Sensible heat flux over a wheat canopy: optical scintillometer measurements and surface renewal analysis estimations

Two methods, namely the optical scintillation measurements using a displace d beam surface layer scintillometer and the surface renewal analysis are em ployed to derive turbulent sensible heat flux over a wheat canopy. The scin tillometer provides refractive index structure parameter, which in turn is used to obtain the temperature structure parameter and turbulent fluxes. Te st comparisons of the scintillometer flux measurements with the eddy correl ation measurements conducted above a bare agricultural field showed good ag reement. Over the wheat canopy, the temperature structure parameter obtaine d from the optical measurements agreed well with those determined from the temperature spectral density, derived from the high frequency temperature m easurements. The 1 min and 20 min resolution sensible heat flux and frictio n velocity estimations correlate well with the net radiation and wind speed variations, respectively. Surface renewal analysis is based on the ramp li ke structures exhibited in the traces of high frequency temperature data an d the mean amplitude and duration of these ramps are used to calculate the sensible heat flux. The estimated sensible heat Aux, averaged over 20 min d uration, are found to be in good correspondence with the scintillometer mea sured values, with the scintillometer values showing smoother variations. T he short term sensible heat flux changes caused by the cloud shadows are di scussed using the 1 min temporal resolution flux data provided by the scint illometer. (C) 1999 Elsevier Science B.V. All rights reserved.