Maximum transpiration of irrigated apple trees hedgerow orchards, in SW France, were used for testing the performance of the Penman - Monteith model, adapted for an hypostomatous leaf, using simplified micrometeorological and biometrical variables. A 6-year old commercial orchard was used in 1988; in 1989, the trees were 3-year and 5-year old. Representatives trees (6 in 1988; 10 in 1989), regarding the leaf area in the orchards, were chosen to estimate sap flow by the heat balance technique. Summation of hourly values of the maximum transpiration of each tree was used in the comparison with 24-h sap flow data.
Parameterization of foliage net radiation (Rnl) and diffusive vapour conductance for the leaf (gl) and boundary layer (gb) are the main obstacle to the use of the big leaf model. Foliage net radiation was calculated as:
Rnl = (Rn.Rga )/ [Rg (1-r)]
being Rn the net radiation over the rows; Rg incoming global radiation; Rga the radiation absorbed by the foliage; and r its albedo. Rga was estimated with a model of solar radiation interception by the foliage, using Rg and sky radiation, sun position, geometrical characteristics of the hedgerows, foliage porosity and albedo of leaves and soil. Leaf conductance was given by
gl = 10 (1-A.D)/ (1+B/RPAR)
being A and B the sensitivity coefficients of gl for D (vapour pressure deficit) and RPAR (photosynthetic photons flux density). Mean values of RPAR for each tree were estimated as RPAR = 2.32 (Rga. LAI).
ga = (1/58).p^(-0.56).(d/U)^(-0.5)
being p the ratio of leaf area to frontal silhouette area of the tree, d a characteristic leaf dimension, and U wind speed at the foliage level.
Micrometeorological measurements (levels from soil surface, for 1988 and 1989, respectively): a) air temperature, 3.0 and 4.0m, and 2.0 and 5.12m; b) D: at 6.0m and at 2.0 and 5.12m; c) Rg(1 - r) at 6.50m and 5.42m; d) Rn at 9.0m and at 4.73 and 4.62m; e) U at 4.09m and at 3.20m.
In spite of the many approximations for the estimation of Rnl, gl and gb, and of the problems of the representativity of D measured in different ways in the two years, the values estimated by the model in 1988 agreed well with the sap flow values, mainly when Rnl was not divided by the LAI. A lesser degree of agreement was observed in 1989; the degree of agreement dependend on the tree age and period of measurements. The results are discussed in terms of the approximative approach used, the sources of errors in the measurements, including those of the sap flow, and the differences between the two orchards.