The impact of in-canopy wind profile formulations on heat flux estimation in an open orchard using the remote sensing-based two-source model

Abstract

For open orchard and vineyard canopies containing significant fractions of exposed soil (>50%), typical of Mediterranean agricultural regions, the energy balance of the vegetation elements is strongly influenced by heat exchange with the bare soil/substrate. For these agricultural systems a “two-source” approach, where radiation and turbulent exchange between the soil and canopy elements are explicitly modelled, appears to be the only suitable methodology for reliably assessing energy fluxes. In strongly clumped canopies, the effective wind speed attenuation inside and below the canopy layer can highly influence the partitioning of energy fluxes between the soil and vegetation components. To assess the impact of wind attenuation on model flux estimates, an analysis of three different wind extinction formulations is presented, including algorithms from Goudriaan (1977), Massman (1987) and Lalic et al., (2003). The wind extinction formulations are applied to the thermal-based Two-Source Energy Balance (TSEB) model developed by Norman et al. (1995) and modified by Kustas and Norman (1999). High resolution airborne remote sensing images, collected over an agricultural area located in the western part of Sicily (Italy) comprised primarily of vineyards, olive and citrus orchards, are used to derive all the input parameters need to apply the TSEB. The images were acquired from June to October 2008 and include a relatively wide range of meteorological and soil moisture conditions. A preliminary sensitivity analysis of the three wind extinction algorithms highlight the dependence of wind speed just above the soil/substrate to leaf area index and canopy height over the typical canopy properties range of these agricultural area. It is found that differences in wind extinction among the models is most significant under sparse and medium fractional cover conditions (20-60%). The TSEB model heat flux estimates are compared with micrometeorological measurements from a small aperture scintillometer and an eddy covariance tower collected over an olive orchard characterized by moderate fractional vegetation cover (≈ 35%) and relatively tall crop height (≈ 3.5 m). TSEB fluxes for the 7 image acquisition dates generated using both the Massman and Goudriaan wind extinction formulations give close agreement with measured fluxes, while the Lalic et al. equations yield poor results. The Massman wind extinction scheme slightly outperforms that of Goudriaan, but it requires an additional parameter describing the roughness of the underlying vegetative surface. This parameter is not directly obtainable using remote sensing, hence this study suggests that the Goudriaan formulation for landscape applications is most suitable when detailed site-specific information regarding canopy architecture is unavailable.