A litre over 15 miles isn’t that much for me. Again a lot depends on weather, terrain and whether you’ve been out on the piss the night before.
The reason I say this is because I did a ride around some of hills around Kettlewell on one of the few hot days in July 2011. Those hills are big for my riding. I’d also had six pints and a curry the night before.
At the top of the first climb one of my mates simply said “look at the plight of him” to our other mate. I felt fine, but the sweat was dripping off my nose at about two drops a second and I could feel more running down all around my face and neck. I got through five litres of fruit juice and water in a camelbak (refilled at pub where I also had a pint of lager shandy). I don’t recall going for a slash at all.
The fact of the matter was hot day + steep climbs + big hills + no tree cover + ales and curry night before = torrential sweat for whole ride. Five litres in and I wouldn’t mind betting the same out.
My water balance sometimes seems a bit odd to me too (after a big ride). I’ll do the usual very dark yellow wee first time of asking. Then I’ll rehydrate with water or whatever, but after a while (but seemingly not long enough to fully rehydrate) that will start going straight through me and I’ll do copious wees of very clear (good) stuff.
I dunno really, but pissing a lot of clear urine is infinitely better than getting dehydrated, so I’d always err on the side of drinking too much (within reason).