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Early fault detection of problems in hydroponic production systems necessities looking at rate variables such as water uptake. Crop transpiration rate is directly related to the quality of the crop. By placing a small production system on a scale, it was shown that a measurable change occurred within 20 minutes. It was found that air velocity must be manipulated or measured when trying to model evapotranspiration in greenhouses. Using the simple Penman-Monteith evapotranspiration equation gave a reasonable prediction for the 4-hour evapotranspiration data (R2 = 0.70) using constant resistance terms. Dynamically determined resistance parameters would improve this correlation and are necessary for the 1-hour and 20-minute data. Preliminary results support that using the evapotranspiration rate of a crop, it is possible to detect a sudden copper toxicity in the root zone.