Etude in situ de l'evapotranspiration reelle d'une culture de ble

The evapotranspiration of a wheat crop was studied over two consecutive years and determined by an automatic system of energy balance (B.E.A.R.N.). Results were analysed to examine the role played by meteorological factors (excluding indirect biological effects), and separately by physiological factors, on the variations in evapotranspiration. Even with the large difference in rainfall, and consequently the water balance, in the two years 1975 and 1976, it clearly appears that the critical evapotranspiration (P′Rn/(P′ + γ) plays dominant role together with the regular physiological development related to the ageing of the crop, as indicated by the mean crop resistance. The good relationship between this mean crop resistance and the critical resistance defined by meteorological factors alone, may be justified by the classical response of stomatal aperture to light and atmospheric humidity. Nevertheless, this relationship possibly suggests the definition of a specific ratio between actual evapotranspiration and the critical value of evapotranspiration, a result which can be very useful in practice.