2020-04-02_Science_Illustrated

the end of January: “A wonderful family

day,” reported the Mayor, “gingerbread houses,

sheep, and green grass in the main arena.”

Water pressure

Prolonged drought puts ground water

supplies under pressure, supplies that are

already depleted by the continued high

consumption in private homes, by agricul-

ture and for industry. If global ocean levels

rise as expected, more drinking water in

coastal areas will be polluted by inflows of

ocean water, making it no longer drinkable

by humans or plants. Rising temperatures

and longer droughts are expected to

continue in various parts of the world. In

some places underground deposits have

become the only source of fresh water.

The development has been documented

by NASA’s GRACE (Gravity Recovery and

Climate Experiment) satellites. From 2002-

2017, the satellites carefully observed the

world’s ground water deposits from their

orbits. In 2018, the space agency launched

The extreme heat in Australia makes kangaroos invade farmland,

consuming crops and drinking from farm water tanks.

Satellites measure distance

by means of microwaves

1

The GRACE-FO mission consists of two

satellites that orbit 220km from each

other and communicate by means of

microwaves. If Earth were a perfect sphere,

the distance between them would be constant.

Earth’s mass affects satellites

2

Mountains, valleys, and water cause

slight differences in the gravitational

pull that influences satellites. The

differences are measured by means of micro-

waves and read as changes in the distance

between the satellites.

Falling water levels cause

weaker gravitational pull

3

As mountains very rarely move,

the changes in measured

gravity over brief periods will

usually be due to moving water – such

as changing ground water deposit levels.

Satellite measures water reserves

One third of the world’s ground water deposits are threatened by climate change

and excess consumption by people, agriculture and industry. NASA’s GRACE-FO

mission is mapping out the ground water developments very accurately.

High ground water level

Longer

distance

Shorter

distance

Low ground water level

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40 | SCIENCE ILLUSTRATED

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