Shelter and water are the two most important essentials when it comes to survival. Regulating your body temperature can prevent hypothermia and hyperthermia. You can construct some pretty cool makeshift shelters using a tarp, which are great for stowing away in a backpack because they’re relatively light and quite versatile in how you use them.Having a tarp as part of your bug out bag is essential, it’s lighter than a tent, easily carried on your backpack and so versatile. Tarps are also good to use because of their effective waterproofing unlike some natural shelters. See this cool graphic you can save or print out of more than 60 shelters you can make using a tarp.
Also check out this handy Rain Catcher by Roy Dredge: