TURBULENT EXCHANGE OF CARBON DIOXIDE, WATER VAPOR, HEAT AND MOMENTUM OVER CROP SURFACES (NEBRASKA, SOYBEAN, SORGHUM).

摘要

Turbulence was measured over soybean and sorghum crops at Mead, Nebraska. Measurements were made with a prototype, fast response CO(,2) sensor employing infarared beam absorption techniques, a Lyman-alpha hygrometer, fine wire thermocouples, and with drag and one-dimensional vertical sonic anemometers. Calibration and operational procedures were developed for the use of the CO(,2) sensor in the field.; Daytime and nocturnal CO(,2) fluxes measured over soybeans and sorghum were found to be consistent with profile and chamber measurement results reported in the literature. On a clear day with moderate temperatures and wind speeds, peak CO(,2) flux rates over well water soybeans (leaf area index, LAI (TURNEQ) 4.0) ranged around 1.0 mg m('-2) (ground area) s('-1) when flux density of photosynthetically active radiation (PAR) was about 1,750 (mu)Ei m('-2)s('-1). Under similar environmental conditions with PAR = 1,350 (mu)Ei m('-2)s('-1), peak CO(,2) flux over sorghum (LAI (TURNEQ) 4.0) ranged around 1.4 mg m('-2)s('-1). Dark respiration rates (crop + soil + roots) for soybeans (LAI (TURNEQ) 4.0) were between 0.1-0.2 mg m('-2)s('-1). Respiration rates for sorghum (LAI (TURNEQ) 4.5) were 0.12-0.25 mg m('-2)s('-1).; Non-dimensional statistics were computed. The scaled standard deviation of CO(,2) was found to be about 2.9. Statistics for water vapor temperature and wind velocity were also computed.; Normalized CO(,2), humidity, and air temperature spectra had similar shapes and peak frequencies. The shapes of these spectra were found to be broader than the longitudinal velocity (U) or vertical velocity (W) spectra and their peak frequencies were found to lie between those of U and W spectra. Spectra shifted to higher frequencies and lower spectral density as thermal stability increased from neutral to stable regimes. The reverse situation occurred as stability decreased from neutral conditions.; CO(,2), water vapor and heat cospectra were found to be similar while momentum cospectra were centered over lower frequencies. A strong similarity was found between CO(,2) and water vapor cospectra.; Spectral correlation coefficients and Kolmogorov constants of CO(,2), water vapor and momentum transport were calculated. Spectral correlation coefficients decreased with increasing non-dimensional frequency and with increasing thermal stability from neutral conditions. Kolmogorov constants compared well with values reported in the literature. The constant for CO(,2) fluctuations was found to be 0.78 (+OR-) 0.11.