A comparison of methods for estimating aerodynamic resistance of heather (calluna vulgaris (L.) hull) in the field

This paper presents measurements of r_av the aerodynamic resistance to water transfer for heather moorland in North Yorkshire, Gt. Britain. Values of r_av obtained by measuring the evaporation from a weighing lysimeter after rain vary inversely with wind-speed and are consistent with a simple theoretical description in which the zero plane displacement and roughness length are respectively 0.63 and 0.13 of the vegetation height. Estimates of r_av obtained by applying a theoretical 'bluff body' correction to momentum transfer resistance, measured directly using the eddy correlation technique, have a similar wind-speed dependence but are significantly higher. The difference is shown to have little effect in models of long term evaporation from heather, so that a simple expression of the form r_av = 140/u may be adequate in models which predict the evaporative response to a change in land use in heather catchments.