Application of the surface renewal technique in two types of screenhouses: Sensible heat flux estimates and turbulence characteristics

While the usefulness of screenhouses in crop improvement and water savings is rarely disputed in semi-arid regions, reliable evapotranspiration (ET) estimates in screenhouses remain a challenge. The surface renewal (SR) technique utilizes high frequency readings of air temperature near or above the vegetation by low-cost fine-wire thermocouples, to indirectly estimate the sensible heat flux from which the latent heat flux can be estimated assuming energy conservation. The SR technique had been successfully used in open fields, and this research investigates its applicability in screenhouses. Experiments were carried out in two field campaigns, in banana and pepper screenhouses covered with shading and insect-proof screens, respectively. The SR technique was employed by installing several miniature thermocouples at different heights. Reference measurements of sensible heat flux were conducted using the eddy covariance (EC) technique. A calibration coefficient, α, was calculated using linear regressions between SR and EC outputs. In agreement with observations in open fields, in both experiments, α was found to decrease with height, and values were smaller for stable than unstable conditions. In a separate validation period, R2 reached 0.93 and 0.65 for measurements above the shading and insect-proof screen, respectively. For both the shading and the insect-proof screens, best coefficients of determination (R2) of these regressions were obtained just above the screen, at 1.02 and 1.12 of the screen height, respectively. Turbulence characteristics were observed to be affected by the porosity of the screen, which is conjectured to be the main cause for reduced performance of the SR technique in the insect-proof than the shading screenhouse. In addition, it was concluded that in screenhouse conditions sampling frequency can be relaxed to as low as 2 Hz with little effect on performance. The specifications provided here allow for a low cost deployment of the SR system in shading screenhouses. Such systems can be available for day-to-day use by farmers to improve irrigation management.