I was sitting here thinking (like i do most days) and I came up with an idea for your system.. It may or may not be something your interested in. It is a way to collect the water your system drops out while running. Also I think it would increase the effectiveness of your cooling (more soil contact area). It may or may not be suitable for you, but I thought it prudent to share it.
Intead of using the perforated 4" piping you can use PVC tubing burried instead. A 4" pipe runs down and 7 1-1/2" pvc tubes slit off of that at a slight downward slope away from the inlet tube. It takes 7 1-1/2" PVC tubes to hold the same volume in a 1 ft length. This will go out for a distance and join at a 4" pvc pipe on the other side of the "inlet" tubes and the water will drain into that portion. you can run a 1/2 PVC to run the water to a central point from the base of the 4" PVC. This PVC is joined to a parrallel 4" at the tops where the 1-1/2" PVC slopes back up (also with a 1/2 drain line to the central water point from the return 4" PVC). The central collection point can have a little
pump that you can use to pull the water out as needed to water your garden.
I will draw something later to show what I am thinking. It will probably be a bit more expensive, but can collect a ton of water daily with the right humidity. and with more soil contact with the 1-1/2" PVC there is a good chance that you will get a lot more cooling as well.
I will draw a picture of it later to make sure my idea is going through well. :P