Jon, trust me, the solar still converting urine back into water works. How do I know? Well, maybe because I taught the subject for the US Navy. I completed jungle Warfare school at Fort Sherman Panama and lived in Asia for 5 years, 2 years in the jungles of South and Central America. I did cold weather survival in Alaska and subject trained on desert survival techniques. During one of my tours I was assigned as a combat instructor training members from various units of the Army mostly at Ft Hood, Texas, The Navy and the 4th Marine division. Our Naval Special Warfare unit specialized in survival. OK, I'm a subject matter expert but it's been years since I have long retired.

So here is some run down on Cactus. Cactus can provide a water moisture source, if edible and taste good. Wow, edible, you might ask. Some, but not like you think. You are referring to Barrel Cactus, which it takes hours to drain into it's water fluid after cutting and to can often lead to making one have diarrhea and or vomiting if digested. That is not a good thing if your stranded with out water. The body will loose roughly a gallon per day just by sweat and another by urinating. If you eat it will take water to digest, but plants has juices which will aid staying hydrated. Chew on them to get the most out of them. As a common rule, if the stem or leaf has a milky slime to it, don't eat it.

Keep in mind individuals are effected differently due to health, build, etc...I have drank some cactus juice before but not from a barrel cactus. Your right how most do believe the barrel cactus is filled with water, and your right, it's not but it does have a slimy juice that can be made into a drink.

However, here's the problem, there are three types of barrel cactus in North America, if I recall and one will flat out make you sick. The species Ferocactus covillei barrel cactus is the one which you likely have heard about being highly toxic.The Indians call it the killer plant. The other two may or may not effect you. So you have a 1 in 3 chance to not poison yourself. That is not good enough and is why survivalist recommend to stay away from drinking cactus water. You can not survive on 1 in 3 chance...You need to be 100 present staying focus while keeping your wits.

Nevertheless, while water is very difficult to find in dessert there are some forms of vegetation. If so, plan waking before sunrise and wick up as much dew using anything, t-shirt, bandanna or what ever works to absorb as much dew then squeeze in a mouth full of water. Look for wash outs from rain where water may be just feet below the surface. Again, it won't be a river and might only be wet seepage to just moisten your lips.

Move at night and find some form of shade during the day. Save your urine. You may fine the need to drink it. IF you drink it straight your are returning the toxins back into your body that can lead to making you sick, that is why the solar stile works. You can also place vegetation in the same hole that you dig to make the stile, but this is to answer your question, not to write a book on this matter, but urine after being process turns back to just water. Odds are, you can't make a stile because it requires a large plastic sheet, a cup to collect it in and your not in military garb. I say drink the urine straight and cheers to you.

However, your best bet is to think ahead. Planning ahead and being aware that even the best of us can make one mistake finding our self perhaps stranded, be it due to off roading in a 4x4, out riding and some how the horse runs off with out you, or what ever it may be, plan a head. Oddly, there are very few places left in the USA that does not have cell phone coverage. Check your resources before any out door planing. Have a compass and mini survival kits, or as in the Navy, we call it ours Escape and evasion kit. I hope that answers your question. Oh note, there are numerous types of cactus being made into jellies, food sources plus long been used for drugs and the making a various alcohols. If your in a survival situation, keep your wits and size up your situation. Hope this helps you with that question.

Whhooowee! What's with all that McGyver stuff and urine? I don't know about how many cactii it would take to quench your thirst either. I do know that cactus can keep you from drinking too much. Fall off your horse on a cholla once and your ability to reason will be greatly enhanced. As will your ability with tweezers and needle nose pliers. You will also have learned not to drink too much. That's about all I know about cactus. Sorry.