Water is hands-down the most important essential of the 10 Essentials. Nearly 60% of your body is water, and while you might be able to last up to three weeks without food, you can only survive three days without water under regular conditions – not taking into account the fact that you are exerting yourself through hiking.
So how much water should you take with you on a hike? The general rule of thumb is half a liter (16.9 oz) for every hour of moderate activity in moderate temperatures. But this is only a general rule. Each individual will have different water requirements based on:
Sweat rate
Gender
Age
Pre-hike hydration levels
External environment conditions
In hot weather, especially when you are exposed to the sun, you are going to want to have more water than you would if you were in colder conditions (although water is equally important in the cold conditions as well). The strenuousness of the hike will also affect the above-mentioned general rule. I personally always pack 2 liters at a minimum and carry a filter. That's one full two-liter standard bladder, or four regular sixteen oz plastic water bottles.
There are a few tools that can make carrying your water supply a little bit easier, and it is completely up to the individual on how they prefer to carry their water.
Bladders
One of the most convenient ways to store your water is in a bladder (sometimes called reservoirs). This is a lightweight way to pack water and allows you to sip on it as you hike to keep yourself continually hydrated. There is a limit to how much the human body can absorb at one time, so chugging water can actually make you run through your supply faster (you pee it out vs absorbing for use) and can cause a “splashing” sensation in your stomach that is uncomfortable. Bladders are a great tool to maintain constant hydration.
Bottles
Using a water bottle is a more durable way to store your water and can more easily be used to scoop up water from a stream for chemical treatment. They are generally cheaper than a bladder hydration system and can more easily be cleaned at the end of the hike. But depending on the bottle, it can also add additional weight to your pack and will require you to have to dig it out of your pack to drink from.
Filters & Chemical Treatments
If you are going to need more than 3 liters of water, you might want to look into a water filter or chemical treatments. Water filters like the LifeStraw or Sawyer filter are lightweight, inexpensive and perfect for you to have in case of emergency. They provide instant access to clean water straight from the water source without waiting for chemical treatments to work.
But chemical treatments are still a good thing to have with you. Just ensure you are following instructions carefully and waiting the full time before drinking the treated water. At a minimum, I would recommend always having iodine and neutralizer tablets in your pack. For a mere $10, those tablets could save your life.
Pro Tip: Use a bandana to cover the opening of your water bottle when scooping water from a water source for treatment – the fabric acts as a filter for sediment and rocks.
It is completely up to you on how you want to store your water, but it is more important to make sure you have enough before heading out on the trail. Many hikers fall short in their planning when it comes to how much water they really need because they are unable to accurately estimate how long they will be on the trail. And because water is heavy (1 liter is just over 2 pounds), hikers will skimp on their water supply. This can be detrimental to both yourself, and those you are hiking with. When in doubt, throw in one more plastic water bottle for safety.
Dehydration can kill.