It is very important to drink enough water while backpacking, to avoid dehydration. Don't wait to drink water until you feel thirsty. Thirst comes after the body is already low on fluids. Keep your water bottle near you all day long, and sip from it frequently.
By doing this, you will avoid some of the common problems that afflict backpackers who are dehydrated: headaches, fatique, irritability.
During hot weather backpacking, plan on drinking three to four quarts of water a day. If you are at high altitudes, above 8,000 feet, add another quart to this. The body loses more fluids at high altitudes.
More than once I have neglected to drink adequate water on a backpack trip. The symptoms for me are headaches and fatique. Once I notice this I start to feel better about an hour after drinking plenty of water.
Its best not to wait until you get in this condition, and sip small amounts of water during the day.