Calculation of barometric efficiency in shallow piezometers using water levels, atmospheric and earth tide data

The variation in atmospheric pressure at Baldry in New South Wales, Australia (32°52' S, 148°32' E), has been determined using a 4-year record of hourly measurements. Fourier analysis of this data shows clear diurnal and semi-diurnal variation in atmospheric pressure caused by atmospheric tides. The semi-diurnal component of the tide changes little with the seasons and induces a barometric response in shallow confined piezometers. The piezometer response is shown to be contaminated by an earth-tide component that has to be removed before accurate barometric efficiencies can be calculated. Data from three piezometers installed in a region of fractured and weathered granite are described in this paper. The results show that piezometers installed in shallow bedrock can have barometric efficiencies of close to unity and that only a small quantity or thickness of lower permeability material is sufficient to cause confining conditions. This observation has important implications for the interpretation of recharge processes in shallow aquifers.