One billion tonnes of rain falls to earth every minute — some 500,000 cubic kilometres of water annually. If this fell evenly, it would cover the globe to a depth of 1m every year.
But of course, rain does not fall evenly. It is irregularly distributed to say the least, both geographically and over time. In the wettest place in the world, Meghalaya state in north-east India, annual rainfall can top 12m. But while 8-10m is not unusual in the tropics, desert regions routinely receive less than 1cm in a year. Parts of the Atacama Desert in South America are thought not to have seen rain for at least 500 years.
So since the earliest times it has been the job of hydrologists and water engineers to wrestle with rainfall’s extremes and inconsistency, to mitigate the effects of drought and flood, to secure a reliable supply of clean water, and to organize the safe removal of waste.
Their work is not getting any easier. A fast-growing global population demands more and more water for domestic use, and burgeoning cities are drawing increasing amounts of water from their surroundings. Under pressure to feed more people, agriculture needs more water for irrigation, while industry too consumes more water than ever. Often these sectors are already competing for a limited supply, and they are coming up against the challenge of global warming too. Rainfall is becoming even more inconsistent and unpredictable, with both mega-droughts and mega-floods becoming more frequent.
“Climate change is here right now,” says Grant Gabriel, an associate water engineer with WSP in Australia, and manager of its water and wastewater partnership with City of Gold Coast. “I’ve spent most of my career dealing with droughts and floods and it is only getting worse. Cities — countries — have to work out how they are going to deal with that. The answers will be different for different places.”
Given Australia’s recent experience of heatwaves, lack of rain and rampant wildfires, it is no surprise that drought strategies are a particular focus. “I have twice been asked to analyse the potential for towing an iceberg from the Antarctic to help out with our water supply,” says Gabriel. “And when working for municipal authorities I have had sadly to identify, in times of drought, which industries were going to have to shut down for the duration. You can’t cut off water to people, so you cut off businesses. It’s grim because the damage to confidence in the region, and the cost to the economy, is huge.”
Having dismissed the iceberg option, Australia, like most water-scarce regions, is left with three main supply-side solutions: pumping more groundwater, building dams to create more capacity in reservoirs, or constructing desalination plants.
The limits of groundwater are now much better understood, as is the need to maintain a sustainable balance. Pumping too much from coastal areas, for example, can cause seawater to be drawn into the aquifer so the source becomes briny and useless. Aquifers must be allowed to recharge too — pump beyond a sustainable yield and they can run down, with consequences for local river systems.