Winter Rains Are Boosting Cape Town’s Water Supply—But for How Long?

Only a few months after Cape Town faced the threat of its city water supply running dry, there’s a risk of flash flooding across the Cape Town region late this week. A multiday stretch of wet weather is predicted to bring widespread amounts on the order of 2-4” across far southern Africa. The return of winter rain is boosting spirits as well as reservoir levels, but it’s too soon to know for sure whether Cape Town will avoid a water situation as dire in 2019 as the one it faced earlier this year.

After three years of brutal drought, the “Big Six” reservoirs that feed Cape Town the bulk of its water supply dropped to alarming levels in early 2018, dipping to just 20% of total capacity by April. That figure is worse than it sounds, because water can’t be easily withdrawn at reservoir levels below 10%. City officials warned in January that Day Zero—the point where municipal water supplies are forced to be shut down for an indefinite period—could arrive by April (see our post from January 19).

Shaken by the potential impacts of Day Zero, including having to schlep each day to retrieve a limited water ration, Cape Town residents and business heeded the call to conserve, and that went a long way toward averting Day Zero. Reservoir levels held steady in May, a time when they normally drop (see Figure 1). The arrival of winter rains has given another boost, pushing the reservoirs above 30% of capacity in early June. This week’s imminent rains should bring the Big Six reservoirs even higher.

Figure 1. Conservation efforts implemented in early 2018 helped the Cape Town region keep its reservoirs from dipping into the failure zone that would have eventually prompted Day Zero. Image credit: City of Cape Town.

Piotr Wolski, a hydroclimate expert at the University of Cape Town, is cautiously optimistic. “So far it looks like the region is receiving rainfall in the normal category, which becomes increasingly indicative that we are getting ‘locked’ into a normal year, and that the risk of a dry year is becoming lower and lower,” said Wolski in an email. Analyses by Wolski and UCT colleague Peter Johnston show a tendency for generous rains (or a lack of them) early in the wet season to continue through the subsequent months.

The definition of a typical wet season has itself been in some flux for Cape Town. Average annual rainfall has slipped over the past few decades, especially since the 1980s (see the reservoir inflow data in Figure 2, which corresponds closely to rainfall). The years 2015–2017 were the three driest on record, and 2017 was the single driest year.

In its water outlook for 2018, last updated on May 20, the City of Cape Town noted: “If runoff from rainfall equates to an average year, dam levels will be just over 80% at end of October. However, should only half of 2017 runoff from rainfall flow into the system, dam levels will be just below 23% at the end of October, and drop to 13.5% early in 2019. For the system to recover, rainfall at least equivalent to 2017 is required, while restrictions remain in place.”

Figure 2. Annual inflow into the “Big Six” reservoirs that serve the Western Cape Water Supply System, measured in terms of water volume (millions of cubic meters). The average inflow for the three years 2014–2016 (circled) was less than 60% of the long-term average. Image credit: City of Cape Town.

Human-produced climate change is expected to tilt the odds toward drier conditions across the Cape Town area, which gets most of its rain from midlatitude fronts that brush past the southern tip of the continent. 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.

Avoiding Day Zero, at least for this year, has given the Cape Town area some much-needed time to develop supplemental water supplies for its growing population. Among the options being considered are modular desalination plants (modest in size, to allow for turnkey operation as needed) and withdrawals from two large aquifers that lie beneath the region. One marine salvage expert has even proposed hauling in massive icebergs from the south as needed, although the city’s acting executive mayor, Ian Neilson, threw cold water on the idea in May, calling it “both complex and risky with an anticipated very high water cost.”

Figure 3. Volume of water (in millions of liters) stored in the six reservoirs that make up the bulk of the Western Cape Water Supply System, measured by week (1 through 52) for the last decade. Image credit: City of Cape Town.

Conservation remains a critical part of the picture. “Getting through the drought in 2018 requires that demand be reduced,” warned the city in its May water outlook. The city continues to encourage residents to aim for using no more than 50 liters of water per day (about 13 gallons) for drinking and other needs. Among the other tactics being used under the current Level 6B restrictions (in place since February 1) are reducing water pressure to city zones where per-capita usage is especially high and installing household flow regulators at homes where usage is more than 10,500 liters per month. “Every person in the city needs to realise that this is a crisis,” said the report.

According to Mark New (University of Cape Town)—citing recent observations from his UCT colleague Martine Visser, who tracks household-level water usage—“it appears that usage is creeping up again, as people psychologically respond to the rain we have had. The fact that a normal rainfall season seems so wet to them (anecdotal from several conversations) tells us how dry the last few winters have been.”