Abstract
The accuracy in determining sensible heat flux (H) of three Kipp and Zonen large aperture scintillometers (LAS) was evaluated with reference to an eddy covariance (EC) system over relatively flat and uniform grassland near Timpas (CO, USA). Other tests have revealed inherent variability between Kipp and Zonen LAS units and bias to overestimate H. Average H fluxes were compared between LAS units and between LAS and EC. Despite good correlation, inter-LAS biases in H were found between 6% and 13% in terms of the linear regression slope. Physical misalignment was observed to result in increased scatter and bias between H solutions of a well-aligned and poorly-aligned LAS unit. Comparison of LAS and EC H showed little bias for one LAS unit, while the other two units overestimated EC H by more than 10%. A detector alignment issue may have caused the inter-LAS variability, supported by the observation in this study of differing power requirements between LAS units. It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H. In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit. However, the results obtained do not show clear evidence of inherent bias for the Kipp and Zonen LAS to overestimate H as found in other studies.
Highlights
It is known that estimation of evaporation and transpiration on varying spatial and temporal scales is important, whether on a field or farm scale for irrigation water management or a watershed scale for basin water management
The Large Aperture Scintillometer (LAS) units at the study site were set up within a period of a few hours and, following manufacturer recommendations, the units were aligned and the power was set at the transmitter to achieve a signal strength at the receiver of 50%
During a site visit on 21 July (2011), the alignment was restored in LAS-2 and LAS-3, a storm the same afternoon appears to have caused the subsequent misalignment observed with LAS-2
Summary
It is known that estimation of evaporation and transpiration (evapotranspiration, ET) on varying spatial and temporal scales is important, whether on a field or farm scale for irrigation water management or a watershed scale for basin water management. The eddy covariance (EC) method is a direct method of measuring latent (evaporative) and sensible heat fluxes using high frequency measurements of water vapor concentration, air temperature, and vertical wind speed. Despite the common issues with failing to close the surface energy balance [1,2], use of the EC method is prevalent likely because of the advantage of continuous, direct measurement of turbulent sensible (H) and latent (λE ) heat fluxes. A LAS yields a spatial average of H (W·m−2) over path lengths up to 4.5 km, and relies on ancillary measurement of net radiation (Rn, W·m−2) and ground or soil heat flux (G, W·m−2) to solve the land surface energy balance for λE (W·m−2) as a residual
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