I have heard of an old wives tale that rain water is good for hair. Wether it's true or not has yet to be tested by myself. I would think it would be beneficial on the logic that it's the better option over tap water to use whith gardening as it is normally brown/green and full of life because of it's softness. I'm not completley swept by the Paleo lifestyle but I do agree with the concept that if we did something 10,000 years ago we could do it now. Back in paleo times there was no chemicaly infested water supplies and definatly no shampoo so the only way we would be able to rinse our hair would be at a natural source of water, or maybe we didn't even bother. Unless the water actually contained anything harmfull in it rain would be an excellent source of external nutrients for our scalp so I personnaly would think it would be a great idea. The only drawback though is that aside from maybe good nutrients there could be algea, insects and other horrid things i'm sure your not in a rush to put in your hair. If you could control that though, use it on your hair then rinse it again with tap water (or shampoo) you could be on to something. It's a different approach I know but if your concerned about hair loss i would think you were a little more open minded. Definatly worth a discussion though. Fire away.