Every drop of water on Earth is on a journey. Heat from the sun evaporates water from oceans, lakes and soil, and plants release more through their leaves in a process called transpiration. The vapour rises, cools and condenses into clouds. Eventually it falls as precipitation: rain, hail or snow. Some of that water runs off the land into creeks and rivers; some soaks into the soil and seeps down to become groundwater; and almost all of it, sooner or later, returns to the sea to begin again. No new water is being made. The water in this morning's shower has been cycling through the planet for billions of years.

The same cycle, a different continent

In Australia, the cycle has a particular character. Across the continent, average annual rainfall is only around 470 millimetres, well below the global average for land, and only Antarctica is drier. But the average hides the real story, which is variability. Year-to-year rainfall in Australia swings more wildly than in almost any other region of comparable size. On top of this, the continent is hot and flat, so evaporation is extreme. Over much of inland Australia, the amount of water that could evaporate in a year is several times greater than the amount that falls. The result is that only a small fraction of rain ever reaches a river.

El Niño and La Niña

Much of this variability is driven by the Pacific Ocean. Normally, trade winds blow from east to west across the Pacific, pushing warm surface water towards Australia and Indonesia, where it fuels cloud and rain. During an El Niño event the trade winds weaken, the warm water drifts back towards South America, and eastern Australia is left drier and hotter, with a higher risk of bushfire. During La Niña the opposite occurs: the winds strengthen, warm water piles up near Australia, and the eastern states receive heavy rain and sometimes severe floods. These phases typically last around a year and alternate irregularly, which is why farmers speak of 'boom and bust' rather than of good and bad seasons.

Water beneath the surface

Not all of Australia's water is visible. Beneath roughly a fifth of the continent lies the Great Artesian Basin, one of the largest underground water reserves in the world. Rain falling along the Great Dividing Range soaks into porous sandstone and travels slowly westward through the rock, a journey that can take thousands of years. Pressure within the basin forces this ancient water to the surface through natural springs and drilled bores, supplying towns and cattle stations in country where surface rivers rarely flow. Because the basin refills so slowly, water taken from it today may not be replaced for many generations.

Living with the swings

Australians have responded to the cycle's extremes by trying to smooth them out. Dams and reservoirs capture water in wet years to be used in dry ones. Yet even large storages can be exhausted. During the Millennium Drought, which gripped south-eastern Australia from roughly 1997 to 2009, reservoirs serving major cities fell to alarming levels and households lived under strict water restrictions. In response, most capital cities built desalination plants to turn seawater into drinking water. Then, in 2010 and 2011, a powerful La Niña brought record rainfall and devastating floods to Queensland and Victoria. Within two years the national conversation had flipped from saving every drop to coping with too many.

Understanding the water cycle in the abstract is simple enough. Understanding it in Australia means accepting that the question is never only how much water falls, but when, where, and whether we can hold onto it.