Here's what I did:
First, I weighed myself butt nekkid before going for a run. The scale read 75.84 kilograms (167.2 lbs). Then, I ran for one hour at my 10k race pace (high Zone 3 heart rate) and I took a .59 liter (20 oz.) bottle of pure water along. While on the run I downed half of the bottle at 15 minutes and the second half at 45 minutes.
Weather.com said the temperature was 27.22 c (81 f) and the humidity was 61%.
Finally, after the run I undressed and dried myself, including my hair, and weighed in again. This time the scale read 74.93 kilograms (165.4 lbs).
Soooooo, how much water did I leave on the street?
(a) Ending weight = 74.93 kilograms
(b) Starting weight = 75.84 kilograms
(c) Water drank = .59 liters (.59 kilograms)
(d) Urination = 0.00 liters ... Ummm, yeah you gotta measure that too. It's probably easier if you hold it in until after the run and your second weighing... I'm just sayin'.
(e) Time in hours = 1.0
(a - b - c + d) / e = sweat rate
(74.93 - 75.84 - .59 + 0.00) / 1 = -1.50 kilograms (1.5 liters) per one hour.
This means I should drink 1.5 liters (50.72 oz.) per hour under similar conditions.
50.72 ounces per hour? What?? That's over 2 water bottles per hour! Jeeze, what am I, some kind of water buffalo? I have never drank that much during a race or during training. Perhaps here's a piece of the puzzle as why I tend to melt down, like here, in hot conditions, eh?
I'll be doing this experiment again at higher and lower heart rate zones and for both shorter and longer times to see how consistent the data is produced.
So, 'BoomerNation, go ahead and try this little experiment yourself.
REMEMBER: TAKE WATER ALONG WITH YOU.
REMEMBER: TAKE ELECTROLYTES WHILE EXERCISING.
Let me know your results.