A cup of coffee depletes your body of one glass of water.
HEALTH Up to three quarters of our body weight is water. Dehydration occurs when the amount of water leaving our body is greater than the amount taken in. Humidified breaths, perspiration, urination and bowel movements routinely deplete the body of essential water that must be replaced. 
Symptoms of Dehydration
Are you dying of thirst? Insufficient water intake can cause a host of unpleasant symptoms. Coupled with urine conservation, thirst is one of the most apparent responses to dehydration. Thereafter, more obvious symptoms may develop [1,2]:
Hair can be comprised of up to 25 percent water, so dehydration negatively affects its texture and growth.  If you're a healthy adult, you can usually treat mild to moderate dehydration by drinking more fluids. Get immediate medical care if you develop severe signs and symptoms such as extreme thirst, no urination for eight hours, shriveled skin, dizziness and confusion. Go to the nearest hospital emergency room or call 911 or your emergency medical number if you think a child or older adult is severely dehydrated. 
People often associate dehydration with hot weather. We can dehydrate in the winter. Feelings associated with a typical hangover are largely due to dehydration. Both the common "cure," coffee, and cause, alcohol, are diuretics. They increase urine output. Hence, it is advisable to drink one glass of water for each cup or glass of diuretic beverage just to remain at baseline without even addressing the daily needs for intercellular function. 
Electrolytes are used to conduct electricity for the normal function of our bodies. These essential ions include sodium, potassium, chloride and bicarbonate.  Too much or too little water upsets the balance. This is why athletes generally drink fluids enriched with electrolytes following strenuous exercise.
Living, working and exercising at high altitudes (generally defined as 8,000 to 12,000 feet, or about 2,400 to 3,600 meters) or very high altitudes (12,000 to 18,000 feet, or about 3,600 to 5,400 meters) can cause a number of health problems, including dehydration. The body tries to adjust to high elevations through increased urination and more rapid breathing — the faster we breathe, the more water vapor we exhale. 
How Much Water Should We Drink?
You have likely heard the general rule of 8 (8-ounce) glasses per day is sufficient. In most cases it is plenty. But the amount of water required actually depends upon environmental conditions, body weight, and daily exercise routine. Hence, an online calculator suggests 10 glasses for someone living in a warm climate with a daily 20 minute workout. At 250 pounds and no workout in warm weather, more than 13 glasses is recommended.  Fortunately some water is aquired through foods. Fruits and vegetables can time-release H20.
It is important to spread water consumption over the course of a day. A potentially fatal condition known as hyper-hydration or water intoxication occurs when someone drinks too much water — typically somewhere far above 2 liters (8 cups) within a short period of time  without proper elimination. It dilutes sodium levels, causing coma, brain damage, and death. So if you're drinking water regularly, that's good. But if you are not eliminating through perspiration (aerobic exercise) or urination — particularly none within 8 hours — you may need to consult a medical professional or dietitian. If you simply forget to drink water and have an iPhone, iPod Touch or iPad, check out Waterlogged  in the iTunes Store.
- Dehydration. MedicineNet.com
- Dehydration. MayoClinic.com
- Can Dehydration Cause Hair Loss? eHow.com
- Effects of Alcohol Dehydration. SymptomsofDehydration.com
- Electrolytes. MedicineNet.com
- Human Water Requirement Calculator. CSG Networks.com
- Water Intoxication. Wikipedia
- Waterlogged. Shadel Software, iTunes app
- Dehydrated Skin. Skin-Beauty.com
- No Water In Calif. School Cafeterias? NPR.org
- Trying to lose weight? Drink more water. CNN.com