Comment by saalweachter

> How much rain did fall on Hamburg in 2022 given a city size in square kilometers, and an average fall of cm/day.

Oh, now you're in US customary unit territory.

Let me introduce you to the acre-foot.

The acre-foot is the unit of measure for reservoirs. For instance, the Ashokan reservoir is 8300 acres with an average depth of 46 feet. Its volume is 381,800 acre-feet.

If an area 10,000 acres received 3 inches of rain, you need 2500 acre-feet of reservoir to put the outflow in.

How big do you need to make your 2500 acre-foot reservoir? Well, if you're working with 100 acres, you make it 25 feet deep.