Unfortunately you can't rely on drinking when thirsty to avoid dehydration and being dehydrated has a significant adverse effect on pilot performance. As we get older thirst or the realisation of being thirsty diminishes so we dehydrate more easily. If one flies several bright sunny warm days in a row it's easy to get into a downward spiral without some dedicated fluid intake. Pee should be very pale.
Finally found the URL I was looking for:
http://www.danlj.org/~danlj/Soaring/...oaringMag.html