J.D. Kalma

Measurements are given of radiative and aerodynamic characteristics of a typical mature orange orchard in Israel's coastal plain. Net radiation to the canopy can be calculated adequately for weekly periods and longer from global radiation and three radiative crop characteristics. The role of aerodynamic crop characteristics, e.g., in crop water use studies, has been discussed.

Rainfall, wind run, air temperature and humidity of the air were measured at equivalent heights both in a climate station outside the orchard and above the tree canopy at the experimental orchard site, and the microclimatic modifications associated with the above characteristics are discussed. Rainfall differences were negligible, whereas wind run at the climate station is shown to be significantly under-estimated. Absolute differences in temperature never exceeded 2°C, while relative humidity of the air above the canopy was almost constantly 8% higher. Relationships are given for calculating the relevant crop and climate parameters from standard climatic data.

The distribution of net radiation, wind flow, air temperature and vapour pressure throughout the day has been illustrated graphically.

The canopy structure of the orchard is not homogeneous horizontally. The horizontal variations especially affect the transfer of mass, energy and momentum within the canopy. Attempts to simulate these processes within the stand are mostly based on one-dimensional models, assuming horizontal homogeneity.

In this study one-dimensional models of Cowan (1968) for the absorption of net radiation and the alternation of windflow within the canopy have been tested. Reasons are given for model breakdown and the need for general three-dimensional models is stressed.