I agree. The 'magic' number is 500ML a day. Why is that important - why not 400 or 300? Remember, earlier this year it was 700 or 600. So, politicians like De Lille, Limberg and polymaths like Zille (refer her crazy suggestion in 2015 to throw out the system of 'government' audits because universally accepted audit methodology inconvenienced her administration) are throwing numbers around, and in common with many non-specialised people, don't really know what it means or know enough to ask the experts what it means. This is why I think the city has so badly managed the disaster, and that Limberg is too inexperienced and lacks maturity for the post De Lille catapulted her into. 
I worked as an engineer years ago. For any system there's limits or tolerances that cannot be breached. I was not a civil engineer but I think it's not possible for a city of 3.7m to use less than 500ML/day and still function properly. It's like asking people to breathe less air. This they can't understand.