What are our trigger levels, and what happens at each?
Why it matters: pre-agreed storage levels for each restriction stage remove guesswork and lobbying when the pressure is on.
Interactive tool · scarcity & resilience
How many days of supply are left?
In 2018 Cape Town published a date, "Day Zero", when most taps would be turned off if its main dams fell to 13.5% of capacity. The date moved as people saved water, and it never came. This calculator does the same arithmetic for any system: how long storage lasts under three scenarios, and how much time restrictions and loss reduction buy.
Runway to the critical level
Decide before the crisis
Eight questions to settle while the reservoirs are still full
Drought decisions made in a hurry fall hardest on people with the least voice. These are the governance choices to write down in advance, and to test against the calculator above.
Who is protected first if supply must be cut?
Why it matters: hospitals, care homes, schools and people who rely on communal taps need written priority, as in the Dutch priority order (verdringingsreeks).
How often will we publish the forecast, and in what form?
Why it matters: Cape Town's regularly updated Day Zero date made the risk concrete and helped cut use sharply.
What do restrictions do to the poorest households' bills?
Why it matters: drought tariffs can push low-income households over the affordability line. Test it before adopting them.
How much of the shortfall can we close by cutting our own losses?
Why it matters: asking residents to save while the network leaks undermines trust. Know your real losses first.
Which other users share our sources, and what have we agreed with them?
Why it matters: farms, industry and neighbouring utilities draw on the same basin. OECD principle 2 asks for co-ordination at basin scale.
What is our emergency supply, and how long does it take to bring online?
Why it matters: groundwater, transfers or desalination take months or years. The decision point comes long before the reservoir is low.
Who has the authority to declare each stage, and is it recorded?
Why it matters: slow co-ordination between national, regional and city government cost Cape Town valuable time. Log each step in the decision log.
Learn from a city that faced Day Zero
Cape Town cut its daily use by more than half between 2015 and 2018. Read what worked, what it cost and who it hurt.
Read the Cape Town case