The heat load in Arizona deserts can e unbelievable, especiallu if you are not acclimatized.

Doing a SAR mission in Tucson on a dy when the temp "only"reached 90F, may companion and I ran through 3 1/2 gallons of water, starting about noon, hiking swiftly uphill over 6 miles on a south facing slope, finally reaching a known dependable water source. We were dehydrated and drank deeply. That was an extreme situation, but you get the drift.....