The removal from water level data fluctuations due to barometric forcing at subtidal frequencies is studied using transfer functions. In a poorly confined well-aquifer system these transfer functions are, in general, dependent upon air diffusivity through the unsaturated zone and diffusivity from the water table into the unsaturated zone. The stability in time of estimated transfer functions from selected water well sites near Parkfield, California, is examined, and an average transfer function is computed for each site that demonstrates reasonable sability over time. Two of these transfer functions that show marked frequency dependence in the subtidal frequency band are used to filter water level data in this frequency range. A comparison is made with residuals after subtracting the best fitting multiple of band-passed barometric pressure from the water level data, and transfer function filtering is shown to be superior. Comparing the estimated transfer functions with previously derived theoretical ones shows that in one case the well-aquifer system can be reasonably well modeled by assuming that the effects of air and water diffusivity are nonnegligible, while the remaining frequency-dependent transfer function can be better modeled by assuming minial air diffusivity through the unsaturated zone.