[Actual evapotranspiration of a maize crop (Zea mays) as function of the canopy conductance]
1993
Steduto, P. (Centre International de Hautes Etudes Agronomiques Mediterraneennes, Bari (Italy). Institut Agronomique Mediterraneen)
The response analysis of crop evapotranspiration (ETE)to variation of canopy resistance (rc), and the relative influence of such resistance on the evapotranspiration flux, has particular relevance in modelling the water balance of cropping and agroenvironmental systems. To contribute in gathering experimental information on such sort of analysis, the results concerning the relationship between ETE and rc of a field-grown corn are reported. Three hectars of such crop were regularly irrigated until approaching the tasseling period, after which irrigation was witheld allowing the crop to continue his cycle relaying only on the soil-stored water, and to undergo a continuous increasing water stress. During the development of this stress, the soil moisture content, the leaf area index (LAI), the water potential of the apical sun-exposed leaves, the crop ETE (by means of the Bowen ratio method), the aerodynamic resistance (ra) using the log-law of the wind-velocity profil, and the canopy resistance (rc) derived by the Penman-Monteith equation were determined. The efficiency of the gas-exchange between crop and atmosphere was estimated by the coupling factor "Omega" (McNaughton and Jarvis, 1983). Although, rc reached the minimum value of 25 s/m, the threshold value beyond which rc started to have influence on the crop evapotranspiration, normalized relatively to the reference evapotranspiration ETo (ETE/ETo), was of almost 50 s/m. Furthermore, a 5 fold increase of such rc values was needed to observe an ETE/ETo reduction of 50 per cent. Though the low aerodynamic resistance, such reduced influence of rc over the water vapor flux points out a low efficiency of the gas-exchange between crop and atmosphere. The obtained results then would support the indications of McNaughton and Jarvis (1991) on the modelling simplification of the evapotranspiration according to the spatial scale considered.
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