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I hike in western ghats of Maharashtra India, where temperature in the afternoon varies from 35 degree Celsius to 40 degree Celsius with high humidity (more than 50%).

On my last hike I perspired a lot, even though my ascending speed was very slow during a steep ascend.

How much water should I carry on such hikes, if I know I am not going to get near any source of water till evening?

How should I regulate hydrating myself? I can just keep drinking water until I run out.

Is there a way to control perspiration, so that I lose less water?

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    You cant, and shouldnt, control perspiration, thats an important mechanism for your body and it tells you a lot. When you stop sweating its not a good sign. I love cold and very used to it, in one month backpacking in Cambodia/Thailand I was immediately soaked in sweat within a few minutes out of the hotel and was going on 5 liters a day. Once acclimated things changed noticeably. If your body is used to that climate its perspiration is what it needs to be. In high humidity the whole process of cooling through perspiration is already more difficult on its own. – Erik vanDoren Apr 4 '16 at 13:04
  • @ErikvanDoren I'm curious about this statement: In high humidity the whole process of cooling through perspiration is already more difficult on its own. Could you elaborate? Does high humidity on it's own prevent cooling? I've always thought we sweat more when humid and figured dehydration might be an issue, but not cooling. – Roflo Apr 4 '16 at 20:37
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    @Roflo, you cool when the sweat evaporates off you, it works the same if you put a wet bandana around your neck etc. In a very high humidity environment your sweat has more difficulty in evaporating off your body (and you realize that even when you hang the laundry to dry and it feels like it takes forever). The efficiency of our natural cooling system is already reduced in that situation. So, for our body, a dry hot climate is not the same as a humid hot climate, even if the temperature was the same in both, thats why we have indexes like the "humidex". – Erik vanDoren Apr 5 '16 at 13:39
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The amount of water you require can vary a lot depending on the conditions and your level of exertion and conditioning. Dehydration can affect your physical and mental performance quite quickly so it should be a high priority.

Similarly, if you are hiking between water sources you need to make a careful judgement on how reliable those sources are. Can you be sure that you will find water where you expect to and will you have enough water to reach an alternative source if the intended one fails?

Perspiration is an important way of regulating your body temperature so you want to look at ways to keep cool rather than directly stopping perspiration. Heat injuries are potentially fatal so you need to be especially careful when the ambient temperature feel hot even when resting.

Two litres per day (including the water content of food) is often cited as the minimum required for survival but with heat and exertion this can go up a lot. Certainly two or three times this amount might not be unreasonable in some circumstances, also allowing some margin for safety.

Making sure you have plenty to drink when you do have ready access to water, e.g. in the mornings and evenings. This will help a lot as you will be fully hydrated when you set out and should need less during the day.

While you are walking it is best to drink when you are thirsty and as much as you feel you need, this is easier to achieve if you drink little and often. There isn't much point rationing water during a day as it will give you more benefit in your body than in a water bottle. You might want to save a small amount until you reach your next water source for morale reasons but there is no point setting yourself an arbitrary limit.

Similarly, if water sources are uncertain it is best to err on the side of caution. In the conditions you describe I would carry as much as I could manage, bearing in mind that the weight will go down throughout the day as you drink and sweat.

The best guide to how much you need will be experience. For example go on a walk in similar conditions, staying close to water sources or go for one day and take a full load made up with water and make a note of how much you use.

Ultimately you need to balance the extra effort of carrying the water with the benefits of staying well hydrated in terms of comfort and performance as well as the risk of getting into more serious trouble.

Also consider the temperature when you plan your route. What route will offer the most shade? How long will it take you to reach the next water source and can you comfortably make it in a single day? What if you get delayed or injured or the route is blocked?

It may also be helpful to plan to rest during the hottest part of the day, even if that means starting a bit earlier or finishing a bit later.

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    I hope you dont mean he should just chug as much water he feels like without distributing it through the day, since he knows that what he carries is all the water he has. A bit of discipline when drinking helps to not exaggerate, swishing water in the mouth before swallowing its a way to slow down and consume less water than just emptying one liter in one sitting. – Erik vanDoren Apr 4 '16 at 13:18
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    Also saving a good drink for with any daytime food can give a greater opportunity for our to be absorbed as well as reducing the chance of being thirsty at lunchtime. – Chris H Apr 4 '16 at 14:37

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