It will take three months and cost R350m for a small-scale temporary desalination plant to produce enough water to raise the level of Cape Town's supply dams by just one percent.
Yet the small amount of rain over the long weekend in April raised the dam levels by more than one percent - free of charge.
Gisela Kaiser, the City of Cape Town's executive director for water and waste, said this had brought home just how expensive desalination could be as a water source.
"Surface water is significantly cheaper than any other source - but it is not resilient to drought," she told delegates at the Adaptation Futures 2018 conference in the city on Wednesday.
The international conference focuses on finding solutions for global society to adapt to the effects of climate change. Kaiser was addressing a conference workshop on ways cities can become more resilient to future changes in water supplies in the face of climate change.
Large-scale permanent desalination plants were more cost-effective than small temporary plants, but both desalination and groundwater abstraction were expensive and complicated, she said.
Kaiser said the availability of surface water – the run-off from rainfall into rivers, lakes and dams – in the Western Cape water supply system had been reduced because of climate change and the invasion by alien vegetation in the water catchment areas. Alien vegetation consumes far more water than indigenous vegetation, so reduces run-off into storage dams.
Kaiser said, from 1850 until today, Cape Town had relied on surface water, increasing the number of storage dams through time. However, there was uncertainty about the reliability of the surface water supply in the future.
"At this point, there is still uncertainty. Could this be a step change like Perth?"
'We're doing okay, but we're still in a drought'
The Australian city's annual rainfall has been declining by 3mm a year on average, while average temperatures in the region have increased by 1°C in the last 40 years. The city now relies more on desalination and groundwater than on dams.
Kaiser said Cape Town's demand management programme and water restrictions had managed to keep water in the dams throughout the three-year drought.
However, the water crisis had created a lot of uncertainty, anger and fear.
"The question people were asking was 'what if it rains only half as much as last year?' A grip of fear hit the city and hit the administration. In 2017 it rained even less and really elicited a fear response. If it had been only an average year, it would have saved us."
Kaiser said the dams in the Western Cape water supply system were connected with tunnels and canals, so the national Department of Water and Sanitation could balance the flows. Cape Town used 64% of this water, agriculture 29% and other smaller towns 7%.
As the water crisis intensified, national government instructed agriculture to cut water consumption from this supply system by 60% and urban areas by 45%.
Kaiser said the early winter rains this year had brought some relief and the supply dams were now 40.3% full. This time last year, they were 23% full. Earlier this year they dropped to 20%.
"We're doing okay, but we're still in a drought. This was the first year agriculture was ever cut off."
'One in four of the world's large cities are water stressed'
The City is trying to build resilience in Cape Town's water supply. Apart from the augmentation schemes, the City has now taken storm water management away from the road's department and brought it under the control of the water department. The City intends to use the storm water - rainfall that runs into drains and out to sea.
Kaiser said that, in the face of climate change, there was a movement worldwide to use storm water in cities.
As the change in departmental control had happened only last year, the City has not implemented use of storm water yet.
Hastings Chikoko, regional director for C40 Cities in Africa, said the urban water crisis might appear to be unique to Cape Town, but Sao Paulo, Manila, Lima and Tehran had all suffered from extreme water shortages.
"Many urban water systems are already challenged by unreliable supply. One in four of the world's large cities are water stressed," Chikoko said.
Because of climate change, 25% of cities would be stressed by 2025 because of lack of reliable fresh water supplies.
"Had it not been for the work the City of Cape Town had done over the last five years to reduce demand, it would probably have been much worse. It shows how much Cape Town has to teach the world," Chikoko said.