Downwind evolution of scalar fluxes and surface resistance under conditions of local advection. Part II: measurements over barley

Abstract

It has been suggested in Part I of this paper that, after a dryland-to-irrigated transition, the surface sensible and latent heat fluxes may exhibit little change with downstream distance from the leading edge. It was argued that such step changes in surface fluxes could be caused by a feedback mechanism between surface resistance and changing saturation deficit. A simple model was derived for describing the streamwise change in surface resistance leading to such a step change in flux. The aim of the present paper is to test these assumptions against experimental data acquired specifically for this purpose. Measurements were performed over a transition between dryland and irrigated barley. Eddy covariance determinations of sensible heat flux at a height of 1 m showed little variation downwind of the transition and, once corrected from the footprint effect, exhibited patterns roughly compatible with a step change in surface flux. Transects of radiative temperature, and air temperature and humidity above the crop, also showed downwind changes in good agreement with the expected variation. The most convincing results were obtained from measurements of stomatal and canopy resistance, whose downwind variation was close to that required by a step change in evapotranspiration.