The effects of spanwise canopy heterogeneity on the flow field and evaporation rates

The effect of spanwise canopy heterogeneity on the turbulent wind flow field and associated evaporation rates was investigated by combined stereoscopic particle image velocimetry (stereo-PIV) and evaporation rate measurements on a model canopy in an atmospheric boundary layer wind tunnel. The model was designed to simulate a mature corn canopy, based on a similar wall-normal distribution of the projected frontal area index. Stereo-PIV measurements were performed in wall-parallel planes across the spanwise heterogeneity at several elevations. For comparison, measurements were also performed for a homogeneous canopy layout. For the heterogeneous layout, large spanwise gradients of the mean streamwise velocity at the canopy edge were observed, resulting in high shear and high peak values of the in-plane Reynolds shear stress component. Local evaporation rates were measured directly utilizing a specially designed wicks based system. Measurements were conducted at three locations positioned in a line, perpendicular to the incoming flow, across the heterogeneity. The spanwise heterogeneity did not alter the well-accepted sweep-ejection mechanism, sweeps dominated flow was recorded at the sheltered wick positions. Evaporation rates increased with local mean streamwise velocity and power-law based correlations for the Sherwood number versus Reynolds number were derived for sheltered and exposed evaporating wicks. Turbulent flow structures, in the form of sweeps and ejections, were important both in wall-normal and transverse directions (for a heterogeneous canopy). The results indicated that in arid regions, dry air transported in the transverse direction into the canopy is likely to lead to increased evaporation rates at canopy edges.