If I'm in the wilderness in a dry, hot climate without much vegetation and I'm running low on water, what land features and other potential signs can I look for to guide me to a drinkable water source?
I never tried, but one option I read about is to tie a transparent plastic bag around a branch of a large leaf tree, and let water condensation from the plant to stick to the inner wall of the bag. Cut a small hole in the bag and let the liquid drip out.
An alternative is to dig a hole, put anything having a water content in it, add a cup at the bottom of the hole, cover the hole with a transparent plastic sheet, put a rock in the middle, and let the sun evaporate the water. This technique is called a "solar still". One note of warning with the solar still: digging a hole requires sweat, and the amount of water a single solar still provides is small. A solar still in a very hot climate may be a net loss of water, but I speak from gathered info, not experience.
When it comes to territory, clearly you will likely find liquid water below the timberline. Water follows the gradient, so keep into account the watershed and look for features that form confluences, such as V-shaped bends.
Look at the surrounding terrain and think what water would do when it rains.
No place is perfectly flat for very long, so suface water will flow away from some place and gather in others. Someplace dry with little vegetation will usually have obvious channels that water made when it did rain. Whether you can see these channels directly or have to imagine them, follow them downstream to where more of them meet. After you've gotten to what would be a stream bed with sufficient rain, look for dips where water would pool for a while after it stopped flowing. Now dig there.
Another hint is the vegetation, although that can be misleading too. If there are mostly suguaro and cholla around, then you come to a clump of willow trees, you know that water pools there underground. However, that could be deeper than you can realistically dig, and the trees are actively pumping what is there into the atmosphere. These kind of vegetation clues are better after a recent rain, and become less useful after the trees have had a few weeks to pump down the reservoir.
Water can be surprisingly close to the surface, even in a desert, if you look in the right places.
This answer is based on my experience in the Mojave desert, where open running water is very rare, especially in the summer, and springs have a moderate chance of being poisonous due to chemicals in local soils.
- The easiest thing way to find water is to have the water come to you. Make sure people know where you are going in advance, and when to expect you back. That way, you can expect a rescue, and the rescuers will know where to look for you. Stay in the shade as much as possible, doze during daylight when it's hottest, and limit activity at night. Depending on how hot it is, you can survive one or two days like this without drinking anything. If finding shade means leaving the trail, mark where you leave the trail, and mark a trail to where you find the shade to make it easy for your rescuers.
- If you've planned well, you will have a map with springs and manmade ponds for livestock marked on it. These are not perfectly reliable; springs dry up, ponds get abandoned, and especially in remote places this may not be reflected in the maps for years. It's also entirely possible that you are 20 miles or more from the closest water source.
Now here's the answer that assumes you went on a hike without telling anyone, got lost without a map, and now need to survive an indefinite period of time.
- The best place to find water is in the mountains. Solid rock has nooks and crannies that will hold water after it rains. If it rains on a flat, the water runs off or is absorbed in minutes in most cases. If you can get to north facing slopes where there's been a thunderstorm in the last week or two, you have a very good chance of finding enough water to not die. The longer the time since the last rain, the fewer water sources, but there are (rare) natural holding tanks that can hold water for years.
If you remain in the desert flats with no expectation of rescue, you will almost certainly die unless you happen to be close to a safe spring. You can't cut open a cactus and drink.
In the Mojave, if you see dense greenery, it means there's water nearby, but it may be underground or poisonous. If you know your plants, you can recognize the ones that can't tolerate high mineral contents. Easier would be to watch for wildlife. Insects and other animals using the water is a good sign, as is green algae.