Accurately predicting the timing of developmental stages, or phenology, is essential in using simulation models and decision support tools to assist in crop management.  PhenologyMMS (Phenology Modular Modeling System) is a decision support tool that predicts when developmental stages occur under irrigated or rainfed conditions for multiple crops.   A critical component in simulating phenology is determining the amount of thermal time (or growing degree-days, GDD) between developmental events.  PhenologyMMS, as with most simulation models and DST, uses fixed discrete values for the thermal time parameters.  These values can be grouped into a “generic” category representing the mean of all known varieties, or sub-groups such as those viewed as early-, medium-, or late-maturity.  If desired, varietal-specific parameters are also provided.  Measuring phenological traits, particularly under field conditions, inherently has error and several sources of variation.  Uncertainty exists about how the traits vary across environments and among genotypes.  This paper examines phenological data for 24 winter wheat genotypes to assess the variation and uncertainty of phenological parameters for environments differing in water availability.  The analysis was based on a 3-yr field experiment with five levels of water deficits precisely controlled by drip irrigation in northeast Colorado.  Measurements included the date of jointing, flag leaf blade complete, heading, anthesis, and physiological maturity.  The irrigation scheduling included full irrigation, limited irrigation at specific developmental stages (e.g., jointing and anthesis), and dryland treatments.