It’s a relatively straightforward point: when there is less water to irrigate farmland, there will be less irrigated farmland. For example, OtPR last year:

As groundwater sustainability agencies have to bring irrigated acreage in line with the sustainable yield of the groundwater basin, they will be retiring irrigated lands (Dr. Burt: 1-1.5 million acres; Dr. Lund: up to 2 million acres). I say 3 million acres, because so far everything we’ve predicted for climate change has been an underestimate.)

But this will be really, really hard to do, because the people irrigating land now would prefer to continue to do so, for perfectly understandable reasons (profit, way of life). And there will be hard tradeoffs in deciding how much less water, and which bits of land. Use groundwater now at the expense of future subsidence and loss of resilience? How much of the environment to protect and enhance with water that could otherwise be used to irrigated crops? Those are questions of values, not science, much as people love to scientize them in the political debates.

But however we do it, and however big the reductions are, they are coming. Jay Lund:

• The San Joaquin Valley will have less irrigated agricultural land. The Central Valley south of the Delta is a huge productive agricultural region that currently relies on water from the Delta imports, groundwater overdraft, and reduced outflows from the San Joaquin River. Reductions in those sources will decrease water available to this region by 2-5 million acre feet per year, requiring the fallowing of 500,000-2 million acres of this region’s 5 million irrigated acres. Some of this land will be retired due to salinization and urbanization. Continued shifts to higher value crops, especially orchards, will help maintain agricultural revenues and jobs, as they have during the drought.