In the heat I can sweat a LOT. So I carry 2 liters in hydration pack and fill it to the brim any time I can. I also have 1 liter Gatorade bottle that I can fill when I expect it to be dry for a while or just as a backup. One problem with a hydration pack is that it hard to tell just how much water you have left. I hate being 1/2 way up a hill in the sun and run out of water. I know one can go a long way without water but I would rather be safe than sorry. Then again, I have also dumped extra water when I want to lighten my load towards the end of the day. The Gatorade bottle also serves as a bottle to mix powered drinks ins or exchange for a REAL bottle of Gatorade if you can get one. At camp, I have a 4 liter tote bag that I haul to to the water source and fill. From this I pump water for dinner and the next day.