RADIATION CLIMATE OF AN IRRIGATED ORANGE PLANTATION

Regular measurements of incoming and reflected global radiation and net radiation were taken both above and below the canopy of an irrigated orange plantation during one year. The results have been analyzed to show the diurnal and seasonal changes in the main components of the radiation balance. The mean albedo of the plantation throughout the year was 0·16, ranging from a minimum of 0·12 in late spring to a maximum value of 0·21 in late autumn. The reflectivity of a single layer of orange leaves was more than twice that of the entire canopy, indicating the importance of internal absorption of reflected radiation within the canopy. Over the year, 0·28 of the incident global radiation was radiated to space in net long-wave flux, slightly more during the day than the night. The seasonal variation in net long-wave loss at sunrise and sunset was relatively small and showed no clear seasonal trend. Summertime profiles of net radiation from above the canopy to the soil surface beneath the canopy, together with canopy leaf area measurements, were used to compute the extinction coefficient for the plantation. Measurements of the net and short-wave radiation fluxes at ground level, made at different times of the year, were analyzed to show the extent of seasonal changes in the transmission coefficient for both long- and short-wave radiation. Measurements of the soil heat flux show this to be a minor component of the energy balance; seasonal and diurnal variations in this flux are discussed. The annual sum of the radiation balance available to the canopy amounted to 56 per cent of the incident global radiation. The changing seasonal partition of radiation energy between latent and convective heat fluxes has been calculated for both above- and below-canopy conditions and the results show that the leaf surfaces offer a greater resistance to water-vapor diffusion than the bare soil surface beneath the leaf canopy. © 1969.