Day Zero

Day Zero signifies that drinkable water will run out by that day for a region, if no rain falls or consumption change takes place.

By the way, Montevideo is hitting Day Zero these days. To keep the system running, the city is adding salt water from Rio Plate to the drinking water network.

Source: https://www.dw.com/en/uruguay-drought-capital-montevideo-hit-by-water-shortages/a-66075991