This weight is used to calculate how much water the character must intake on a daily basis in order to maintain ideal hydration. If a character loses 1% of their water weight, say, through daily respiration, this would be a total of 1.1 lbs ... which must then be drunk by the character that day to maintain perfect health.
In most environments, this is not difficult; water can be obtained from wells in urban and most rural areas, or from streams in places that are well-watered. However, where water is scarce, hydration can become a problem.
Perspiration depends on the actual temperature. To understand the table below, see Temperature Grades.
The hour of perspiration loss should be counted if the character is moving about, travelling, working, or resting directly under the sun. If the character is resting in the shade, the total body water lost should be reduced by 60% (so that in a baking ambient temperature the water loss would be 2% per hour).
The water loss column above indicates the present time circumstance of the character ~ which is to say, if the character has lost 3% of the water weight, without replenishing it in this exact moment, then the effects associated with a 3% dehydration will be in place (-1 ability stats, -1 attack die, etcetera) until the character actually drinks enough water to counteract the effect (and a 5 to 10 minute effect rule might be considered).
The character's effective temperature indicates how hot the character feels ~ it does not show the actual temperature the character's body is subject to and therefore there should be no recursive effect between the character's perceived comfort and the actual temperature. The dehydrated character at -5% during a balmy day may perceive that the day seems sweltering, but the character will continue to perspire at 0.8% water loss per hour.
The ability stats symptom is applied to all stats indiscriminantly. If, due to dehydration, any of the character's ability stats drop to zero or less, the effects can be severe. A zero or less strength, constitution or dexterity will indicate that the character has laid down and died. A zero or less intelligence or wisdom will indicate that the character has gone mad. They will wander away, gibbering, and most likely make errors in judgement that will amount to death: falling off a cliff, drinking poisonous standing water, eating sand and so on. A zero or less charisma will cause the character to commit suicide to end their misery.
Thus it can be seen that any character with any ability stat less than 17 will not survive a water loss of more than 25%, even for five minutes. Most characters will die at a 20% loss. Many will not survive more than a 15% loss. With this system, a bowel movement can kill you (though the body may not have sufficient means to defecate, apply the water loss just the same).
Attack rolls and saves work normally.
See Water Discipline