Sap flow in the trunk of two different‐sized apple trees [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf. cv. Splendour/MM.106 and Braeburn/M.9] was measured using the compensation heat‐pulse method. Supporting measurements were made of the total photosynthetic photon flux (QP) and the total all‐wave radiation (QN) absorbed by each tree. These data were used to test the output from a three‐dimensional model of light interception that approximated the orchard as an array of nonoverlapping, truncated ellipsoids, with each tree having a uniform density of green leaves that were randomly distributed within the canopy volume. Experimental observations, together with model predictions, were used to demonstrate how transpiration responds to changes in the aerial environment. Model testing was rigorous in the sense that the model was compared against complete and independent data collected on the same time scale. Agreement between measured and modeled values was generally very good; all correlation coefficients were large (r2 > 0.95), and the linear relationship between measurements and simulations of QP, QN, and transpiration has a slope that was within 5% of 1:1. A sensitivity analysis revealed that light interception was influenced most by changes in leaf area and leaf optical properties while transpiration was influenced most by changes in leaf area and leaf conductance. On a leaf‐area basis, results from the Braeburn tree (leaf area = 8.65 m2) were very similar to those from the larger Splendour tree (leaf area = 35.5 m2). A smaller, more compact fruit tree is more efficient at intercepting the sun's energy, yet it may require more irrigation water per hectare to sustain productivity.