AERODYNAMIC AND SURFACE RESISTANCES AFFECTING ENERGY-TRANSPORT IN A SPARSE CROP

Aerodynamic and surface properties of the soil and canopy affect energy transport within sparse crops. A study was conducted with cotton (Gossypium hirsutum L.) to evaluate the feasibility of using both surface and within-canopy resistances to describe heat and water vapor transport from a row crop at partial cover. Bowen ratio measurements of the field energy balance and stem flow measurements of transpiration were coupled with detailed radiation measurements to determine sensible and latent heat flux from the soil and canopy separately. Flux measurements were combined with point measurement of temperature and vapor density to calculate the surface and aerodynamic resistances to heat and vapor transport using a one-dimensional model. The characteristics of the soil and canopy were described with bulk parameters, and a hypothetical within-canopy airstream was employed to model transport between the surface and the within-canopy air. Micrometeorological estimates of canopy surface resistance to vapor transport were similar in magnitude and behavior to data reported for full canopies. Computed values of soil surface resistance to vapor transport increased as the soil dried, reaching a maximum value of 1578 s m-1. However, results suggest that a sophisticated soil surface model wil be needed to accurately quantify a diffusive soil resistance. An alternate approach for describing soil evaporation yielded more favorable results, in which soil water potential and temperature were used to estimate the vapor concentration at the immediate soil surface, eliminating the need for a diffusive soil resistance. Within-canopy aerodynamic and surface resistances had equal influence on vapor transport from the canopy. Aerodynamic resistances to transport from the soil surface were greater than those for the canopy at low wind speeds. All aerodynamic resistances tended to decrease as wind speed increased. However, calculated within-canopy aerodynamic resistances were highly variable and could not be adequately described by average wind speed, wind direction or canopy size. This study indicates that within-canopy transport from the soil and canopy cannot be partitioned or quantified using simple gradient diffusion relationships in combination with standard meteorological data.