I thought these crazy Americans wouldn't surprise me anymore with another strange unit of measurement and here's the acrefoot
Of course it's a bit odd because it uses acres and feet for the area and depth, instead of reasonable units like meters, but those are just the usual strange units.
Whereas in a more rational system, the unit for volume would be based on the unit for length in such a way that one could easily convert square-kilometer-inches into cubic meters without a calculator, for example.
Second, the direct "depth of flooding" value is useful both on the production side (rainfall) and for two of the largest consumers (agriculture and lawn care). The conversion therefore needs to happen twice, which is one of the usual thresholds for delaying the conversion into M-K-S.
If you don't have a good, easy way to convert between acres and square feet, you instead need to imagine some unit custom built for the application. So rainfall is typically measured in inches, crop areas in acres, simple we will just invent an acre-foot and now people can convert their rainfall needs into water amounts. Maybe.
There's probably a lesson for system design here.
It makes thinking about large volumes of water easier - especially in the context of lakes and rivers. I have no clue how to begin thinking about 5,000,000,000 kiloliters or 130,000,000 gallons. But I can visualize an acre, and I can visualize a foot.
Plus, think about rates of change. If 5 acre-foot of water is moving through a channel every hour, and you have a 500 acres reservoire, then the water level drops by 3 inches a day.
The acre-foot is a useful measure only for certain scales of volumes, but there's a need to convert between small and large scales of volumes all the time.
If we didn't have the metric system, sure, the acre-foot makes sense to use. And in the metric system, perhaps people will occasionally talk in terms of hectare-cm instead of liters or m^3 (I don't know enough about non-US water systems to know what they typically use). But regardless of liters or m^3 or hectare-cm, these can all be converted by moving a decimal point rather than multiplication or division with multiple significant figures.
We have a better system, we have just chosen not to use it.
What's strange is chopping off the head of a bunch of French aristocrats because ergot poisoning drove you mad, and then replacing old emergent domain-specific measurements with something new because those old measurements just aren't rational, and you're clearly very rational, obviously, because what could be more rational than tearing a bunch of people from their homes and chopping off their heads? Their world order was clearly arbitrary and irrational, and evil! So with that oppressive old world order extinguished, you're now impelled to conjure new, more rational unit definitions like "one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris", which is in no way arbitrary, and is of course very rational and sensible.
And from this day henceforth, the ploughman shall no longer measure his land in terms of the area that a pair of oxen can plow in a day (1 acre), but in terms of the square of "one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris" - the meter! The meter is rational, scientific, even noble, yes indeed.