It’s definitely important to keep your fluid intake up on long hikes. Dehydration occurs when greater than three percent of body weight is lost. It is not uncommon for athletes to lose between two and six percent of body weight as fluid during strenuous activity. The best general advice I can give is to find fluids that go down easy and to be sure to replace what you’ve lost.
To what you should drink, weigh yourself before and after your next backpacking trip and calculate a rate of intake based on the total losses. As effort during backpacking is significantly less than that of a marathon, greater fluid intakes should be possible though not necessary.
Fluids should be cool (15-21°C) for optimal palatability. If the taste of water doesn’t do it for you, try diluted juice or a sports drink. Look for low calorie versions of drinks such as Gatorade Propel or G2 to avoid lots of empty calories. You can also try making your own sports drink at home.
Give this one a try:
32 ounces of cold bottled water
Freshly squeezed orange, lemon, lime or grapefruit juice to taste
2 tablespoons table sugar (or equivalent of sugar substitute)
A pinch of salt