Like you, I do a lot of backpacking in desert mountains where water can comprise a significant part of my pack weight. The gear I take with me normally weighs about 11 pounds. In addition to the gear, I take about 24 oz of food per day and from three to four liters of water per day depending on weather and how much I expect to sweat.

I try to plan trips around reliable water sources such as seeps or tinajas but also carry emergency water in case a source has dried up. But, I have started several three day trips carrying as much as ten liters (22 lb) of water. This puts my starting pack weight between 35 and 40 pounds. But, by the last mile back to the car I am only hauling a bit over 11 pounds.

With a gear load of 20-25 pounds I would have started these three day trips with about 45-50 pounds: a load like that sort of takes the spring out of my step. I think the point about going light is that while one is starting at a lower base you still have a load of "consumables" to carry. By going light you are reducing the "dead" weight.
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May I walk in beauty.