Climate and Rainfall
The years of 2010–2012 were marked by a double-dip La Niña event and record-breaking rainfall in several parts of Australia19–21. The consecutive occurrence of the La Niña events contributed to placing April 2010 to March 2012 as Australia’s wettest two-year period on record. The rainfall surplus in the study region led to a pronounced groundwater recharge event at the first half of 2012 (Fig. 2-middle panel). This was followed by a period of around average rainfall (on a yearly basis) for 2012 through to 2015. Though in early 2013, heavy rainfall led to floods in several catchment areas of interior NSW (Fig. 2a), however those events were more localised and intermittent than the persistent rain associated with the previous La Niña event.
El Niño is generally associated with dry conditions in east Australia, however, despite the occurrence of the extreme 2015–2016 El Niño event, rainfall was not atypical that year 22. Instead, the region experienced significant rainfall surplus in 2016 due to the appearance of a record negative Indian Ocean Dipole event in the same year. There was significantly above average rainfall in 2016, caused by the occurrence of this record negative Indian Ocean Dipole event. In the following three years, east Australia suffered from significantly below average winter rainfall which led to a severe drought across the Murray Darling Basin from 2017 to 20198,23, and the subsequent devastating 2019/2020 Black Summer bushfires24. An additional La Niña event also occurred over the 2017/2018 summer, albeit too weak to alleviate the dry conditions in the region. The 2017–2019 Murray Darling winter drought finally broke in 2020 after the return of wet conditions following the development of a La Niña event. The region experienced significantly above average rainfall, including heavy rain events and floodings in 2020 that contributed to the observed groundwater recharge (Fig. 2c). The La Niña-related wet conditions persisted through spring and 2020/2021 summer, rising soil moisture, runoff and water storage levels and increasing the risk of floods in the region. Heavy rainfall and floods affected many parts of southeast Australia in March 202125, which was exacerbated by the appearance of a weak negative Indian Ocean Dipole event in winter 2021. November 2021 was the wettest November in 122 years for New South Wales and Australia as a whole26.
Groundwater Recharge
Despite all the heavy rainfall associated with the climate indices over the 9-year period of study, there was only one large recharge event detected in all groundwater bores, in 2016. This reinforces that diffuse recharge is episodic and in water limited environments, such as Wellington, infrequent. This further confirms that Australian Water Landscape Model 27 deep drainage term does not reflect reality at this and probably similar sites, as this model shows continuous, but low recharge over time, with no distinct recharge events (Fig. S2).
For this study the drip timeseries from Cathedral Cave, Wellington Caves were compared with groundwater bores at the nearby Wellington Research Station (Fig. 2). There was good agreement between the timing of identified potential recharge events observed at -25 m in the caves and observed groundwater level increases at the Wellington Research Station bores, with ten events in common (Fig. 3 and Table S1). However, there are a few days difference (0–5 days) between the cave and groundwater recharge events. There was one additional recharge event identified (July 2013) in the groundwater bores that did not appear in the cave, though it was observed in bores adjacent to the cave. There were also two additional recharge events observed in the cave (September 2013 and July 2020).
Based on the measured rainfall at the nearby BOM Wellington weather station a minimum threshold for recharge is determined to be 54 mm in the 21 days prior. Over the study period this threshold is exceeded on 27 separate occasions, however, on 14 (51.9%) occasions groundwater recharge is not observed in either the groundwater bores or the caves. This reflects the role that antecedent conditions play in controlling times of groundwater recharge, with drier periods requiring more rainfall to overcome the soil moisture deficit.
All recharge events are associated with broad cloudbands28–38 and widespread high rainfall totals, and recharge would therefore be expected at both the cave and bore sites. Despite this, there are three events that were only observed at either the cave or in the groundwater bores at the Wellington Research Station. For the majority of the events observed in the bores there was not recharge observed in every bore. This reinforces that diffuse recharge can have spatial variability.
Climate drivers
Recharge events are generally associated with low pressure troughs (12 out of 13 events), often slow moving and interacting with cold fronts or upper-level systems, resulting in extensive rainfall 28–38. In five cases, recharge occurred in some of the wettest months in the instrumental record (March 2012; June-September 2016). The cluster of recharge events in 2016 occurred during a period of negative Indian Ocean Dipole. One event (April 2020) was generated by rainfall associated with a cold front.
The strongest recharge occurred in 2016, which was due to a negative IOD event in the absence of a La Niña event and in the same year as an El Niño event. This negative IOD event was the strongest negative IOD event observed in June-September in 60 years 39. Anomalously warm sea surface temperature in the eastern Indian Ocean (i.e., off northwestern Australia) during a negative IOD provide a warm tropical atmospheric moisture source that is conducive to atmospheric circulation changes and moisture advection and rainfall from the northwest to southeast Australia 8,10. In addition to warm water to the north of Australia, the sea surface temperatures of the Coral and Tasman seas were the warmest on record through austral autumn 2016 and warmest on record for May40. It is well known that the moisture that contributes to precipitation in east Australia comes primarily from the Coral and Tasman seas41. The warmer waters during the negative IOD event increased moisture availability for precipitation, which in association with a series of East Coast Low events in June42 intensified moisture transport from the east seas into southeast Australia leading to floods and high river flows in many parts of NSW.
Negative IOD events (as well as La Niña events) have an important drought-breaking role in southeast Australia 8,43, and here we provide evidence for the strong negative IOD event in 2016 to generate sufficient rainfall to generate rainfall recharge of two fractured rock groundwater systems.
To further explore the climate drivers associated with recharge, the timing of past recharge was determined using a soil moisture model that was developed for the Wellington Caves site 44. Based on available rainfall and potential evapotranspiration data this model was applied from 1900. There was good agreement between the timing of recharge events from this soil moisture model and recharge observed in the groundwater bores for the 2012–2021 period. The soil moisture model identified 130 potential recharge events since 1900 (Fig. 4 & Fig. S3). The recharge events were accumulated on a monthly basis and compared to the relevant monthly IOD and ENSO values. More recharge events were associated with negative Dipole Mode Index than negative Niño3.4 index (89 vs 79 events) (Fig. 5). For the IOD timeseries, recharge events occurred more often when the Dipole Mode Index was on its negative phase than positive phase (89 vs 31 events). 36% of recharge events on the negative phase of the Dipole Mode Index were strong events (recharge above 1 standard deviation), while only 19% of recharge events on the positive side of the index were strong events. 26 recharge events occurred during negative IOD events (i.e. index below − 0.5°C, See Methods), while only 4 occurred during positive IOD events (index above 0.5°C).
The impact of ENSO on Australia rainfall and soil moisture is typically amplified when it co-occurs with IOD events10, more specifically La Niña and negative IOD increases the chances for extreme rainfall over southeast Australia, while El Niño and positive IOD are generally associated with stronger drying over the region. Negative IOD enhances the sea surface temperature gradient in the Indian Ocean, increasing the atmospheric thickness gradient that drives strengthened onshore moisture flux, thus raising the chances for precipitation in southern and southern Australia. Indeed enhanced tropical moisture flux and above-average rainfall over southeast Australia has been observed during negative IOD events10, simulated in global climate models45, and also verified in a Lagrangian model 41. Risbey et al.46 also note that the enhanced meridional thickness gradient during La Niña and negative IOD is favourable for the development of cutoff low pressure systems, which in turn increase rainfall over southeast Australia. The amount of rainfall produced by cutoff low systems increase during negative IOD and La Niña, although the frequency of these cyclones seems to be less affected 42 Here we show that the impact of La Niña and negative IOD, as well as their combined occurrences, extends to recharge events in NSW (Fig. 4).
At present, it is unclear how the frequency and strength of negative IOD events will change in the future 8,47, though strong positive IOD events are projected to become more frequent in a greenhouse warming scenario48,49. It is also unclear whether less strong negative IOD events would generate conditions conducive to the rainfall recharge of groundwater at our fractured rock sites and elsewhere. This is obviously a very important question to answer to predict the impacts of climate change on groundwater resources that support remote communities, farming and groundwater dependent ecosystems in southeast Australia and other semi-arid environments.