Vertical variations in airflow and turbulence in a large banana screenhouse.

摘要

Shading banana and other orchard crops with screens is increasingly popular due to the resulting decreased water use and increased fruit quality. This study focused on vertical variations in airflow, turbulence characteristics and eddy fluxes in a large commercial flat-roof banana screenhouse in northern Israel. Screenhouse dimensions were 300 m long, 230 m wide and 6 m high, and the measuring tower was located approximately at the center of the screenhouse, at a point with a minimum fetch of 110 m in all directions. Two measurement campaigns were carried out in the same screenhouse in different periods of summer 2006. In the first campaign measurements were made with two 3-axis sonic anemometers and in the second campaign, two eddy covariance (EC) systems were deployed on the same tower. Results show that within the screenhouse the logarithmic wind profile usually prevailed and airflow direction was independent of height. Friction velocity, normalized to the local mean air speed and the efficiency of momentum transport, were both larger at the lower level, closer to the canopy, than in the upper level. Evapotranspiration measured by the lower and upper EC systems averaged 5.34 and 3.13 mm day-1, respectively. Average daily ratios of evapotranspiration to irrigation were 0.68 and 0.4 for the lower and upper EC systems, respectively. Energy balance closure analysis gave a slope of 0.93 and intercept of 25 W m-2, for the lower EC system and 0.67 and 7 W m-2 for the upper one. The significantly better energy balance closure at the lower as compared to upper level suggests that in the screenhouse environment there is an optimal height for valid evapotranspiration measurement with the eddy covariance technique.