Analysis of parameters for evaluation of canopy and aerodynamic resistances over turfgrass.

Abstract

The estimation of the surface roughness parameters and the choice of the non-dimensional gradient functions for stability correction are important steps in characterizing the transfer of momentum, heat, and water vapor over vegetated surfaces. The analysis of hypothetical and experimental wind profiles indicated that the zero-plane displacement (d(m)) is an unnecessary parameter in the log-wind profile model. The inclusion of d(m) in the model causes a drastic underestimation of the roughness length for momentum (z(0m)). The analysis of data collected over bermudagrass indicated that z(0m) is virtually constant at wind speeds larger than about 2.5 m/s but increases at lower wind speeds. The roughness length for heat (z(0h)) was found to be about 1/7.6 of z(0m). The calculation of the roughness length for water vapor (z(0v)) was not possible because there is no practical method to measure the specific humidity at the surface (q(s)). The calculated canopy resistances from the humidity profiles (assuming z(0h) = z(0v)) were variable over time. Minimum canopy resistance was about 78 s/m which produced a minimum stomatal resistance of about 360 s/m. The fitting of the non-dimensional gradient functions for momentum (φ(m)) and for water vapor (φᵥ) for unstable conditions, and φ(m) for stable conditions agreed very well with regression lines from the literature. A large scatter was apparent in the measured values of non-dimensional gradient function for heat (φ(h)) under unstable conditions. Under stable conditions large scatter was found in the measured values of φ(h) and φᵥ. The results seem to indicate that φ(h) is equal to φᵥ but different from φ(m). Further research is needed to determine the roughness lengths for various crops without the inclusion of d(m) in the log-wind profile model. The effect of wind speeds on z(0m) should also be studied more carefully. Also, more research is needed in order to better characterize the fluxes of heat and water vapor under stable atmospheric conditions.