Not alone, I think it must be associated to daily growth of biomass. Infact excess of evaporation should saturate the green house and lower the photo-sinthesis  with negative effects on growth.

assuming you use a shade house that is open to the atmosphere, thereby not saturating the air, you can weigh a well watered pot ( soil field capacity ) in the early morning, and weigh that pot again in the evening to determine actual ET.  You can then set up a series of watering treatments, by giving progressively lower water amounts to diff pots, and see the difference. In the end, normalize ET by biomass. However, these would be estimates of ET and not water use efficiency. For the latter, one would also need controls - pots of soil with no plants, to measyre soil evaporation, and subtract t hat from ET estimates of each pot.

No, its required percolation ET, Soil moisture, deep drainage, percolation