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A field drought stress monitoring system was developed for New Guinea Impatiens production in greenhouses. The system used an empirically determined crop water stress index (CWSI) for the assessment of the plants' water status based on a linear Lower Limit function. Instead of the linear function, the Lower Limit of the empirical CWSI function was found to be a group of quadratic functions of temperature history as well as of two other commonly accepted parameters: temperature difference between ambient air and plant canopy, and vapor pressure deficit. The quadratic Lower Limit estimation is a better reflection of the evaporation potential that plants are expiring than the linear estimation. The linear Lower Limit function will cause more pump reaction theoretically.