Thanks for the reply!  I'm glad to hear that you are using & like the camelbak 
system.  My two concerns are the straps slipping down (almost as bad 
as loosing a bra strap) and feeling too hot under the pack.  I saw a 
3/4 size one at a local bike shop (don't remember the # of ounces) 
that I thought I may try.  
Unlike in the desert, we do have water for the horses along most trails, but 
I wouldn't drink it myself.  (?Double standard?)  Our heat + humidity in VA 
means DRINK!  It's to cool us as much as to rehyrdrate us.  I often 
add ice cubes to my first bottle, or bring frozen juice packs in a 
cantle bag  for a quick refreshment at a stop :)
Like many horses, my gelding will drink from a soda can, sports 
bottle, or muddy hoof print if it is all that's available.  You said 
you carry water for the horse.  Do you bring a collapsible bucket, or 
does he drink from the containers?  Do you ever use a spritz bottle & 
cooling cloth on his poll?
Linda Flemmer
Blue Wolf Equestrian Supplies/ Blue Wolf Ranch
Chesapeake, VA
   "In case of emergency - Fur side up, steel side down!"