Tracking freshwater depletion in Northern Australia: A multi-satellite approach

Accurate freshwater accounting is critical for effective local water planning and governance, but this is limited by availability of in-situ data. In Australia’s Northern Territory, significant climatic and hydrologic knowledge gaps have brought into question the sustainability of groundwater regulation amidst expansion of agricultural and mining activities. We demonstrate how multi-satellite remote sensing data can enhance understanding of human and drought impacts on changes in freshwater storage to underpin management strategies. By integrating historical Landsat data and in-situ groundwater levels, river discharge, rainfall, and climate data (1986–2022) with satellite gravity observations and output from a hydrological model (2002–2022), we develop a framework to document changes in surface and sub-surface water storage systems over the Cambrian Limestone Aquifer (CLA). We assess the response of these systems to hydrological drought, quantify water deficits and assess how much water may potentially be missing due to drought or extraction. Our satellite analyses show that changes in freshwater storage (or terrestrial water storage-TWS) in the CLA started to significantly decline after 2014. The CLA continued to experience variations in TWS deficits in the last decade (2014–2023) with average time to recovery increasing considerably from less than 5 months in 2014 to more than 15 months in 2021 when water storage deficits reached $\sim 150{\mathrm{km}}^3$. The CLA appears to be getting drier, evidenced by recent (2011–2022) and significant depletion in groundwater storage (−3.88 ± 0.29 km³/yr) and TWS declines (ranging from 6.28 ± 0.75 km³/yr to 8.20 ± 0.75 km³/yr), coinciding with large losses in aerial extent of open surface water features. While acknowledging that drought-affected areas and occurrences during the 2011–2022 period were significantly less than those experienced between 2003 and 2010, it is noteworthy that the recent drying coincided with the commencement of licensed groundwater extraction in the CLA. Building on these findings, we highlight the unique capabilities and benefits of combining satellite gravity and optical data to enhance our understanding of hydrological processes, while also advancing water governance and management responses to droughts and pressures from water extraction.