For some reason, when I was a kid, I made it my business to know as many bushcraft skills as I could possible learn. One of the first I picked up was actually from a young-adult fiction book called Deathwatch. In the book, the protagonist is being pursued through the desert and must rely on his survival knowledge to evade and finally dispatch his pursuer. One of the things that allowed the book’s hero to do this was a device called a solar still from which he “made” fresh water. The book did an inadequate job of describing the still so I was forced to beg and plead my uncle (a member of the ANG) out of a copy of the military’s survival manual.
As I held those pages in my hand reading about how to build a solar still I knew immediately that I had learned one of the most valuable survival skills possible.
A solar still is nothing as magical as perhaps stories of my childhood makes it seem. Simply put, it is a hole in the ground with a cup at the center and bottom of the hole. Over the hole plastic is draped and a small rock is placed in the center. The plastic is secured at the edges. Newer versions actually have a drinking tube from the cup to the user. Who has a drinking tube at a time like this?
The solar still works through properties of evaporation and condensation. The water in the air and in the ground evaporates and forms a mist which floats upwards. The plastic traps the mist and the water collects on the plastic until it becomes heavy enough to drip off the center and into the cup. This process can be amplified by placing moist plant material in the bottom of the hole as green plants transpire and release water through pores. If I understand correctly you can even place saltwater in the hole and the still will desalinate it as it evaporates.
It must be said that there are disadvantages to this system. For one, it takes a lot of energy to build one. For another, the water you gather is only in amounts that will barely keep you alive in most cases. In addition, it must stay put for some time (overnight is best) to allow it to work, so its hard to travel.
However, in dry climates or in areas where freshwater is hard to find the solar still provides a means to sustainable fresh drinking water.