Parched land areas covered in withering flora can be a pretty sure indicator of drought, but the state of affairs beneath the Earth's surface is a critical factor too. Advanced scientific instruments in orbit around the planet have given us the ability to track its stocks of groundwater resources, which researchers are now figuring out how to tap into to build more accurate predictions of drought and the risks that it brings months in advance.
Launched in 2002 as a joint space project between Germany and America, the GRACE (Gravity Recovery and Climate Experiment) mission placed a pair of satellites into polar orbit with the intention of mapping the Earth's gravitational field.
More specifically, the twin satellites work together to map variations in the Earth's gravity. With the ability to measure the distance between each other down to a few microns, or a fraction of the width of a human hair, the instruments are sensitive enough to reveal tiny fluctuations in the Earth's gravitational field as changes in mass underneath cause them to speed up and slow down ever so slightly.
These might be caused by mountains, shifting magma beneath the crust or the impacts of earthquakes. But what drew the attention of scientists at Australian National University's (ANU) Research School of Earth Sciences was the ability to measure changes in the amount of water stored in the ground, as larger masses of water will result in a slightly larger gravitational pull.
Though the GRACE satellites were recently decommissioned, in their time of operation they collected a wealth of useful data for scientists to pore over. Researchers studying this in 2012 revealed an unsettling pattern of the Earth's groundwater reserves on the decline, but as study author Dr Paul Tregoning explains, there is plenty more to learn.
"What GRACE doesn't tell you is whether it is deep water, soil moisture of surface water," he tells New Atlas. "It does give you the change in the total water column. The other measurement that we've used is from a soil moisture satellite mission called SMOS (Soil Moisture and Ocean Salinity) and that gives you a measure of the amount of water in the top five centimeters (2 in) of soil.
"Other studies have used either GRACE or SMOS, but no one has yet used them both," he says. "What we found was that using either of those two satellite measurements improves the accuracy of the hydrology modeling, but using both improves it to another level."
This provided Tregoning and his fellow researchers with a much clearer picture of how much water was present at various depths, if any at all. They then analyzed how different kinds of vegetation around the world are able to access the groundwater at different depths, information they were then able to use to predict the state of that same vegetation months down the track.
"So if you looked at say August 2016 and how much water was there in the shallow soil or in the deeper soil, and then we looked at which particular level of water was important for grassland or for forested regions, we would predict the state of vegetation up to many months in advance," Tregoning says. "And these predictions turned out to be much more accurate than what had been done previously."
The researchers describe the accuracy with which the satellites can measure the presence of water on Earth as "mind-boggling" and "unprecedented." Combined with computer modeling, the scientists say it can help simulate the cycles of plant growth and water distribution, helping them predict declining conditions of grazing areas, crops and forests in advance, and even indicate areas at heightened risk of wildfires.
"We have always looked up at the sky to predict droughts – but not with too much success," says Professor van Dijk from the ANU Fenner School of Environment and Society. "This new approach – by looking down from space and underground – opens up possibilities to prepare for drought with greater certainty. It will increase the amount of time available to manage the dire impacts of drought, such as bushfires and livestock losses."
The research was published in the journal Nature Communications.
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