Surface eddy fluxes and friction velocity estimates taking measurements at the canopy top
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Agricultural engineers, crop physiologists and hydrologists are often forced to deploy instrumentation close to the canopy top to estimate surface eddy fluxes. This is a widely recognized problem either at farm-level or research-level. The eddy covariance (EC) method is grounded on hypothesis, simplifications and relationships valid in the inertial sublayer and alternative methods may require either calibration or inputs that are not commonly available. The present study enables a method that combines similarity, transilient and surface renewal theories in conjunction with the analysis of small eddies (Surface renewal of small eddies, SRSE, method) to estimate the friction velocity and surface fluxes of different scalars (sensible heat, latent heat and carbon dioxide) taking measurements at the canopy top. SRSE minimizes estimation of canopy parameters (only the zero-plane displacement is involved), allows implementing a primary flux quality control and to guide instrumentation deployment. For a mature maize crop, SRSE and EC surface energy balances indicated that EC was slightly more reliable than SRSE during daytime. For unstable cases, the normalized mean absolute error (MAE) to the available net surface energy (ANSE), MAE/ANSE, obtained by the EC and SRSE methods were 9% and 13 %, respectively. For stable cases during the day, 10 % and 11 %, respectively. For stable cases at night SRSE performed better than EC, MAE/ANSE were 23 % and 16 %, respectively. Regardless of the half-hourly eddy flux (including the friction velocity), the EC and SRSE were highly correlated (the minimum coefficient of determination was 0.91) and, in average (for all data) flux of scalars nearly matched including water-use-efficiency. To avoid acquisition of expensive and stringent instrumentation, the closure of the surface energy balance equation was forced to derive affordable approaches to estimate the latent heat flux for different stability cases.
Is part ofAgricultural Water Management, 2020, vol. 241, p. 106358
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