Why Cape Town’s drought was so hard to forecast

Cape Town’s drought and associated water shortage has officially escalated to the level of a disaster. The hope for a natural solution ended with the close of the main rainy season in September, and it is clear that water in the dams supplying the city will not last until the next rains in May-June next year.

The city had promised alternative sources of supply, the plans are not entirely realistic. Its main strategy now is to severely restrict water use through rationing.

Rainfall data visualisations show that 2017 was one of the driest years in recent decades. But the region’s predicament wasn’t caused by the low 2017 rainfall. It’s in trouble because 2017 followed two successive dry winters. Such severe multi-year droughts are very infrequent, occurring perhaps as rarely as once in a millennium. Water supply systems are not designed to withstand their impact.

What drives extreme such extreme droughts? Is is just bad luck in the weather lottery? Or is the drought the price for humanity’s unabashed addiction to fossil fuels?

There is no clear, research-based, defensible answer to these questions. But our analysis suggests that the possibility of extreme drought is increasing in the Western Cape. Future climate projections show a possible shift towards a drier, more drought-prone climate.

This means that it is possible that man made climate change has contributed to the severity of the current drought, and even though it is an extremely rare event, similar droughts may not be rare in the future. On a positive note, there will still be wet years, but likely not as many.

Are droughts predictable?

Predicting rainfall for the rest of the country – which gets its rain in the summer – is relatively straightforward because rainfall is related to El-Niño and La Niña episodes. Normally El-Niño brings drought to South Africa, while La Niña brings wet conditions.

But the relationship between these events and the Western Cape’s winter rainfall patterns is weak and inconsistent (and actually inverse compared to the rest of the country). The well-below-average rainfall of 2016 and 2017 occurred during a weak La Niña and a weak El-Niño respectively. This does not reflect the expected El-Niño-rainfall relationship.

Season’s rainfall can be forecast using dedicated climate models. The models are, however, not equally skillful every season and at every location. For example, seasonal rainfall forecasts for June - August 2016 and 2017 from the South African Weather Service predicted a higher chance of wetter conditions compared to that of normal or drier conditions.

Forecasts by the world’s premier seasonal forecast institutions European Centre for Medium-Range Weather Forecasts and the International Research Institute for Climate and Society, also failed to provide an accurate forecast (here is the former’s forecasts for 2016 and 2017, latter’s for 2017).

It’s possible to find a glimpse of the drought in those forecasts. But on their own they didn’t give a defensible basis for any preemptive drought actions.

If the numerical seasonal forecasts were not useful what about expert opinion? In May 2017, at a Western Cape Winter Rainfall Outlook Summit could not agree on a consensus message about the forthcoming winter. Their final statement was:

[...] the Western Cape needs to consider the full range of possibilities which comprise that the 2017 winter season may be drier than normal [...] or normal [...] or wetter than normal.

But also:

There is little doubt that the Western Cape needs to prepare, in the longer term, for a drier climate.

The experts too appeared to be flummoxed.

Looking ahead

There is good evidence to expect similar, possibly more severe, and likely more frequent drought events in the future, in Cape Town, but also in other cities. This definitely requires making Cape Town’s water supply system more resilient by expanding to water sources that are less drought-prone, such as groundwater, desalination and water reuse. But these sources are also more expensive which is not inconsequential considering the vast economic disparity of Cape Town’s population.

What’s needed is a flexible management strategy for supply and demand to reduce the impact of future (possibly multi-year) droughts and avoid disasters in a way that’s cost-effective to water users. We should be taking advantage of years of sufficient water to prepare for sustaining the city through years of deficit, and be able to react early to the forthcoming drought. For this, however, we need to know what to expect in near future.

Although available forecasts are not really promising at this stage, mileage can be gained through improved near-realtime monitoring of climate and hydrological conditions and a continuous risk-based assessment of a season’s outlook. In that, forecasts must be taken into account as well as defensible statistical relationships between drought and possible global drivers, with clearly articulated uncertainties.

Creating a basis for a flexible, proactive approach to resource management, requires more broad, transparent, and innovative collaboration between the city and climate and hydrological research communities. Drought should not be a water disaster if the risk is managed preemptively and jointly.