Determination of turbulent heat fluxes using a large aperture scintillometer over undulating mixed agricultural terrain

• Published in: Agricultural and forest meteorology 2012-12, Vol.166-167, p.221-233
• Main Authors: Evans, J.G., McNeil, D.D., Finch, J.W., Murray, T., Harding, R.J., Ward, H.C., Verhoef, A.
• Format: Article
• Language: English
• Subjects: Apertures; Complex terrain; Eddy covariance; energy balance; Enthalpy; evaporation; Fluxes; Grasses; heat transfer; Large aperture scintillometer; Latent heat flux; pastures; Scintillation counters; Sensible heat flux; soil; soil quality; Stations; Topography; Triticum aestivum; uncertainty; Upscaling; Vegetation; vegetation types; winter wheat
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2012.07.010

► Scintillometry yields sensible heat flux over complex landscape with mixed crops. ► Evaporation is accurately estimated as the residual of the surface energy balance. ► Highly variable crop heat fluxes are areally averaged by scintillometry. Scintillometry is an established technique for determining large areal average sensible heat fluxes. The scintillometer measurement is related to sensible heat flux via Monin–Obukhov similarity theory, which was developed for ideal homogeneous land surfaces. In this study it is shown that judicious application of scintillometry over heterogeneous mixed agriculture on undulating topography yields valid results when compared to eddy covariance (EC). A large aperture scintillometer (LAS) over a 2.4km path was compared with four EC stations measuring sensible (H) and latent (LvE) heat fluxes over different vegetation (cereals and grass) which when aggregated were representative of the LAS source area. The partitioning of available energy into H and LvE varied strongly for different vegetation types, with H varying by a factor of three between senesced winter wheat and grass pasture. The LAS derived H agrees (one-to-one within the experimental uncertainty) with H aggregated from EC with a high coefficient of determination of 0.94. Chronological analysis shows individual fields may have a varying contribution to the areal average sensible heat flux on short (weekly) time scales due to phenological development and changing soil moisture conditions. Using spatially aggregated measurements of net radiation and soil heat flux with H from the LAS, the areal averaged latent heat flux (LvELAS) was calculated as the residual of the surface energy balance. The regression of LvELAS against aggregated LvE from the EC stations has a slope of 0.94, close to ideal, and demonstrates that this is an accurate method for the landscape-scale estimation of evaporation over heterogeneous complex topography.