"One inch of water" [View all]

[IDemo]

This is often given as a unit of measure for how much water is required by a given plant. But it's a subjective term, not like "acre-foot". It apparently means the equivalent volume of water applied to an area if a container has one inch of water after a rainfall or sprinkler irrigation. But how is that in any way more useful than simply stating the volume directly in gallons per tomato plant?