The energy-budget evaluation of the micrometeorological transfer processes within a cornfield

Abstract

The energy budget approach was utilized to evaluate the vertical fluxes of sensible and latent heat by layers within a fully grown corn crop. Small net radiometers were used to measure the height distribution of net radiation. Small aspirated psychrometers were constructed and used to collect data on the temperature and water vapor pressure profiles within the crop. The smaller storage terms were measured, and included in the calculations. All the measurements were made within 10-min periods throughout the daytime hours of 1 day. An exponential relationship with leaf area approximated the extinction of net radiation within the crop. The largest recorded leaf-air temperature differences were found to be near, but not at, the top of the crop. Midday air-temperature profiles were characterized by the establishment of a warm air layer in the center of the crop. A diurnal trend in the movement of the level of highest air temperature was evident. The calculated exchange coefficients spanned a range of three orders of magnitude. The average of the midday exchange coefficients at the top of the crop was 4,400 cm 2 sec −1. Below 100 cm, the sensible heat flux was found to be downward. The latent heat flux was used to determine the transpiration per unit leaf area from each level within the crop. The calculated values ranged from 0 to 3.8 · 10 −6 g cm −2 sec −1. The transpiration rate was expressed as a function of three micrometeorological parameters: net-radiation absorption, saturation-water vapor-pressure deficit of the air, and a leaf-to-air transfer coefficient; and two plant parameters: leaf wetness and leaf-area density. It was found that four of the parameters varied with time and height within the crop. The fifth parameter, leaf density, varied only with height. Each of the parameters took part in regulating the transpiration rate. The calculated values of the leaf-wetness parameter ranged from 0.07 · 10 −4 to 2.77 · 10 −4 cal. cm −2 (leaf area) sec −1 mbar −1. For the entire crop, 46% of the net radiation was used for transpiration, 13% for soil evaporation, 32% for sensible heat flux, and 6% for soil heat flux. Measurements indicated that 16% of the net radiation reached the ground under the dense crop.