A method of measuring the aerodynamic resistance to the transport of water vapour from forest canopies.
Describes a method of calculating the aerodynamic resistance from the ratio of the humidity gradient to the evaporation rate of a forest canopy. The humidity gradient is obtained from the difference between the humidity of unsaturated air at a reference height above the canopy, and the absolute humidity of saturated air at mean leaf temperature. The evaporation rate is obtained from measurements of canopy water balance during intermittent rain (i.e. two periods of rain separated by a period during which some intercepted water is evaporated). The relation of aerodynamic resistance to wind speed was investigated on the basis of measurements in adjacent stands of Pinus nigra var. maritima and Pseudotsuga menziesii (ca. 40 years old) near Farnborough, Hants., UK, in 1967. Differences between the two species were associated with their different canopy structures.