Pear trees are a commonly grown crop in arid climates and, while temperature is the main factor affecting phenological development, other environmental (e.g., rainfall) and grower management factors (e.g., irrigation) are also important for determining development rates. Knowing the phenological development rates of pear trees is useful for growers to manage pests and to determine crop coefficients for irrigation scheduling. In general, degree-day (i.e., heat unit) models provide a simple temperature-based method to estimate phenological development. However, with the same air temperature, plants that experience water stress are known to have higher temperatures than well-watered plants, and, hence, they develop more rapidly. Phenological and climatic data were collected near Oristano (Sardinia), Italy and were analysed to identify the best threshold temperatures and the cumulative degree-day requirements to reach bud break, bloom, fruit setting, and ripening dates of pear trees. Although degree-day based phenological predictions were generally better than using the mean calendar date, the advantage from using degree-days alone was small. Sometimes the phenological stages could not be explained by heat unit accumulation. During early growth (i.e., before the irrigation season), development was slowed somewhat by rainfall although the heat accumulation was similar for dry and wet years. For example, the air temperature trends were similar, but bud break came 14-28 days earlier in drier years. Since the pear trees were dormant during this period the earlier bud break cannot be explained in terms of water stress. More likely differences are due to the combined effects of the rainfall and temperature on growth regulators. Later in the season, development was also faster in drier years, and the difference is likely due to water stress effects on plant temperature and growth. The results from this experiment show that considering only temperature is insufficient to explain phenological development of pear trees. In this paper, the methodology used to determine threshold temperature, calculate degree-days, and account for rainfall and management factor effects on pear tree phenological development will be discussed.