The Energy Budget at the Earth'S Surface: Basic Concepts of the Spectral Analysis by Digital Means

摘要

An instrument to measure the rate of gas movement from beneath the surface of soil or other porous media was constructed. Subsequent measured rates of gas movement from beneath surface coverings of coarse and fine gravel, very coarse and medium sand, Chenango silt loam and straw were correlated against concurrent measurements of wind and air pressure fluctuations. At a depth of 2 cm gas movement in silt loam was not significantly affected by air turbulence but in coarse gravel was increased 10-fold by wind velocities of 400 cm/sec. Intermediate effects were observed for media of intermediated pore sizes. Mathematical equations for calculation of soil air mass flow are derived. They are based upon the spectrum of air pressure fluctuations at the ground surface. Air pressure in the field was measured during both day and night with and without a corn crop. The spectra of air pressure were calculated and all were roughly straight lines with a slope of about -6/3 on a log-log plot. Spectral density decreased from 10 to th 8th power to 0.10 sq microbar/cycle/sec over a frequency range from 0.0001 to 100 cycle/sec. Soil air mass flow was calculated from the spectra for several soils. Using previous work, an attempt was made to evaluate the increase of soil gas movement beyond diffusion from the soil air mass flow. (Author)