Two consecutive days of Stage 4 load shedding at key pumping stations will run dry its reservoirs, Gauteng water supplier Rand Water says.
But it can't survive any level of electricity rationing indefinitely, and places at the far end of its distribution network will feel the effects of blackouts sooner.
Thanks to other factors, water reservoirs are already at the lower end of the ideal range.
It would take just two consecutive days of Stage 4 load shedding at two of its most sensitive pumping stations to lead to a water shortage, Gauteng's bulk water supplier Rand Water says.
Until this week Stage 4 electricity rationing was almost unheard of, but Eskom implemented the measure on both Sunday and Monday, with no clear answers on when it would be able to fully meet demand again.
That makes water shedding a distinct possibility, even though Gauteng dam levels are high, with the Vaal Dam at 71% and the Sterkfontein dam 93.9% full.
Reservoirs are already below the ideal 60% to 80% level due to the recent heatwave and capacity constraints, spokesperson Justice Mohale said in response to Business Insider questions.
"Any disturbance with load shedding therefore affects pumping which is designed as a 24 hour continuous operation."
Rand Water has a deal with Eskom that its pumps are the last to be blacked out during electricity shortages, Mohale said. But this applies only to pumps directly supplied by Eskom, not the many connected via municipal grids – and only applies up to Stage 3.
"Stage 4 means Eskom can ignore the agreement and load shed all our pumping stations that are supplied directly by them," Mohale said.
Two days of consecutive Stage 4 rationing in two key spots – a main pump station at Zuikerbosch and main pump in Vereeniging – will run reservoirs dry, he said.
Though the entire pumping system for Gauteng is affected, areas at the end of the supply chain would be hit first. That includes Bethal, Rustenburg, the region north of Pretoria, and the east of Ekurhuleni.
In August last year Joburgers scrambled for bottled water and rental toilets after a substation explosion affected the key Rand Water Eikenhof Pumping Station, leading to dry taps.
If power cuts continue to be a daily occurrence, then water restrictions will be imposed, Mohale said – and now would be a great time to start saving water to avoid complete outages.