With the impending landfall of Cyclone Yasi, we were pondering how much water wo

Question

With the impending landfall of Cyclone Yasi, we were pondering how much water would be dropped by storm after hearing them say there would be about a metre of rain falling.

Assuming that this is the amount of rain falling on one square metre, and is constant across the main storm, and that the storm is a regular circle of 500 kilometres diameter, we arrived at:

3.1415 * 250 000 * 250 000 = 196 343 750 000 cubic meters
= 196 343 750 000 000 litres
= 196 344 gigalitres

By comparison - we have a reservoir which holds about 20 gigalitres.

Even if we assumed a constant falloff from the centre to the edge - we would still get 49 086 gigalitres.

We then figured out how much water was dropped in QLD in December (209 mm average over the month) which caused the severe floods. Queensland covers 1 852 642 square kilometres:

0.209 * 1 852 642 000 000 = 387 202 178 000 cubic meters
= 387 202 178 000 000 litres
= 387 202 gigalitres

Which means that this storm will dump a significant amount of water on a very small area...

Is there anything wrong with our assumptions - are these calculations right?

Explanation / Answer

I don't think there is anything wrong, but depending on the application you should consider studying a more complex dynamics. Consider how much water enters your system (per unit of time) and how much is drained.

You see that 1 meter of water in 1 square meter sounds a lot! but if it's in a period of one week then the water will probably drain or evaporate at almost the same rate. No flood in this case.

1 meter is a lot! Even when averaged for a week. Here in central Europe we would name some cm within a day a catastropy.

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