Yes, metric is nice.

In the meantime, the acre is the standard unit of area for farms, fields, residential property, other property, parks, and certain other uses of land. A house on a quarter-acre lot generally has a backyard, and would fit in most suburbs. A house on a half-acre lot has a big backyard with plenty of room for your kids to run around. A full acre is getting pretty decently big — it's more like the size of a small field that you'd use for growing crops (which is the origin of the measure: 1 farmer-pair-of-oxen-plow-day.)

Rain is measured in inches and feet.

Just multiply two values people use in practice and get another that they can use in practice: acre-feet. Are you forecast for three inches (0.25 feet) of rain, on your forty-acre property? That's 10 acre-feet of water you can get from the sky instead of the irrigation authority. Like most things customary, it's easier to use in some ways, harder in others.

I mean c'mon, it's so simple. One acre is just one chain by one furlong, then you multiply that 12 inches and that's an acre foot.

Ok fine, that might be too complicated, an acre is merely however much land a single man and an oxen team could plough in a single day. Just take that value in your head and multiply it by the length of a medieval king's [citation needed] foot and there you go, acre-feet.

Googling research papers on the subject of European irrigation, I'm mostly finding water usages quoted as mm/yr. Cultivated land area seems to be discussed, explicitly but often implicitly, in hectares. But I'm having trouble finding statements or graphs that show mm/yr/ha or mm/ha.

Unless I'm missing something (and I very well may be), I'll take acre-feet over mm/yr. acre-feet seems a more useful standard compound unit for discussing large scale (agricultural, industrial, etc) water usage, even if the underlying units are more archaic. If you're going to drop or leave implicit one of the three dimensions, time seems like the much better choice than area.

But maybe I'm biased by familiarity (and utter lack of expertise), or maybe I'm just wrong about the standard terminology. I need to get back to hacking, otherwise I'd keep digging....

You should get out more:

http://www.buyinchiangmai.com/land-measurements-in-thailand....

[Edit: And in Hungary, which is also European, you usually use hektár (ha) and négyzetméter (m2) now but you'd better know your hold ( ~ acre) and négyszögöl ( ~ square fathom) if you want to buy agricultural land.]

Being American, same.