This is one of those cases in which knowing the technical term to google is very useful. The term you’re after is acre-inch: which is one inch of rainfall, or the equivalent amount in irrigation, over one acre. That’s how the water needed for crops is usually measured.
The amount of water needed is going to vary by crop, by weather, by soil type, and by other factors; but here, along with some discussion, is about the first figure I came across for corn:
In Nebraska, total corn water use ranges from 28 inches per year in the southwest to 24 inches in the east.
so, taking that as a rough approximation, the next thing you need to know is how many gallons in an acre-inch? In US gallons, approximately 27,154.
So say about 25 acre-inches times about 27,000 gallons: about 675,000 gallons for an acre of corn crop in Nebraska.
And there’s 640 acres in a square mile. So a square mile would need about 432 million gallons.
Many parts of the world ordinarily get that much or more from natural rainfall (including where applicable the water in snowfall); though I don’t know if Nebraska’s one of them. – hmmm. Seems to depend on where you are in Nebraska, not too surprisingly. Some places more, some places less.
Average Yearly Precipitation for Nebraska - Current Results)
15 to 30 inches a year, on average; which is definitely on the dry side from my perspective. Most places in New York State, where I am, get between 35 and 46 inches per year on average; corn is almost never irrigated around here, and I’m sure hemp doesn’t need to be either, or marijuana if they decide to let us grow it outdoors.
Those rates are annual, of course; it’s not all going to fall during the growing season. But most soils can hold at least some water for some time.