How 'bout a sunken
hugel bed inside the hoop with a trench to catch the water as it runs off the poly and direct it under the edge into the
wood of the bed. I think this would work if you are wanting help with the watering of your plants in the tunnel. If you are just wanting to collect water for emergencies or something then you could attach a gutter through the outside of the poly to the frame and seal the top with some sort of flashing (or duct tape)to direct the water off the poly into the gutter. You would want to put this high
enough so that you could easily drain the gutter into your catchment (barrel, tank, etc.). So just sandwich the poly between the gutter and the hoop framing and your good to go, make it as high as you want, direct the flow into whatever you want and your done.....easy (if not a bit butch)!