Long-term groundwater storage change in Victoria, Australia from satellite gravity and in situ observations

Analysis based on satellite gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) and land surface models indicates that groundwater storage in Victoria, Australia had been declining steadily, until a trend reversal around early 2010, attributed to two wetter seasons in 2010 and 2011. In situ groundwater level measurements (from a network of 1395 bores in Victoria) also indicate a steady groundwater depletion since the early 1990's, and show remarkable agreement with GRACE estimates for the 10-year period (2003-2012) in common with the GRACE mission. Groundwater depletion rates for 2005 to 2009 are relatively large as indicated by both GRACE estimates (8.0 ± 1.7 km3/yr) and in situ measurements (8.3 ± 3.4 km3/yr). Over the same period (2005-2009), GRACE measurements capture significant groundwater depletion in a wider region covering much of the southern Murray-Darling Basin, and the total groundwater depletion rate in this region is about 17.2 ± 4.7 km3/yr. Annual groundwater storage changes are strongly correlated with precipitation anomalies, but only about one-fifth of anomalous precipitation contributes to groundwater recharge. The strong correlation suggests that this groundwater depletion is primarily related to drought with related groundwater pumping for agricultural and domestic consumption. The remarkable agreement between GRACE estimates and in situ measurements demonstrates the great potential of satellite gravity observations in combination with land surface model estimates to quantify changes in regional groundwater resources, especially when in situ measurements are limited or unavailable. This study shows the importance of reducing leakage bias in GRACE observations and the effectiveness of the forward modeling iterative method used.