Hey all,
Well, I can't vouch for all the parks, but I know about some of the attractions in MK. Well, the water for all the park is collected from Reedy Creek which runs along the back of the park and is then treated and pumped into 20K lagoon over that waterfall in the far left hand side of the lagoon. Then it follows a set of pipes running through the utilidors to the waterfall right next to cosmic rays. Mind you this is still the same water from 20K. Ever wonder why that waterfall seems to appear from nowhere? Well there you go. It travels along around the castle and into adventure land where it ends up in Swiss family Robinson Treehouse lagoon and then is sent into Jungle cruise. In the jungle cruise, it is cycled through a pumping station near the falls on that ride (the backside of water one). There they add that really musty color we see when we ride the ride. From there, that same water that was in jungle cruise, swiss family robinson, castle, and 20K, is sent to frontierland through and undergroud piping system again and into tom sawyer lagoon. Lastly, the water exits that lagoon by a dry dock on the far back side of the lagoon, is filtered, and sent into the water pageant canal and into seven seas lagoon. This is all so we could enjoy the beauty of water.
Pirates gets their water from a small pipe system that runs directly from the castle and into it. Once it is full though, that pipe is shut off and it has it's own resevoir. Splash mountain is completely seperate from everything else. There is a 470,000 gallan resevoir on the backside of the ride that holds the entire amount of water that the ride needs to operate. Believe me when i say this thing is huge. There they treat the water, get all the debris from the ride out of it (hats, glasses, jewelry, etc) and send it back in the ride. You wouldn't want the things that come out of that resevoir though. The chemicals they use basically ruin anything, and i do mean anything, that goes in there, but are harmless to us.
Hope that helps out a bit.
Thanks,
Brian