The general rule used to be that west of the 100th meridian, you cannot do commercial agriculture without irrigation (because not enough falls from the sky over

The general rule used to be that west of the 100th meridian, you cannot do commercial agriculture without irrigation (because not enough falls from the sky over the arable land). That boundary has now shifted east a bit, and is closer to the 98th meridian at this point. It's a "rule" that applies to all areas west of that line, with the exception of the zone west of the Cascades in OR and WA.