
Personally I always feel this dehydrated fatigue after a long tedious afternoon of running errands without drinking any water.
To ensure adequate hydration I recommend for most people to simply divide their body weight in half and that number is roughly the amount of ounces of water a person needs daily. For instance a 150 pound woman would need 75 ounces of water daily.
You can also divide your body weight in to thirds and then add 8 ounces for each vice (coffee, tea, alcohol) and then another 8 oz for every 20 minute period of exercise. So again a 150 pound woman would need 50 ounces plus 8 ounces for a cup of coffee and 16 ounces for 40 minutes of exercise totaling 74 ounces. Hydration is not just about water, it is also about electrolytes.
Sodium, potassium, and trace minerals are important for preventing dehydration. Emergen C packets can be added for electrolytes, or a pinch of organic sea salt. The simplest way to stay on top of your water consumption is to bring a Klean Kanteen stainless steel water bottle everywhere with you wherever you go. Since the recent banning of BPA in plastic water bottles, stainless steel is the safest option for transporting your water.