[btilly]

As irrigation becomes more efficient, the majority of water used in agriculture does indeed go to plants and then to evaporation. And while California does get some water back from the Colorado river, most does not. The water coming to California from other sources, such as the Pacific, is not materially affected by the amount of water flowing into the Pacific from California.

Therefore water given to California agriculture in any given year is unlikely to come back to California within that year. So for planning purposes, water given to agriculture should be considered mostly consumed and available to no other use within California.

And as the climate becomes hotter, this becomes even more true. A hotter climate means faster evaporation and an easier time for it to blow over the Rockies. Which is part of the reason why the trend as the climate changes is for land to the west of the Rockies to become ever drier, while land to the east to become wetter. (Most of the water to the east comes from the Gulf of Mexico.)