Models of CO 2 and water vapour fluxes from a sparse millet crop in the Sahel

Abstract

Canopy fluxes of water vapour and CO 2 from a sparse millet crop are simulated on the basis of a leaf scale model designed to predict stomatal conductance, leaf temperature, transpiration rate and photosynthetic rate for millet on a diurnal basis. The modelled leaf fluxes are extrapolated using two different big-leaf approaches. In the traditional big-leaf approach, all leaves are exposed to the same microenvironment which is different from the environmental conditions above the canopy, whereas in the modified big-leaf approach the canopy is regarded as a partly shaded big-leaf. In the sun/shade big-leaf model, soil reflection, diffuse radiation and separate evaluations of the radiation load on sunlit and shaded leaf surfaces are taken into account. Due to the low fraction of shaded leaves in the sparse canopy, the two types of big-leaf models predict both canopy fluxes equally well. The sensitivity of the modelled fluxes to the various input parameters was ranked for the identification of the most important parameters controlling photosynthesis and transpiration. This information is used for identification of more simple scaling models aimed at predicting daily canopy fluxes. The influx of sensible heat to the leaf was found to be an important energy source for transpiration. It was confirmed that daily transpiration can be parameterized by the air humidity gradient using only the leaf area index (LAI) for the evaluation of seasonal changes in bulk stomatal conductance. The photosynthetic rate was found to be most sensitive to radiation and leaf temperature. It is shown that the daily canopy photosynthesis can be estimated on the basis of LAI and midday values (1200 h) of incoming radiation density and leaf temperature.