Satellites aid in the monitoring of underground water storage..

 

Groundwater is an unseen but critical resource to our daily lives.

 Fresh water makes up only 2.5 percent of the total amount of water on the planet. About a third of the 2.5 percent is available as groundwater, or subsurface water reserves. Fresh water makes up around 1% of the Earth's surface, in rivers, lakes, streams, and reservoirs. The remainder is encased in ice.

For billions of people, groundwater provides a source of drinking water. Aquifers provide drinking water to roughly half of the world's population. Groundwater is also an important source of crop irrigation water. Much of the water needed in sewage systems and industrial processing comes from underground sources as well.

 

It's important to track changes in groundwater since even minor changes in volume can have significant negative implications.

 Despite the fact that there is a lot more water underneath the ground compared to the water on the surface, only a small quantity of it can be pumped without having detrimental consequences. Overuse of water can induce subsidence, which causes the land surface to sink. Depletion of groundwater threatens rivers and ecosystems that rely on water from subsurface sources (through baseflow). Excessive groundwater pumping in coastal areas can restrict freshwater flow to the ocean, allowing saltwater to creep toward the land and infiltrate the aquifer. People who rely on groundwater for personal consumption risk having poor quality water or no water at all if aquifers become contaminated with seawater or depleted due to over-pumping.

Changes in the volume of water underground were impossible to measure until recently. Groundwater is found under the surface of the Earth, sometimes in deep aquifers or in isolated, difficult-to-reach locations. In high-income nations, measuring this resource required expensive and time-consuming equipment, whereas in low-income countries, it was simply not possible.

This challenge has been solved by taking measurements of Earth's gravity from space.

 The Gravity Recovery and Climate Experiment satellites were launched in 2002 by the US and German space agencies (GRACE). Microwave pulses were utilized to monitor any change in the distance between these two satellites, which were orbiting at the same height but 137 miles (220 kilometers) apart. When the first satellite passed over a mountain, for example, the landmass would increase Earth's gravitational attraction on the satellite, forcing it to accelerate. The microwave pulses would trace the change in distance between the two satellites as the first one accelerated and then slowed as it passed through the gravitational attraction of the mountain below. The difference in distance would then be as the second satellite passed over the same peak. The satellites' exact locations were determined using GPS devices. The combined microwave and GPS data were used to create maps showing how the world's gravity changed, if it moved at all, with each orbit of the planet. Because landmasses like mountains do not migrate, Earth's gravitational force does not alter substantially. However, water moves, and the satellites' monitoring equipment are sensitive enough to detect it. As a result, scientists can identify where subsurface water supplies are rising or diminishing by monitoring minute variations in gravitational force.

These data, together with hydrological models, have been used by scientists to determine where groundwater is being depleted. For example, GRACE data from 2003 to 2013 revealed that people were quickly draining 13 of Earth's 37 major aquifers, with little or no natural replenishment.

 

GRACE satellite observations gave several new insights into the Earth's water cycle. In 2017, the first set of satellites were turned off. In 2018, a new set of satellites was launched to collect data on groundwater, the amount of water in lakes and rivers, changes in ice sheets and glaciers, and changes in sea level and ocean currents. These measurements help us follow the melting of ice sheets, monitor drought, and understand the effects of climate change on the water cycle.