if, in short, a river in California has a mouth, then the water coming out of it is part of that oft quoted 50%.

(..)

Agriculture is absolutely important to California's welfare, but is it four times as important as everything else combined?

Ok, so even if we reduce the 50% somewhat, it still takes up about twice as much as the water used for farming. Are the various environmental causes twice as important as agriculture? Also, as far as agriculture, it is using water for economically productive purposes, most of the other uses, e.g., lawns, swimming pools, aren't.