Water Use Efficiency (WUE) in SPAC Analytics is defined as the ratio between the accumulated plant biomass gain and accumulated water lost by the plant through transpiration over a selected period of time.
Equation: WUE = Accumulate (Plant Growth) / Accumulate (Daily transpiration)
The WUE is the slope of the linear regression from start to end of the accumulation time.
In the following example, the WUE was calculated over a period of 5 days, in which plant growth and daily transpiration had positive values each day:
To get valid WUE values, several consecutive days of proper night irrigation are needed, in which the "Daily transpiration" and "Plant growth" parameters are positive. To achieve that, at least 2-3 pulses of irrigation per night are necessary while making sure that drainage occurs between pulses.
Weight at dawn (after irrigations are completed and before sunrise) should not decrease between two consecutive days. To avoid oxygen deficiency and possible root damage, the medium should drain well.
For additional information about WUE and plant weight calculation, please watch this video from 39:48 up to 1:08:00.