I see this argument often in the discussion around meat (beef, mostly) consumption. I also see the counter that most of that water goes straight back into the ground and most of the rest goes back in after it goes through the cow.
The counter seems logical to me, but I am probably missing something?
For example just looking at the prevailing winds in the San Joaquin valley I'd expect a lot of water that evaporates there to end up as rain in the Sierra Nevada mountains. Does that in fact happen?
If so, where does that water go after it becomes rain?
If not, where does it go?
The Earth's water systems as a whole are a huge collection of cyclic paths, with many branches. If you artificially take water from someplace and drop it someplace else you have almost always just borrowed the water and it will eventually end up back where it came from.
"Eventually" could be a very long time though, and if we borrow enough water for long enough it is possible we could permanently change some of the cycles so that some places never recover their water.
I've not been able to find any analysis that looks at this from a whole systems approach and figures out if we are borrowing enough to permanently change things, or how long it takes borrowed water in specific cases to effectively return.
With pastures, none of that applies.
Like GP I am also more interested in specific details of the water cycle. A strictly X uses Y gallons of water, therefore bad, is entirely incomplete to deal with nuances like GP is talking about, where the water ends up. If evaporation and wind up to the Sierra means 90% comes right back down as groundwater, that net usage is an order of magnitude less, and some other area is still benefiting from more water.
Zoom in a little bit, and that single amount of water is unevenly distributed but still part of a cycle that is global in scale.
Zoom in a bit more, and it's extremely unevenly distributed and a part of regional cycles in which it is relocated in the short term but restricted in the long term.
Zoom in a bit more, and it's extremely unevenly distributed and a part of local cycles, where it never leaves a watershed (for example).
Which zoom level(s) you choose depends on your purpose and your agenda.