I would reiterate that a gallon a day is a minimum. Tweak conditions a bit and you can easily require more water.
A somewhat extreme example from SAR in southern Arizona. My partner and I started off around noon, hiking uphill on a steep south facing slope with a clear sky and full sun. We carried 2 and one/half gallons between us and hiked at a steady, deliberate pace. Our individual packs weighed a bit over forty pounds. By five PM we reached a water source which we knew was good. This was fortunate because we had consumed all our water in reaching this stream. In fact, we were dehydrated and drank deeply, refilling our containers. Continuing on, we encountered our victim, whose main problem aside from three fractures, was dehydration, although by this time we were above 7000 feet elevation in cooler temperatures. We were accustomed to these conditions and in fact I usually ran about four or five miles at noon in the summer in order to maintain acclimation.
All kinds of events can influence your water consumption - shade versus sunshine,talking versus remaining silent, and any kind of trip delay. A full brimmed hat, an early start, and a mid day siesta can all help you decrease your water usage.
Bottom line, I don't recall ever carrying too much water but I have taken too little more than a few times.