source of fresh water for over two billion people worldwide; a lack of it puts global ecosystems, as well as food and water sources, at risk.
Terrestrial Water Storage fluctuates at times of high or low rainfall, making it a useful metric in measuring floods, droughts and groundwater levels, as well as their long-term impacts.
The GRACE satellite measures Terrestrial Water Storage
In collaboration with JPL/NASA, the German Research Centre for Geosciences (GFZ) has launched satellites that monitor TWS. The first mission, GRACE, ran from 2002 to 2016. The follow-up mission is currently in progress and the GFZ team is preparing to launch its third mission in 2028.
The missions use two satellites which orbit the Earth measuring spatial changes in its gravitational field. From this data, GFZ generates monthly maps that display changes in the global water cycle. The maps and data are available free of charge to researchers worldwide via their online portal.
The GRACE satellite has advantages over other measuring methods
Taking near-surface soil moisture measurements only indicates the impact of droughts and floods on near-surface soil layers. Because the GRACE satellite measures deeper soil layers and groundwater, it can monitor how floods and drought impact water levels in the longer term.
As for groundwater, monitoring it via observation is a costly and time-intensive process. Using the GRACE satellite’s data, scientists can calculate groundwater levels by measuring TWS and subtracting all other water storage apart from groundwater.
As floods and droughts increase and groundwater levels decline, tracking Terrestrial Water Storage is especially crucial. Thanks to its open-source data, the GRACE satellite is a key puzzle piece in helping scientists understand and mitigate the risks of climate disasters on our planet.
