THE ESTIMATION OF TRANSPIRATION FROM SPARSE DRYLAND MILLET USING STOMATAL CONDUCTANCE AND VEGETATION AREA INDEXES

The stomatal conductances of the leaves and panicles of a sparse dryland millet crop grown at ICRISAT Sahelian Center, Sadoré, in southwest Niger were measured using a diffusion porometer and an infrared gas analyser, respectively. Leaf conductances were found to be high, up to 12 mm s-1 or 480 mmol m-2 s-1, and varied according to the leaf surface, age and position in the canopy. These data were combined with measurements of leaf area index to calculate canopy conductance. Because of the low leaf area (maximum 1.3), canopy conductances were low and varied both diurnally and seasonally. Transpiration was calculated using the above canopy conductance values using the Penman-Monteith and Shuttleworth-Wallace models. In comparison to the Shuttleworth-Wallace model, it was found that the Penman-Monteith equation underestimated transpiration when the soil was dry and overestimated it when the soil was wet. These differences in transpiration arise because of the modification of the in-canopy vapour pressure deficit caused by heat and water vapour fluxes from the soil, a mechanism which is only present in the Shuttleworth-Wallace model. © 1990.