Many farms in the US run off a municipal water supply. Only in the west do farms have irrigation systems. When you get east of the Missouri river there is enough rainfall to water the crops and so you don't have any irrigation needs (I don't know why the climate changes so drastically on this line but it does). Even if you have a well, if you are east of the river it is only for household and livestock use, not irrigation.
I live in the midwest and irrigation is definitely used around here. Not for all crops but there has been a huge increase in the usage of irrigation over the past decade. There's usually enough rain but irrigation still helps make yields a lot more consistent.
