[A shorter version of this article appears in the Feb 2020 edition of Tableland Talk]

Groundwater levels are showing a predictable downward trend as we head into high Summer. Not surprising, given the district has had less than 250 mm rainfall since August.

Overall, 2019 was a pretty dry year with barely 700 mm recorded in most districts and on the back of a similarly dry 2018. Over the last 24 months the Tableland has had a rainfall deficit of about 600 mm; not only have we seen failed Spring and Autumn breaks, but the groundwater aquifers haven’t received their ‘normal’ recharge either. More worrying is the long-term regional trend. Between 1999 and 2018 April to October rainfall across SE Australia declined by 11% which is the most sustained large scale change in rainfall since records began (see chart below & BOM), even with the wet years of 2010 and 2016 and this has translated into significantly reduced stream flows.

‘Difference from mean’ analysis of stream-flow in Seven Creeks downstream of Polly McQuinns, showing the differences between a specific year and the yearly mean over the period of the record. The bigger the red/blue bar, the bigger the difference from the mean. Since the year 2000 stream-flow has been alarmingly low.

Fortunately, groundwater across much of the Tableland still appears to be holding – we’re not in 2006/7 territory yet! In those years many bores began to struggle and shallow bores began to fail.

For this project two bores in the Strathbogie district are automatically monitored. The charts below show that these bores respond very differently to the district’s rainfall conditions – Bore 1 clearly responds fairly quickly to rainfall events (suggesting groundwater in that area is fed by a single recharge area and the fractured granite aquifer that is quite porous/responsive), whereas the groundwater at Bore 2 may be fed from diverse recharge sources (perhaps near and further away).

The regular fluctuations in water level in Bore 1 for the entire time period and becoming more pronounced from October are not responses to rainfall, but are somehow linked to pumping from a separate bore only 4 m away (there is no pumping from Bore 1) – at this stage the reason is unclear.

The small fluctuations in Bore 2 are caused by rainfall, but the deeper drops from October onward are pumping events (this bore is used for domestic watering). This limited pumping appears to have only a short-term effect on the water table.

Comparison of the fine detail of water table changes in these two automatically monitored bores shows how groundwater can vary across the Tableland, something that the other nine bores being manually monitored also reflect (though in much less detail!). Though each monitored bore shows broadly the same pattern of rise in winter and fall in summer, each bore/site has its own, unique water table response to changes in rainfall (see below). For this reason, with rainfall becoming less predictable, it’s worth understanding how your bore behaves.

Water table variation in the nine manually monitored bores on the Strathbogie Tableland between September 2018 and Jan 2020. The vertical axis shows depth of the water table below ground level, with the date/time across the top. Trend lines are hand-drawn approximations. The nine bores are listed at right

Yes, it all comes back to rainfall. Here’s a chart (below) showing the April to October rainfall anomaly for SE Australia. In other words, how much the rainfall varied (compared to the 1961-90 average), during the months SE Australia expects most of its rain. I’ve pointed out a few of the prominent patterns/events on the chart, but perhaps the most important observation is that starting in the 1980’s and certainly since the 1990’s, April to October rainfall has become less and less reliable. We know this is already having serious knock-on effects to streamflow, as the first chart we looked at in this post shows. We are only now beginning to measure and monitor groundwater.


Further reading

Bogies and Beyond Groundwater Monitoring Project is supported by the Victorian Government and the Goulburn Broken Catchment Management Authority.

Access to water table data for Bores 1 and 2 can be found through the Gecko Clan’s Water on farms project.

For more information contact Bertram Lobert bertram.lobert@activ8.net.au 5790 8606, 0409 433 276 or go to: https://strathbogierangesnatureview.wordpress.com/?s=groundwater