Field-based determination of air diffusivity using soil air and atmospheric pressure time series

Air diffusivity in two zones over a 4.6-m interval of the unsaturated zone was determined through analysis of atmospheric pressure and soil air pressure time series. Regression analysis was used to calculate the ratio of amplitudes (admittance) and phase lag between these series at diurnal and semidiurnal frequencies. The admittance for each of the monitoring zones was close to unity for both frequencies. Phase lags between the two time series were statistically equivalent to zero at the diurnal frequency but were as large as 20 degrees +/- 7 degrees at the semidiurnal frequency. The phase lag at diurnal and semidiurnal frequencies for each zone was compared to a diffusional model of soil air pressure response to atmospheric loading. From this comparison a composite air diffusivity was determined for each zone. The absence of large variability in air diffusivity and the trends in phase shift with depth suggest that macropores or fractures propagate through a clay layer at a depth of 0.5-1.2 m. The length of the time series (110 days) allowed for the examination of the magnitude of temporal changes in phase shift and air diffusivity. Temporal and spatial trends in phase lag are smaller than the error bounds in the phase estimates. Air diffusivity determined from the entire time record deviated only slightly from the average diffusivity determined from the analysis of a sequence of partial records.