Correlation and spectral analyses to assess the response of a shallow aquifer to low and high frequency rainfall fluctuations

With the predicted acceleration of drought and flood frequency worldwide, it is critical to build knowledge on how shallow groundwater systems respond to rainfall variability. In this study, we characterise the hydrodynamic response of an agricultural catchment located in southeast Queensland, Australia, to the low and high frequency fluctuations in precipitation that occurred in the past 25. years. Strong interannual variations in the precipitation input affected surface water flow more substantially than groundwater levels (GWLs). There was also a likely influence of groundwater abstraction on stream baseflow and GWLs, although it was difficult to quantify. Several low frequency oscillations were apparent in the GWL records, particularly in the upstream section of the aquifer, which were neither detectable in rainfall nor in discharge records. Statistically significant episodes of coherence were found at the 2-4-year band between the Niño3.4 index and GWLs for those upstream bores, especially during the 1995-2000 interval, which may be related to a strong La Niña event. In the downstream section of the catchment, higher groundwater persistence probably led to higher filtering of these low frequency oscillations. This paper also proposes a methodology for assessing the dynamic response time of GWLs in shallow aquifer systems to the precipitation input. Response time in a downstream borehole was highly variable temporally, ranging from 11 to 121. days. Rainfall amount was found to significantly affect the short-term dynamics of response time, with elevated rainfall leading to a decreased response time. Annually averaged response time was correlated with the annual number of days with rainfall >30. mm, which was interpreted as the potential for diffuse recharge. Higher recharge potential induced longer response times, probably because of the larger amplitude in GWL variations.