I do think that marketizing water will inevitably fallow land, because us farmers can't pay the way the cities can. That might be okay if there was not an externality involved - i.e., re-orienting water from ag to urban does not make things more "efficient"; what it does is send ag offshore to places where it can be done more cheaply, which also (usually) means without the framework of environmental protections that California-grown crops live with. In other words, we run the risk of standing up monoculture in third-world nations where there ain't no ESA, there ain't no water quality laws, there ain't no instream flows to worry about.This perspective combines two opinions: Farmers cannot afford to pay city prices, and (because of this), they will be displaced in the market by cheaper, foreign, less-"earthy" competition.
Let's take these opinions in order. Will "marketizing" water fallow land? Probably yes -- the worst (marginal) land will be fallowed/turned into conservation areas or parking lots when it makes more sense to sell water to cities than grow low-value crops. If farmers have water rights (a likely scenario) and are selling them, this is no bad thing for either side.
Is re-orienting water from ag to urban "less efficient"? No, not if you equate market outcomes (willingness to pay) with efficiency on the domestic side or if food can be grown more cheaply in foreign countries.
Is foreign ag cheaper because of lax environmental safeguards? (Not according to this.) What if locals are willing to bear the environmental costs of growing food for gringos? That will not keep US farmers from claiming that it's wrong to buy goods from producers who are willing to employ kids/pollute land and water/work in "unsafe" conditions in exchange for the economic benefits they receive. Although this moral argument is appealing (as well as self-serving), it is also patronizing. After all, who are we (who am I?) to say that such and such a job is "undignified, unsafe, un-whatever"? I, for one, cannot understand why people sit commute for hours in their cars, but I do not have the right to condemn their judgment or humanity in pursuing their choice.
Bottom Line: Environmental considerations are a matter for those who are suffering fromenvironemntal damages (thus, global warming is a global problem). They should not be used as a means of preventing willing sellers from contracting with willing buyers. After all, the alternative ("picaresque poverty") is hardly a laudable outcome.