It doesn't matter if you're in a desert or a forest. When you're living off the land, you need a ready supply of water if you're to have any hope of surviving more than a couple of days. We lose between half to a full gallon of water a day, just through sweat, urination and being active, and if you don't replace this water loss, you're going to be in big trouble, very quick.
Today, we're going to cover what you need to know about finding water in the wilderness. No matter where you are, with the right gear and mindset you'll be able to get that drink your body needs to stay alive, and keep surviving.
Getting prepared before you head out
The first piece of advice is simply to be prepared. Bring enough water with you so you're not going to get yourself into trouble in the first place. I also recommend being able to properly navigate with a map (and packing one in your kit), so if you do need to find a large body of water you already know where it is and can head directly there. You'll also need to pack a couple of methods to purify any water you find, I carry purification tablets with me as well as a lifestraw filter and a metal canteen I use to boil everything before I drink it. I've been sick with giardia before (a common water-borne parasite) and it's not something I ever wish to repeat. Treat every drop of water you're planning to drink.
Finding water in an emergency
As with everything, even the best-laid plans can be for naught. If you've lost your kit, are disoriented and have no idea where you are, there's still hope.
The first step is to orientate yourself. Look around and see if you can spot any depressions in the landscape or any areas where the trees are greener. Valleys can pool water in the bottom, and lush green growth (especially if it follows a curving line) can indicate a stream you can aim towards. If you've not got a good view seek out a nearby crest or climb a tree and use the higher elevation to scout out the direction you need to travel. As a general rule, you want to be walking downhill, as water is also affected by gravity and you're most likely to find it at the bottom of a valley.
If you do reach the bottom of a valley, or find a dry riverbed yet no flowing water, if it's damp you may still be able to get water out of it. You'll need to dig a seep, which is essentially just a hole you're going to make, to allow the water to collect. I recommend digging it at least 2-3 foot long, and about a foot deep, though you may need to go a little deeper depending how dry it is. Target a low lying area in the river bed, or the outside curve of a bend where water tends to collect.
You can also use the seep technique as a rough filtration method when you find a creek or stream that may be contaminated. Simply dig your seep in the creek bed and wait for it to fill before you drink. It's not 100% guaranteed, but it's far safer than drinking directly from the creek as the dirt acts like a natural filter that can reduce the amount of contaminants you're drinking. You can line the hole with pebbles to reduce the sediment, just be careful about coming back to your seep at a later time. If animals have found it and have drunk from it, your "semi-filtered" water will be contaminated again.
Now if you've dug a hole and there's no water to be found, you can make use of another technique, known as a solar still. You're going to need to make the hole quite a bit bigger (at least 3ft by 3ft, and a couple of feet deep). Ensure it's in a sunny spot, and fill the hole with leafy branches, placing a cup or a wide-mouth bottle in the center. Cover the top of the hole with your emergency blanket or a plastic tarp, and seal all of the edges with rocks and/or dirt. Finally, place a small pebble on top of the cover you've just made so it makes a depression directly above the bottle you placed inside. The sun will cause moisture to evaporate from the leaves, which will turn into condensation on the underside of the tarp, that will run and collect into the bottle you place. You'll get about a bottle of water per day using this technique.
If you're in an area where there's lots of condensation in the mornings you can take a cloth and run it through the grass and leaves to collect the moisture, and wring it out for a morning drink. If you're stuck somewhere and you've only got salt water, you can use a similar method to collect the steam, just be careful about how hot your cloth gets. Use your cloth above a pot of boiling salt water to soak up the steam, then carefully wring it out into a bottle to drink.
There's two things I want every reader to take away from this post. First, make sure you've got a plan to carry, collect and purify the water you need the next time you're heading out into the wilderness. Second, if you do find yourself stuck and are needing to rehydrate, there are plenty of ways to source drinking water in the wild without resorting to Bear Grylls' level and drinking your own urine. Don't do that.
There's plenty of water out there if you know where to look.