“Together we can avoid Day Zero” says the sign on the back of the bathroom door in our crash pad in Observatory, Cape Town. In April 2022, that’s not just a blast from the past, it’s also been buried under two years of signage about masks, handwashing, and ‘social distancing. If you’ve not lived in on near Cape Town you may not even know about ‘Day Zero’ – the day on which the cities taps were, or are, destined to run dry.

Most Capetownians remember well when we suddenly realised that the time had come to make changes: flush occasionally, wear outer garments a few times between washing, shower quickly, install water tanks to catch rainwater, and so on. We had lived in denial long enough and a few weeks of vague references in the press to ‘Day Zero’ suddenly felt real. Water consumption in Cape Town really decreased – a lot! After a few months, to much collective relief, day zero had moved out into the future far enough to lie on the other side of the rainy winter, and at some point we stopped talking about it – but some of our better habits survived, and the sign on the bathroom door remained in place to remind visitors that the luxuries of Cape Town should not all be taken for granted.

Before day zero became largely forgotten, and before ‘models’ about unfolding crises were predominantly about Covid, there was some discussion about what ‘Day Zero’ really meant, why it kept moving, and whether it was actually a scam, a PR exercise, or perhaps really something technically sensible that also spoke to ordinary people about urgency and agency. It was, of course, if we’d known, the perfect opportunity for warm up discussions to prepare us for pandemic-speak.

The city’s engineers have been watching the water supply in Cape Town with some trepidation for many years. Cape Town is prone to very dry hot summers, and not every winter really replenishes the dams as sufficiently to enable us to face the dry summer with confidence. After several poor rainy seasons, 2018 started in the middle of a summer that threatened to stretch the water supply to breaking point. At the beginning of the year, even with ever tighter rules forbidding the watering of gardens and the topping up of pools, the reminders to use water wisely had not really touched a nerve. How to send a message that was technically defensible, and without being hysterical, conveys the seriousness of the situation?

Consider a storage tank with some initial volume of water, which we call V (some number of litres, cubic metres, or whatever units we like to use). Every day, we drain some amount – call this U. How long will it take before we run out of water. Measured in days, the answer is V/U. If you have 60 litres of water, and use 15 litres a day, you have 4 days (60/15 = 4) before you run out. Not the kind of calculation for which you keep a professional engineer on hand, but this was essentially the mathematics behind the day zero model.

Slightly more embellished: imagine a world in which the rain has simply stopped. There is no prospect of further rain. The dams are calibrated – so we can tell, from the level of the water (a height), how much water is in them (a volume). We can measure how much water is used every day, and how much evaporates. No need to monitor every water meter in the city – we just watch the dam levels, and even evaporation and leaks will be taken into account! Water usage varies from day to day, so let’s watch the depletion every day, and keep a ‘running average’ over the previous seven days and call that the ‘current usage’. If the dams go below 15% of their volume, the engineers, nervous about silt from the dams getting into the pipes, will shut the supply rather than gunk up the entire system.

Of course, the engineers knew it would rain again, at some point, and of course the engineers knew that usage would decline as anxiety rose and regulations tightened. There were elaborate plans to provide water from public taps at watering points, with per-person or per-household rationing, which would still keep water flowing from the dams. No one thought that the model world, in which day zero was calculated, somehow captured enough of the real world to seriously predict whether, much less when, the taps would run dry. Probably each dam would face an ad-hoc decision if it came to that.

Nevertheless, the day zero model world constructed by the engineers was a very sensible way to think about the issue. The model details, simple as they were, were barely every discussed, but pretty much everyone intuitively understood what it meant – probably because this really was the one sensibly chosen model world one should seriously be thinking about. People could relate to the daily updates, and how these were linked to changes in practices and regulations. And so it was that Capetownians got on the same page, and collectively woke up, and shifted their behaviour in a really meaningful way – and day zero hasn’t come, yet.