Water
OK we are using about 250 to 300 liter per day so with full tanks well be fine for 100 days (although the tanks aren't suppose to get empty all the way of course). It has rained just 28 mm in the past 5 weeks (that's nothing really). This gives us, with our roof surface, about 6000 liters in the tank. That's about 45 cm per tank and we should be good for 20 days, for showers and cooking.
And of course after all that maths it has to be checked as well. The high tech method over here (that everbody uses) is to knock on the tank and listen to the difference of sound, indicating the water level. And, yes, it matches perfectly - bunch of freaks we're turning into. And very well aware of our water consumption.
At night we hear the possums having a ball on our roof. Where would they go for a piss? Surely not on our roof...