If you know it's gonna be hot on your tournament days, start hydrating yourself about 3 days ahead of time. For hot days, I use the following schedule:
3 days before: 1/2 gallon of water
2 days before: 1/2 to 3/4 gallon
1 day before: 3/4 to 1 gallon. No alcoholic drinks.
During tournaments, I make sure to get one bottle of water (12 oz) in before my first game, then I take one to the field. One or two bottles during each game, and usually a gatoraid/sports drink inbetween games. It's important that you keep your electrolytes up too, so make sure you have a few sports drinks during the day.
I find that the Safeway or Costco bottled water fits nicely into the chain link fence.
Also, make sure you get something to eat around lunch time. (one park here in the Seattle area has a guy who makes awesome cheesesteak sandwiches - half off for umpires! I can never resist temptation!
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