Theoretically, the only water losses from a food production system are evaporation, run-off and water contained in material which is removed (for consumption or composting). Run-off is eliminated in hydroponics, and water in material eaten goes to good use. Evapotranspiration is thus the dominating factor for efficiency. There are ways to reduce it [MBR, Ch. 29]. For example, using a greenhouse traps the water vapor in an enclosed space. Still, we must remember that "sweating" is an important method of cooling for plants. If the air becomes saturated with water, the plants will not be able to cool themselves, as well as being more vulnerable to fungal diseases. We could dehumidify the air without losing water by passing the air from the greenhouse through a condenser (perhaps using seawater for cooling), and capturing the resulting water. Essentially, this is treating a greenhouse like a solar still. We see a pretty good chance that this technique will be desirable.