Hydration is often a tricky balance when hiking, backpacking, or trekking. Finding the right middle ground between drinking enough fluids and not carrying too much weight is something many struggle to find.
A quick internet search will suggest anywhere from half a liter to 1.25 liters per hour, but guessing wrong in that range could leave you carrying 5+ extra lbs of fluid, or thirsty with miles to go. It also doesn't take into account something nearly as important, but often overlooked, when it comes to hydration- electrolytes.
So how much should you drink? That depends of a large number of factors... 1) Temperate- while you need to be sure to keep up with water intake on cold excursions (even when you don't feel like it), your body needs more in the heat. 2) Humidity- when it's humid, the sweat on your skin doesn't evaporate quickly, causing your body to sweat even more, increasing your need for water 3) Weight- the heavier you (and your pack!) are, the more effort it takes to move your body each step, which requires more water 4) Altitude- lacking the same oxygen you'd get at lower elevations, your respiration rate increases at higher altitudes, making you lose water faster. High altitudes can also make you feel less thirsty, and low humidity evaporates sweat so quickly you might not realize your exertion levels, both making you think you need less water then you actually require. 5) Terrain- simply put, the harder the trek, the more water you'll need. 6) Fitness level- the stronger you are, the more efficient your body moves, and the less you respirate (you don't pant like a dog), the less water you'll need in comparison to someone who is less fit than you. Take your abilities into consideration when deciding if you need those extra few ounces. So do you just need water? You know when, towards the end of a hike, you start cramping up, especially in your quads on the downhills? You might think you're dehydrated, so you drink a little more water, but that doesn't make a huge difference. This could be because you're not just dehydrated, but you lack electrolytes and/or carbohydrates. Adding electrolytes in the form of GUs, gels, blocks, etc., or with food such as bananas, oranges, cucumbers, can help balance out the sodium and potassium levels in your blood. Sport drinks with a roughly 6% carbohydrate solution (my personal favorite being Gatorade Endurance Formula Powder) can help provide the right ratio of simple carbs and electrolytes to stave off muscle fatigue and cramping.
So what do I bring?
Let's start with the average 150lb person. If you're much heavier, you'll need a bit more. If you're much lighter, maybe a little less. If you're doing a moderate hike below 8,000 feet, and a temperatures of 65 deg F or below, you'll need roughly 16 ounces of fluid per hour (or 1 liter every 2 hours). If the difficulty increases, if you're above 8,000 feet in altitude, or if the temperate is high, you'll want to increase that to 20-24 ounces of fluid per hour. In extreme cases, above 12,000 feet in altitude, temperatures above 85 deg F, or in an area with a extreme lack of water, I would suggest budgeting a liter of water per hour, or factoring in an additional liter of water as "back up" for each day. I also suggest that half of your fluids have some type of electrolyte or electrolyte/carbohydrate solution in them. That means if you set off with 48 ounces of water, 24 ounces of that has a solution in it.
Additional Tips
+Ditch the heavy Nalgenes for the ultralight ones, or, better yet, plastic water/Gatorade bottles. You'll drop significant weight, and if it comes to it, using a plastic Gatorade bottle as your pee bottle in a cold tent is better than something you spent $40 on. +Socks as insulation sleeves on water bottles work just as well as costly sleeves, and double as extra socks/mittens in the event you need it +Hydration bladders are great for warm weather hikes and reducing stops along the trail, but often don't work on cold days. If you do use them on cold days, get a tube with an insulation sleeve, and always blow air into the tube after each drink- that will force water out of the tube and into the bladder, preventing the tube from freezing solid.