Because the atmosphere is so dry, you may not feel particularly warm, but you'll be steadily drying up. Don't wait until you're thirsty to drink. Carry a water bottle with you at all times. If you are out and active during the hot part of the day, you should be drinking water every 15 to 20 minutes. The color and volume of your urine is a direct indication of your water consumption.
Dark urine is bad, light urine is good.
One gallon per person per day is the rule of thumb. Users of alcohol, caffeine and other drugs are particularly at risk for dehydration, and should pay extra attention to their water intake. Dehydration can cause headaches, stomach cramps, abdominal pains, constipation, or flu-like symptoms. As a Ranger, it is particulartly important to stay on top of your water intake, both to maintain your own health and to set a good example for others. Be on the lookout for people who are walking around without carrying water.