Models of CO2 and water vapour fluxes from a sparse millet crop in the Sahel

Canopy fluxes of water vapour and CO2 from a sparse millet crop are simulat ed on the basis of a leaf scale model designed to predict stomatal conducta nce, leaf temperature, transpiration rate and photosynthetic rate for mille t on a diurnal basis. The modelled leaf fluxes are extrapolated using two d ifferent big-leaf approaches. In the traditional big-leaf approach, all lea ves are exposed to the same microenvironment which is different from the en vironmental conditions above the canopy, whereas in the modified big-leaf a pproach the canopy is regarded as a partly shaded big-leaf. In the sun/shad e big-leaf model, soil reflection, diffuse radiation and separate evaluatio ns 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 rank ed for the identification of the most important parameters controlling phot osynthesis and transpiration. This information is used for identification o f more simple scaling models aimed at predicting daily canopy fluxes. The i nflux of sensible heat to the leaf was found to be an important energy sour ce for transpiration. It was confirmed that daily transpiration can be para meterized by the air humidity gradient using only the leaf area index (LAI) for the evaluation of seasonal changes in bulk stomatal conductance. The p hotosynthetic rate was found to be most sensitive to radiation and leaf tem perature. It is shown that the daily canopy photosynthesis can be estimated on the basis of LAT and midday values (1200 h) of incoming radiation densi ty and leaf temperature. (C) 1999 Elsevier Science B.V. All rights reserved .