Exactly. All your layouts for the individual areas, then another for the overall. Technically your irrigation design (working drawing) should be just that, your working drawing and nothing else. No layouts, just the design in modelspace. Your new file, the sheet set file, will be where you set all your layouts. You might be able to get away with also creating an overall layout in that sheet set file, but I would still suggest making a secondary sheet set file for the overall layout, just so you have full control over what is/is not shown, without hurting the other layouts you created.