Evaluation of the Kipp and Zonen large aperture scintillometer for estimation of sensible heat flux over irrigated and non-irrigated fields in southeastern Colorado
Date
2012
Authors
Rambikur, Evan H., author
Chávez, José L., advisor
Andales, Allan A., committee member
Ham, Jay M., committee member
Gates, Timothy K., committee member
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Abstract
The aim of this work was to assess the performance of the Kipp and Zonen Large Aperture Scintillometer (LAS; Delft, Netherlands) to predict surface sensible heat flux (H). The LAS was introduced approximately 30 years ago and has been marketed as an indirect tool for the estimation of vegetation evapotranspiration (ET). Several tests have shown the LAS to be a fairly robust tool for prediction of H, both over homogeneous and heterogeneous surfaces. However, the Kipp and Zonen LAS has been criticized for overestimation of H and for significant inter-sensor deviation in H. Field experiments were performed in 2011 using three Kipp and Zonen LAS units over two different surfaces to assess the accuracy and inter-sensor variability. Accuracy was evaluated based on reference measurements from eddy covariance (EC) instrumentation, which provides direct measurement of sensible and latent heat fluxes. Notably the EC method has been criticized for systematic underestimation of the sensible and/or latent heat flux, but is nonetheless a common tool used to validate LAS data. The first experimental test site was predominantly dry and uniform grassland located near Timpas, CO. At this site, all three LAS units were deployed together for some time in order to assess inter-sensor variability and an EC system was installed for some duration of the LAS deployment. The EC system was subsequently moved to the second site, which was the Colorado State University (CSU) Arkansas Valley Research Center (AVRC) near Rocky Ford, CO. At the AVRC, one LAS unit was set up over irrigated alfalfa. Results from the inter-LAS comparison suggested that there may be some inherent variability between 6-11% in LAS-predicted H (HLAS) and that the physical alignment of the LAS is critical for maintaining good performance. Testing different methods for estimation of the friction velocity (u*) variable revealed bias between the logarithmic wind profile (LWP) result and the EC measurement. Linear regression slopes between 0.94 and 1.35 were found for HLAS with respect to EC-derived H (HEC) for the Timpas site - dependent on the LAS unit, the LAS alignment, and the u* method. The overall conclusion was that HLAS was reasonably accurate, partially due to the potential of HEC being underestimated on the basis of lack of energy balance closure. For the CSU AVRC (irrigated) site, HLAS was generally observed to be greater than HLAS by 20-30%. However, heat flux source area differences between the LAS and EC units may have contributed to some of the observed biases. Further, the overall conclusion of reasonable accuracy of HLAS was made, again partially due to potential for H underestimation by the EC system. It is recommended, nonetheless, for future applications to calibrate the Kipp and Zonen LAS to a reliable reference on the basis of observed inter-sensor variability. Further, the benefit of the LAS is judged to be higher for a scenario of limited or no irrigation than for one of full irrigation, since the contribution of H to the overall energy balance would be relatively small for a full irrigation scenario.
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Subject
energy balance
sensible heat flux
micrometeorology