Transpiration bags will work, but the typical vegetation you find in the desert are not likely to produce gallons of water. In my experience, we have obtained about a 1 quart maximum per bag under ideal conditions, often much less. Remember that most all desert species are designed to limit transpiration, that's how they survive the desert. In other environments you may well get more; I haven't used them myself except in the desert environment so I cannot speak from experience in that regard. The water you get also tastes pretty bad in my experience, but as long as the vegetation isn't toxic, it is safe to drink, just tastes (and smells) like the vegetation it was taken from. The big advantage transpiration bags have is they take little effort to use, so there is much greater likelihood of a net gain in water and little risk trying.