Bob Henson ·

Above: Bare sand and dried tree trunks stand out on May 10, 2017, at Theewaterskloof Dam, near Villiersdorp, about 40 miles east of Cape Town, South Africa. The dam had less than 20% of its water capacity at that point, and water levels are now even lower. Image credit: Rodger Bosch/AFP/Getty Images.

Three years of unforgiving drought in Cape Town, South Africa, have led to the once-unthinkable: A great world city is about to turn off the tap to its municipal water supply. The long-feared “Day Zero”—the point when the reservoirs serving Cape Town drop below the minimum levels needed to provide water safely—will arrive April 21, according to recent projections from the Western Cape Water Supply District.

Because we’re now in the midst of the Southern Hemisphere summer, it’s the dry season for Cape Town. There is very little chance that winter rains will kick in soon enough to prevent a few days or even a few weeks without municipal water. The city’s impressive water conservation efforts to date haven’t done the trick, and even a few stopgap desalination plants being rushed to completion are unlikely to avert Day Zero. At this point, the hydrologic hole dug by intense drought is simply too deep.

“Day Zero is the day that the water resource system runs out of water,” said Mark New, the AXA Research Chair in African Climate Risk at the University of Cape Town, in an email. What does this mean? “No water coming out the taps. Toilets cannot be flushed. Fire services cannot get water out of the fire hydrants. People will have to walk to water tankers to fill up drinking water bottles.” And there will be knock-on effects, such as schools considering whether they can operate with no water on campus.

Figure 1. Skyscrapers and a shipyard sit below Table Mountain in this 2015 file photo of downtown Cape Town, South Africa. Image credit: Thierry Falise/LightRocket via Getty Images.

According to New, there are three main strategies—over and above the extreme water restrictions already in place—to try to forestall Day Zero:

Drilling and tapping emergency groundwater supplies

Bringing several portable desalinization plants on line

Cutting off water to neighborhoods for periods of each day, to reduce demand even further.

“It is not clear whether the groundwater and desalination options will be operating in time, and with sufficient quantity, to make a difference,” he said. Even then, the supply could still run short, as evident in Figure 2 below.

Cape Town currently supplies its residents with about 600 million liters of water a day, which is down significantly from just a few months ago as a result of demand management. The new desalination plants—which convert seawater into drinking water by extracting salt—would together provide no more than about 10 million liters per day. Tapping groundwater at various depths could help a great deal more, providing as much as 150 million liters/day by April, according to Piotr Wolski, a hydroclimate expert at the University of Cape Town. Even then, Figure 2 shows that Day Zero would still arrive by early May unless conservation is ramped up and/or the winter rains are prompt and heavy.

Figure 2. Five years of declining water supply in Cape Town, South Africa, from 2013-14 to 2017-18. This image shows the gradual drop in storage in the “Big Six” reservoirs that make up more than 99% of the Western Cape Water Supply System. About half of the water is normally allocated to agriculture in the region. Reservoir levels normally drop during the dry season (December to May), when hot weather causes water use to spike. The reservoirs are then replenished by winter rains, especially from June to September. The lowest fraction of the Big Six water supply (area shaded in blue at bottom of graph) cannot be safely used. The winters of 2015, 2016, and 2017 were the second driest, fourth driest, and driest in almost a century of recordkeeping at the Cape Town International Airport. Image credit: Courtesy Piotr Wolski, University of Cape Town. Similar graphics can be created on the fly using the Big Six Monitor tool developed by Wolski.

The fate of a Mediterranean climate in a warming world

A home to indigenous residents for uncounted years, and to Europeans and other arrivals since 1652, the Cape Town area has centuries of experience dealing with a highly seasonal, highly variable water supply. The big concern these days is a rapidly expanding population—the area served by the Western Cape Water Supply System now holds more than 3.2 million people—together with the uncertainties posed by human-produced climate change.

Most of southern Africa is semiarid or arid terrain that gets less than 20 inches of annual rainfall, typically in the form of summer showers. It’s only the southernmost fringe of the continent, including Cape Town, that regularly intercepts frontal systems from storms that spin across the “roaring forties” (the belt of storminess around 40°S latitude).

Cape Town has much in common with other areas that have what’s called a Mediterannean climate, including southern Europe and California. A typical Mediterranean climate has a mild, wet winter and a hot, dry summer. However, South Africa has no large-scale water resource on par with the Sierra Nevada snowpack or Lake Mead. The smaller reservoirs serving the Western Cape Water Supply System rely mainly on winter rains, and they’ve taken a huge hit from the past three years of drought. Complicating the situation further, about half of the system’s water is allocated to agriculture. As in the United States and elsewhere, it can be a political challenge to shift the rural/urban water balance, especially in the midst of a drought crisis.

Nobody knows if the winter rains will again fall short this year. El Niño and La Niña are of some help in forecasting summer rains across parts of southern Africa, but they provide little guidance during the Southern Hemisphere winter. And apart from the typical year-to-year ups and downs in rainfall, there’s an ominous trend: a gradual decline in rainfall in the Cape Town area over the last 60 years (see Figure 3). During the same period, local temperatures have warmed by more than 1°F.

Figure 3. The average annual rainfall since 1933 for three stations representing the area of the Big Six reservoirs that serve the Western Cape Water Supply System (1000 mm = 39.37"). The red line shows the long-term decrease that’s especially evident in yearly data from the 1980s onward. Image credit: Courtesy Piotr Wolski, University of Cape Town.

A warmer climate puts more stress on water supplies and exacerbates the impact of a given drought. When temperatures are higher, more water evaporates from reservoirs, vegetation, and the landscape. Research in California shows that droughts are now much more likely than in decades past to occur with above-average temperatures.

The climate picture painted in recent decades doesn’t bode well for Cape Town, and research points to more of the same as this century unfolds. A warming climate may cause the low-latitude Hadley circulation to expand poleward and more often block the winter storm track from reaching South Africa. The most recent report from the Intergovernmental Panel on Climate Change, issued in 2013, concluded (Chapter 12, Working Group I): “Soil moisture drying in the Mediterranean, southwest USA and southern African regions is consistent with projected changes in Hadley Circulation and increased surface temperatures, so surface drying in these regions as global temperatures increase is likely with high confidence by the end of this century under the RCP8.5 scenario.” (The RCP8.5 scenario assumes “business as usual” emissions.)

Wolski and colleagues have been working to pin down the rarity of the current four-year drought. They’re finding that at the heights above Cape Town where the Big Six reservoirs sit, the multiyear drought has been even more unusual than at sea level. “We are talking return intervals on order of a few hundred years, perhaps up to 800 years, depending on which station you use,” said Wolski in an email. What’s worse, they’ve found, is that climate change may increase the odds of droughts this severe by threefold to fivefold.

Figure 4. Khoisan leader Ockert Lewies begins a prayer for rain at the foot of Table Mountain, on May 25, 2017, in Cape Town, as leaders from various faith communities attend a prayer session on Table Mountain. The mayor of Cape Town, Patricia de Lille, called on religious leaders to come together to pray for rain in the drought-stricken province, where the area's dams had only about 10% of usable water left. Image credit: Rodger Bosch/AFP/Getty Images.

Can cities get a jump on dealing with unprecedented drought?

If Day Zero does arrive, Cape Town residents could face an exhausting set of challenges, even in just getting enough water to stay hydrated and bathed. Eventually, the winter rains will arrive, and the reservoirs will most likely be up and running for at least another few months—thus buying some much-needed time to develop other water supply options. The region’s water crisis may be far from over, though, especially if the winter rains are once again lackluster.

Some observers have claimed that obstruction and mismanagement at the national level (where a different political party is in power than in Cape Town) have kept the region from getting the help it needed. “National government’s numerous spanners jammed up the works of a system that could have managed the crisis quite effectively,” wrote David Oliver (University of the Witwatersrand) in an essay for The Conversation last December. Others have pointed to a regional system that postponed enhancements to the water supply, such as groundwater access or new reservoir capacity, until it turned out to be too late.

“With hindsight, it appears that a sequence of good rainy seasons had taken the pressure off the decision-makers responsible for planning, developing and operating the Western Cape Water Supply System. This allowed them to conclude that Cape Town’s water conservation and demand management efforts had permanently reduced consumption,” wrote Mike Muller (University of the Witwatersrand) in the journal Civil Engineering last June.

Increased development and rising temperatures are going to add to the impacts when drought does occur, regardless of how rainfall evolves in a warming world. If nothing else, Cape Town’s predicament reminds us that we ought to bolster our urban water supplies with extra buffers—from beefed-up conservation to back-up sources—as much as possible, and as soon as possible. In a nonstationary climate, past weather performance is no guarantee of future results.

A useful source for keeping up with science related to the Cape Town drought and water supply crisis is the blog maintained by the Climate Systems Analysis Group at the University of Cape Town. Among the posts there: “Is Cape Town’s drought the new normal? ” and “The Big Six Monitor”.