A solar still will provide plenty of water for survival in a desert environment.
This one has me wondering about a few things. Exactly how do you mean "provide water". Through transpiration, or through purification of dirty/stagnant/bacteria laden water? The solar still is a lot like the use of celestial bodies for navigation. The theory behind it is sound, and tested. How it's taught or proported to work, however, is usually lacking. Solar stills are dependent on a number of factors.
The first of these is method; Like I said, either transpiration or distilling liquid water, which makes a big difference in yield. The second is size; Obviously a 1' square still isn't going to produce as much as a 6' square still, no matter which method. The third is location/how much direct sunlight it's exposed to. These are just the primary three that spring to mind. Then you also have to consider set up.
Most of the online information you see is pretty basic construction. "Dig hole, line hole with vegetation, place container in center, cover hole with clear plastic sheeting and place a pebble over container."
There are a few examples that DO point out that it's best used with a drinking tube or siphon of some sort so as not to disturb the still. I haven't seen one yet that advises you to also have an inlet tube(if purifying water, rather than trying to procure it from plant life). Very few that I've seen tell you that lining the hole with a dark material, preferably plastic will help, or that digging the hole in a "dry" creek/stream/river bed will increase the yield. Even placing it near plant root systems is an improvement over "just dig a hole anywhere and throw some leaves in it".
Now, I'll concede that the solar still is pretty situation dependent.(It's also not a great long term plan, and shouldn't be your primary means of hydration) We get about 350 sunny days a year here, for example. It's also not going to be the well of life either, but in my experience (and yours may very well differ) they DO work, and if you ration sweat, and limit your activity to cooler parts of the day, etc.(things you should be doing already in a desert environment or survival situation) they'll allow you to survive MUCH longer than without.
You may not be 100% hydrated, you may not be very comfortable, but neither of those are really "survival" criteria in my opinion.
The generalizations I gave about short comings are just what I've seen espoused by "survival experts" and online diagrams of solar stills. My opinion though is based on trial and error and heuristic learning in a desert/arid climate. Just thought I'd toss it out there before everybody wrote off solar stills as completely useless.(They're only moderately useless, and usually it's because of one of the above factors

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Gautier