The effect of structure type on the validity of turbulent flux measurements by the eddy covariance technique
The Eddy Covariance (EC) is a common method to directly measure whole canopy turbulent fluxes like evapotranspiration, sensible heat and CO2. The method was originally developed to measure fluxes from canopies in the open air; however, it was shown in recent years to be also valid for flux measurements in screenhouses. To reliably measure turbulent fluxes by the EC technique, several air flow and energy balance conditions should prevail. The purpose of this study was to examine two criteria for the validity of flux measurements in agricultural structures and the energy balance closure. The two tests are the "Integral Turbulence Characteristics (ITC), and the "Steady State (SS). The research was conducted in 3 types of structures with 3 different plants: (S1) A banana screenhouse; (S2) A pepper screenhouse; (S3) A naturally ventilated tomato greenhouse. In each structure an Eddy Covariance system was installed for a measurement period of at least 20 days. Mean fluxes were calculated over half-hourly time intervals. According to the ITC test we found that in S1 80% of all the half-hourly measurements were definitely suitable for the EC technique. On the other hand in S2 and S3 only 63 and 64% were suitable, respectively. According to the SS test we found that in S1 88% of all the measurements were definitely suitable for the EC technique. On the other hand in S2 and S3 only 77 and 79% were suitable, respectively. Reasonable energy balance closure was obtained in the two screenhouses, but in the greenhouse it was poor. We conclude that due to the different resistance to transport through the cover the validity of flux measurements by the EC technique is high in S1, reasonable in S2 and poor in S3.
Achiman, O., Mekhmandarov, Y., Pirkner, M. and Tanny, J. 2017. The effect of structure type on the validity of turbulent flux measurements by the eddy covariance technique. Acta Hort. (ISHS) 1170:345-352
evapotranspiration, sensible heat flux, screenhouse, greenhouse, banana, pepper, tomato