A model was developed that related the first flower phase dates of Syringa vulgaris L. (purple common lilac) to hourly values of temperature and global solar radiation based on data from 1956-1980 at ten locations in western USA. The model takes into account the interaction of effective temperature (base 5-degrees-C) and solar radiation raised to the 0.27 power in the form of Solar Thermal Units (STU). The chill unit requirement of the lilac was determined by the Utah method to be 1049. The relation, which was statistically highly significant, indicated that a larger number of STU were required for flowering at higher elevations. This suggests that under similar conditions of day-time ambient temperature and solar radiation both plant development and water use efficiency at higher elevations are much reduced relative to that at lower elevations. The implication is that irrigation agriculture requires more water to mature a crop at higher elevations and that thermal and solar thermal phenological model requirements for estimating crop maturation should be adjusted to larger values at higher elevations. This also implies that designated water use, in supporting water right claims, in production agriculture should be considerably greater at higher elevations. Factors associated with elevation that may be accountable are discussed.
