Sweat rate variables

From what I've read:

Clothing matters.  Acclimatization matters.  Level of effort matters (specifically, metabolic heat production).  Hydration level matters.  (Menstrual cycle matters -- but I'm guessing from your avatar that that's not an issue...)

RE the menstrual thing, its not a matter for me, but is for the coat   Explain more on metabolic heat production as this might be part of the issue.  I have noticed on hotter days lately that the HR creep isnt as bad either.  If sweat volume is a byproduct of effort or heat transference (which makes sense in that if you slow down vs sprinting, you arent generally sweating as much) that would make sense on a pacing standpoint that reduced effort for same pace vs two years ago would lead to lower sweat levels.  I guess that might be a 'well duh' but since I have had big issues with hydration related meltdowns (post race weight as evidence), just wondering if sweat rate is therefore somewhat 'trainable' at similar paces.  Probably not the right way to put it but trying to compare apples to apples.

I'd venture to say acclimation as well.

BUT ..... usually the more fit you are, the more you sweat as your body becomes much more efficient at keeping itself cool (since sweating is one of the few ways the body can do it).

Your efficiency for cooling your body could have also improved. There are so many factors like everyone else has said, but I wouldn't worry too much about it as you are improving. You can always weigh yourself pre and post-workout to determine how much your net loss/gain of fluids is. Of course it would be approximate, but if you conduct the assessment over several workouts, you will get a better idea of what you need during different conditions.

Just my two cents...

Mike

Bingo.

I gather (I'm not an expert -- just an interested consumer of this sort of stuff) that there is actually a bit of a debate amongst experts about what is directly correlated with sweat rate -- is it actual work produced (e.g., the watts that you are putting out), or is it heat production in the body (which is the result of metabolic activity).  The latter makes more sense to me, but there are studies out there suggesting the former (but I think they have holes in them -- again, though, I not an expert!).  I can't seem to find anything right now except this abstract, but I know there's other stuff out there.

Anyway, if it is heat production, then your scenario makes sense -- you are producing roughly the same number of watts (well, maybe a bit less, as you now weigh less), but you are doing so more efficiently, hence your body does not produce quite as much heat while you are doing it.