[rsj_hn]

Well, yes, water doesn't respect state boundaries so I'm not claiming that California is a closed system, nor am I claiming the majority of water used in agriculture evaporates rather than going into the water table. But just as water evaporates in CA and lands on Colorado, so water evaporates in the pacific and lands on California, and we get ground water and river water back from Colorado. So are you claiming there is some systematic net movement of water out of California and into other states/countries? That would require presenting an entirely different set of facts than has been presented here.

[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.)

[carlob]

> So are you claiming there is some systematic net movement of water out of California and into other states/countries?

Not OP.

The argument made in Cadillac Desert is pretty much the opposite: in order to irrigate California and to give drinking water to its inhabitants, we have damned and diverted many rivers elsewhere with some very large ecological effects.

Another thing to take into account is the salinization of soil as an effect of irrigation: every time water evaporates it leaves behind a salt deposit. This is a process we don't know how to revert and it's considered to be an important historical factor in the rise and fall of civilizations, migration patterns etc.