A HEATED LEAF REPLICA TECHNIQUE FOR DETERMINATION OF LEAF BOUNDARY-LAYER CONDUCTANCE IN THE FIELD

This paper presents a robust field method for continuous estimation of boundary layer conductance by measurement of the temperature difference between a replica millet leaf which is heated with an electric current and an identical replica used as a reference. The measurements of boundary layer conductance in a wind tunnel were compared with conductances calculated using equations from an engineering text. The two sets of values are similar at low wind speeds. At wind speeds above 2.5 m s(-1) replica boundary layer conductances were higher than those calculated for a laminar boundary layer. Errors and assumptions of the technique were analysed. Heating current was switched between the replicas to avoid systematic errors. Using this system, leaf boundary layer conductance was measured in the field in Niger for a millet crop behind a windbreak. The relationship between wind speed (u) and boundary layer conductance (g(a)) was analysed for four distances from the windbreak and three periods at the start of the millet cropping season. The relationship g(a) = au(b) gave a series of values of parameters a and b. Values of b lay between 0.5 and 0.8 and were related linearly to the low frequency turbulence of the air flow (given by the standard deviation of the wind speed).