Results of two-year experiments on leaf area and water flux relations in apple trees of two orchards in southwest France are discussed. Tree leaf area was estimated from the number of leaves in the first year and from shoot girth (5 cm from the base of main or secondary branches) in the second year. The daily water flow rate of trees was estimated by delivering a known and constant rate of heat to a trunk segment then computing the heat balance at 20 min intervals. The determination of conductive fluxes, the heating of the sap and the heat storage in the segment permitted the estimation of the convective heat and the associated volumetric sap flow. A linear relationship between tree leaf area (in the range of 4 to 21 m2) and daily water flow was observed, which was affected by evaporative demand and soil water availability.