Priestley and Taylor provided a practical formulation of the partitioning of net radiation between heat flux and evaporation contained within a parameter-alpha. Their model (PTM) needs verification under a range of environmental conditions. Micrometeorological data sets collected over the Amazon forest at the Ducke Reserve site (2-degrees 57'S; 59-degrees 57'W) gave an opportunity to evaluate-alpha. Evidence presented here and by others shows that there is pronounced diurnal variation in alpha, with minimum values around midday and maximum values in the morning and evening hours. During unstable and stable conditions in the daylight hours, the Bowen ratio (B) varied from 0.10 to 0.57 and -0.71 to -0.08, respectively, whereas alpha varied from 0.67 to 1.16 and 1.28 to 3.12, respectively. A mean value of alpha = 1.16 +/- 0.56 was obtained from daytime hourly values for two days. The daily data sets from three expeditions gave a mean of alpha = 1.03 +/- 0.13. This work confirms that alpha is a function of atmospheric stability over the Amazon forest. Thus the PTM should be applied with caution over time-intervals of one day or less because of the sensitivity to variation in alpha. The calculated values of alpha are in general agreement with those reported in literature.
