Characterization of Changes in Groundwater Storage in the Lachlan Catchment, Australia, Derived From Observations of Surface Deformation and Groundwater Level Data

Global Positioning System (GPS) deformation measurements were combined with groundwater level data to examine the spatiotemporal variability of groundwater storage in the Lachlan catchment located in central New South Wales (Australia). After correcting for effects of glacial isostatic adjustment, non-tidal oceanic and atmospheric loading as well as hydrologic loading using existing models, we show that the seasonal and interannual variability of ground deformation and hydraulic head level data, extracted using wavelet time-frequency analysis, exhibits an in-phase behavior, indicating that the observed surface deformation is the poroelastic response to groundwater pressure change in aquifer system. Combination of GPS displacement and groundwater level change enables the estimation of elastic skeletal specific storage coefficients, which were then used for estimating groundwater storage changes. The estimated groundwater storage changes clearly reflect the four climate events of the Lachlan catchment since 1996: (a) the Millennium drought over 1996-2009, (b) the 2011-2012 La Nina and two significant floods in 2012 and 2016, (c) the drought conditions from mid-2017 to late-2019, and (d) the return of La Nina conditions since early 2020. We also found annual and long-term groundwater storage variations of respectively similar to 25 +/- 2.7 GL and similar to -5 +/- 0.57 GL/yr over over the period 2012-2021. Moreover, we show that groundwater level fluctuations can be predicted from GPS displacement measurements and storage coefficients with sufficient accuracy (80% correlation and 70% RMS reduction when compared in terms of seasonal cycle). This study provides essential information that can contribute to future groundwater planning, management, and control over the Australian continent. Plain Language Summary Groundwater extraction or recharge changes the height of the Earth's surface, which can be obtained from displacement measurements derived from Global Positioning System (GPS). We combined the GPS displacement measurements over the Lachlan aquifer, located in central New South Wales (Australia), with the groundwater level change to study groundwater storage variation. The estimated groundwater storage changes reflect the major climate conditions over the area since 1996 such as the Millennium drought and the significant La Nina events in 2011 and 2020. We further analyze the aquifer response to the rainfall and found a delay ranging from 3 weeks to 2 months between the rainfall and groundwater wells levels, depending on the depth of the aquifer.