Water, Purification, Solar Still, T Brown
"Does an excellent job of purifying bacterially polluted water."

Note: I have several versions, in several books, of illustrations
concerning how to make one of these.  I think this one is the best. -
Paul

Tom Brown's Field Guide to Wilderness Survival   ISBN 0-425-10572-5    
page 52-54

SOLAR STILL 
     A build-it-yourself solar distillery is one of the best ways to
get drinking water in areas where it is scarce. Developed by two
doctors in the U.S. Department of Agriculture, the still is a major
survival tool.  Unfortunately, you have to carry the necessary
equipment with you, since it's all but impossible to find natural
substitutes. But the only components are a six- by- six- foot sheet of
clear or slightly milky plastic, six feet of surgical tubing, and a
container to catch the water.  (In a pinch, you can get by with just
the plastic sheet, since it's the only non-natural element that is
really crucial.)  These pieces of gear can be folded into a neat little
pack along with other survival necessities and clipped onto your belt
for any outing (see "Survival Belt Pack," page 271).
     To construct a working still, use a digging stick to make a hole
four feet across and three feet deep.  Try to locate the excavation in
a damp area, gully, or river basin to increase the water catcher's
productivity. Place a container in the deepest part of the hole.  Then
lay the tube in place so that one end is in the bottom of the container
and the other end runs up and out the side of the pit.
     Next, cover the hole with the plastic sheet, securing the edges of
the material with dirt and weighting the center of the sheet with a
rock. The plastic should now form a cone with sides sloping at
forty-five degrees to the ground. The low point of the sheet must be
centered directly over the container and no more than three inches
above it.
     The solar still works by creating a greenhouse effect under the
plastic. Ground water evaporates and collects on the sheet until
growing droplets run down the material and drop into the container.
When the container is full, you can suck the refreshment out through
the tubing without having to break down the still every time you need a
drink. And on rainy days or nights you can collect a good deal of water
on top of the still as well.
     A good solar still, located in a damp area, should keep collecting
water for four or five days, after which it should be moved to another
area. In drier regions such as deserts, I recommend making the hole
four feet deep rather than three, and moving the still every day or
two.
     You can also place crushed herbaceous plants such as cactus and
thistle in the pit to increase the still's output.  I once used such a
setup in Death Valley and was able to obtain a half gallon of drinking
water a day.  Be careful to use only edible plants for such boosters,
though, as many poisons will evaporate and drip down into your
container even more rapidly than water. For the same reason, I don't
recommend using a solar still to treat chemically contaminated water. 
On the other hand, the still does an excellent job of purifying
bacterially polluted water. Just pour the liquid onto the surface soil
next to the plastic covering.  The water will filter through the ground
and be distilled into a quite safe drinking supply.  In fact, you can
even purify and recycle urine using this technique.