Groundwater change in the Murray basin from long- term in- situ monitoring and GRACE estimates

We present observations of groundwater change in a key, semi- arid, agricultural region of Australia, the Murray Groundwater Basin. Time series of in- situ groundwater levels archived in Government databases were compiled for all the States sharing this groundwater basin. A high quality subset of this dataset in a region affected by dryland salinity provides long-term in- situ observations on the respective impact of (1) increased recharge after deforestation and (2) the recent multi- year drought on groundwater levels. A change in the long- term dynamic of the water table is observed since the beginning of the drought in 1997 (1994 in some regions). The analysis of the bore data first showed a regional rise of the water table by ~5 cm/a from 1980 to 1992 followed by a regional decline at a rate of 17 cm/a from 1997 to 2009. Time series of groundwater storage anomalies obtained from a combination of total water storage using space gravimetry (GRACE) and soil moisture estimates from hydrological models also indicate a strong decline of the water table from 2002 to 2010. From August 2002 to December 2010, GRACE- based estimates indicate a groundwater loss of ~18 ± 1.3 mm/a which equates to a total loss of groundwater of ~45 ± 3 km3 over the ~300,000 km2 hydrogeological basin. These observations suggest that the impact of the drought on groundwater recharge counterbalanced and surpassed the impacts of past land-clearing and has brought a temporary halt to dryland salinity.