In general it takes about 36 gallons of water per minute to make a waterfall 1' wide by 1" deep. Which 1" deep is kinda the standard for residential waterfalls.
So with this info just multiple the width of the waterfall by 36 and that will give you the gallons per minute needed to make a 1" thick water flow over your falls.
(width of waterfall) X 36 = Gallons Per Minute (GPM)
Sample - 3' wide waterfall would require 108 GPM.