The effect of variable fetch on flux-variance estimates of sensible and latent heat fluxes in a pepper screenhouse
Knowledge of crop evapotranspiration (ET) is important for accurate irrigation. The area of cultivation under screens has rapidly increased in Israel and worldwide. ET data are well-documented for many crops grown in open fields, but rather scarce for crops grown under screens or in screenhouses. Consequently, research on methods to measure and estimate ET in protected cultivation systems has increased. We have shown that the eddy covariance (EC) technique can measure whole canopy ET in screenhouses. However, it requires expensive equipment and complex data analysis; hence it is for research only. Aiming at developing a low-cost and simple method that will be available for day-to-day use by growers, we investigated here the flux-variance method (FV), which is based on high frequency measurements of air temperature near the canopy top by a miniature thermocouple. Air temperature standard deviation is calculated and following Monin-Obukhov similarity theory (MOST), the sensible heat flux is estimated. Additional measurements of net radiation and soil heat flux allow extraction of ET from energy balance closure analysis. Since air temperature can be measured near the canopy top we hypothesize that fetch requirements can be relaxed relative to those for EC and a relatively small fetch is sufficient for reliable flux measurements by FV. In the present study the FV technique was examined in a pepper crop in an insect-proof screenhouse in southern Israel. Five miniature thermocouples were installed above the canopy at various distances from screenhouse edges, providing variable fetch from 30 to 150 m, depending on external wind direction. Measurements of stem sap flow in 10 pepper plants provided data on the latent heat flux. Net radiation and soil heat flux were measured to analyze energy balance closure and an eddy covariance system above the canopy measured sensible heat flux. Sensible heat flux from FV was regressed against reference data from the EC. Results showed that the FV similarity constant, CT, varied between 3.5 and 4.4 and was independent of fetch. Coefficient of determination of the EC-FV sensible heat flux regression increased with fetch. ET extracted from FV and energy balance closure was regressed against values calculated from stem sap flow measurements. Results showed deviations of 9 to 23% between FV and sap flow based ET, with reasonable coefficients of determination, mostly above 0.56. We conclude that in the insect-proof pepper screenhouse (i) the FV technique was reliable in estimating ET and (ii) CT was nearly independent of fetch.
Tanny, J., Achiman, O., Mazliach, Y., Lukyanov, V., Cohen, S. and Cohen, Y. (2018). The effect of variable fetch on flux-variance estimates of sensible and latent heat fluxes in a pepper screenhouse. Acta Hortic. 1197, 109-116
evapotranspiration, pepper, eddy covariance, sap flow, energy balance