"Dave Liquorice" wrote :

I believe by the time you actually feel thirsty enough to do something
about it you are already well on the way to dehydration. Noting how often
you are going for a wee is useful as well, if you haven't been for, say 4
hours, and you don't feel the need or if you try there isn't very much
then it's a sign that you are getting dehydrated, thirsty or not.

I remember being amazed at how much water I was drinking when in Ilat one
summer. It was very dry and the low 40's but 6 litres (10 pints) of water
was going in and only the normal amount of wee coming out...

Some of us seem to be, to be blunt, a lot sweatier than others.
I've always perspired heavily, even when I was comparatively slim
and fit. Another one, slightly more famous(!), is Marcus Trescothick -
sometimes when he removes his helmet on a hot day you see the
perspiration literally pour out.

Philip