There have been several near misses in recent years. Eg https://en.m.wikipedia.org/wiki/Cape_Town_water_crisis
Large city, in this market, is defined as population greater than 0.5million.
Run out of water, in this market, is defined by credible media reports of a city turning off the mains and people having to queue for a water ration.
The time of the market has been chosen to give Equal weighting to northern and southern hemispheres. Remember that Manifold returns long term manna bets to you as loans, so your manna will not be tied up for long.
Cases where this is due to pollution or pipe failure will not be considered for market resolution.