[ChuckMcM]

You could grow algae in clear tubes across 15 square miles of the Mohave desert and not consume a single acre of arable farmland.

[dredmorbius]

Where are you going to get the water from?

If it's seawater, how are you going to return it to the ocean. You can't dump salt water on land -- it poisons the soil. Even freshwater evaporating in dry climates leads to salinization.

What's that going to cost you in energy inputs (pumping costs)?

Though the thought occurs to me that the Salton Sea in southern California might make a possibly suitable grow region.

[ChuckMcM]

If we've posited a desert based system, I'm going to guess it is closed loop with respect to water (which is to say the water never leaves the system as anything except perhaps evaporation during post processing.) So the initial water budget probably comes from the aquifer or is imported. Once the system is running an interesting question is how much water would it lose over time.

And while that is an interesting question, it doesn't change the situation that no farmland was harmed in the process. :-)

[dredmorbius]

For a system comprising tens to hundreds of millions of acres, incidental and transpiration water losses are going to be nontrivial.

Hermetically sealing such a system would be ... prohibitively expensive.

You'll be pumping water in from somewhere.