The Tindall Limestone and Oolloo Dolostone aquifers are important groundwater stores. The dry-season flow (May–October) for parts of the Daly River system is mostly dominated by input of groundwater from these 2 underlying aquifers. The aquifers are also a primary source of water for human consumption, with more than 80% of local water use being sourced from groundwater.
A key groundwater level of the Tindall Limestone Aquifer is located near Katherine; a key bore for the Oolloo Dolostone Aquifer is located near the junction of the Douglas and Daly rivers.
In 2020, for the second consecutive year, the normal increase in groundwater levels during the wet season in the Tindall Limestone and Oolloo Dolostone aquifers did not occur because of poor wet-season rainfall (Figures 8 and 9). Except for a few minor increases in groundwater levels following rainfall events, levels in both aquifers declined for most of the year. At 30 June 2020, groundwater levels in the Tindall Limestone Aquifer were the lowest in more than 20 years; levels in the Oolloo Dolostone Aquifer were the lowest on record (since 2006) (BOM 2020f).
Data are lacking on the volumes of water stored in the aquifers; however, information on the total annual change in aquifer storage is available. In the Daly River aquifers, there have been 7 annual drops in storage during the past 9 financial years, reflecting a sustained period of relatively poor wet-season rainfall in the region (Figure 10). The very large increase in storage in 2011–12 was primarily attributed to well-above-average rainfall over a 2-year period associated with the 2010–12 La Niña event (BOM 2020g).