Records show that an average of about 1 inch of rainfall a week during the growing season will insure an average crop of corn. But of course that doesn't tell the whole story.
Barger, G. L.
"How Much Rain Makes Corn?,"
Iowa Farm Science: Vol. 3
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol3/iss12/6