Simply put you can live without food for many days if not a week or more befoere it begins to significantly degrade your survival ability, but 48 hours (2 days) without water and you are probably toes up. In a controlled environment, you should drink about half of your body weight in ounces of water, per day. Physical activity will exacerbate your lack of water problem in a survival situation, so you should conserve or ration your sweat by accomplishing your Survival tasks in periods of low heat and out of direct sun light, which means in the morning or evening hours and at night. Eat less food. The body requires water to digest and absorb food, so eat less of it if your water supply is limited.
Thirst is not a reliable symptom of dehydration, the other symptoms below are more reliable for indicators of a heat injury cause by dehydration.
Heat Cramps. Caused by the loss of electrolytes through sweat, a good indication of initial dehydration is muscle cramps, may be followed by a headache. Get out of the sun and hydrate now. Minimal activity should be allowed until sufficient water is taken in and the cramps go away.
Heat Exhaustion. This is the next level of severity of dehydration. Additional symptoms include headaches, dizziness, pale skin clammy to the touch. Irritability may be present, but if you are in a survival situation, who wouldn’t be irritated? Drink water! Evaporative cooling by pouring water onto the person and letting the air (not the Sun) evaporate would be a good idea.
Heat Stroke.. Again headaches, dizziness probably accompanied by nausea, vomiting, fast pulse and mental confusion are key symptoms for heat stroke. If you are at this point you probably won’t recognize it in yourself and if you are by yourself you will probably die. Immediate hydration by IV fluid and evaporative cooling are crucial and immediately necessary. If the patient is conscious and able, allow him to drink water in small amount frequently, maybe 3-5 ounces every 5 minutes or so.
If you are in a Survival (Urban or Rural) Situation with other people, train each other to watch for these symptoms and use a buddy team concept to monitor each other.