The advent of accurate and stable groundwater logging systems makes possible the resolution of the fine detail in the hydraulic head record at a time-scale not possible given manual observation. The necessity to correct pumping-test data for atmospheric pressure loading, or coastal groundwater observations for ocean tide loading, has long been recognised. However, the recognition of the important groundwater resource information contained in the diel and sub-diel responses to atmospheric pressure and earth tide loading has been far more recent. The atmospheric pressure varies over a range of frequencies from the approximate week-long response to mesoscale movements of high and low pressure (<0.1 cycles per day), to diel and sub-diel (1 and 2 cpd) responses to changes in temperature in the upper atmosphere caused by the rotation of the earth. These pressure changes impact on heads measured in a confined or semi-confined aquifer but not on an unconfined aquifer. Their recognition is therefore fundamentally important in the allocation of storage values into a groundwater model.
The mesoscale processes are of larger amplitude than the sub-diel response and are easily observable. The ratio of the average hydraulic head change to the average amplitude of the atmospheric pressure change is a measure of the barometric efficiency, from which aquifer storage can be determined. However, the variation in frequency of the mesoscale processes precludes accurate barometric efficiency determination. By contrast, the diel and sub-diel responses are far more consistent and occur at a narrow frequency band that leads to accurate analysis using standard Fourier techniques. In this paper, we will give a review of the interpretation process and demonstrate some of the important groundwater system response data that can be determined using these techniques. The methods provide an important new approach to resolving aquifer storage in complex deep semi-confined sedimentary sequences.