Assessing Inter-Sensor Variability and Sensible Heat Flux Derivation Accuracy for a Large Aperture Scintillometer

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.

Measuring and modelling evapotranspiration in a South African grassland: Comparison of two improved Penman-Monteith formulations

“…e Bowen ratio (β) was also used to reject positive uxes when β > 3, since H would be far larger than LE and the latter insigni cant. Any data points where the β was between −0.05 and 0 were removed, due to instability of the solution for this extreme value range (Rambikur and Chávez, 2014). Data during precipitation events were also excluded.…”

“…e study applied well-established and rigorous data quality control protocols in order to accurately derive uxes (Campbell Scienti c, 2005;Bouin et al, 2012;Rambikur and Chávez, 2014) and this helped to reduce uncertainties in the calculated ET. e good performance of the PML model on the Truro farm, where only validation was done using calibrated values of α and g sx from the Somerton site, suggests that the obtained values could be accurate given that environmental conditions also di ered during the calibration and validation periods.…”

“…e availability of micrometeorological equipment in data-scarce areas is useful for locally calibrating and validating models to enable widespread applications with higher con dence levels. However, the application of micrometeorological methods is fraught with many challenges owing to the underlying theory and inherent errors (Savage, 2009;Rambikur and Chávez, 2014).…”

Remote Sensing and Aerodynamic Temperature-Based Energy Balance Models to Estimate Crop Evapotranspiration Rates

“…with sensible heat fluxes measured with two sets of Large Aperture Scintillometers (LAS) that provided T o values (from inverted measured H values) Rambikur and Chávez (2014). presented a LAS evaluation study in which it was shown that sensible heat fluxes derived from LAS systems were comparable to H values obtained with EC systems.…”