Combining the dissipation method and surface renewal analysis to estimate scalar fluxes from the time traces over rangeland grass near lone (California)

The dissipation method, the method preferred for estimating scalar Surface fluxes over open water has not traditionally been used by agronomists, whereas the surface renewal (SR) theory in conj. unction with the analysis of the scalar time trace offers tremendous advantages for estimating fluxes over agronomic crops. For a steady and horizontally homogeneous flow, it is shown that the dissipation method and SR analysis are closely related. As a consequence, a new dissipation-SR analysis expression for estimating scalar surface fluxes was derived. The new equation requires no calibration, and the scalar time trace measured at a frequency capable of identifying canopy-scale coherent structures (typically 4-10 Hz in agriculture) is the only input required. Sensible and latent heat flux estimates obtained from 10 Hz air temperature and water vapour concentration measurements in the inertial sub-layer (2 m height) over short, homogeneous rangeland grass at a site where similarity does not hold gave similar results to those measured with the eddy covariance (EC) method. For unstable cases, the new equation provided a root mean square error of 57 W m(-2) for the surface energy-balance closure. For stable cases, the performance was difficult to evaluate because the EC fluxes were similar in magnitude to the sensor error. It is concluded that the proposed method can contribute to a better understanding of hydrological processes and water requirements by providing an accurate, less costly, alternative method to indirectly estimate evapotranspiration as the residual of the energy balance equation. Copyright (C) 2009 John Wiley & Sons, Ltd.