EXPERT REACTION: BOM declares a La Niña

BoM today declared a La Niña has developed in the Pacific Ocean, and concern about impacts on water storage behaviours in farming re-emerge.

Researchers at UniSA have been exploring farmer water storage behaviours and perceptions the last time this occurred in 2012 when La Niña generated large impacts across Australia, including Australia’s wettest two-year periods on record, and widespread flooding, exasperated by water stored unsafely in farm dams.

As food production demands increase in agricultural catchments, every drop of runoff becomes more valuable. Farm dams in catchments capturing this runoff provide immense societal and economic benefits but can also cumulatively threaten downstream communities and environments.

The cumulative impact of farm dams constructed across a catchment is large because they alter the natural flow regime and capture all the runoff generated from their upstream catchment area aggregating risk across two potential exacerbation extremes: 1) drought conditions during limited rainfall periods; and 2) flood conditions during intense rainfall such as those caused by La Nina.

On-site data was captured by the researchers for 80 physical dam sites across four states so that flood risk and the amount of potentially extra water stored by blocked or under-designed spillways could be calculated, finding nearly half of farm dams (43%) are at high hazard rating to communities living below them and yet less than half (39%) had acceptable flood capability per ANCOLD Guidelines.

The researchers undertook a longitudinal cohort survey of 254 South Australian farmers providing a snapshot of farmer perceptions and behaviours around farm dams in the context of changing policy and environmental conditions from 2012 to 2018.

In the 5 years post the last La Nina event, farmers were more concerned about the future of their dams to continue farming operations and more likely to choose specific aspects of regulation to comply with, rather than conform to all dam management mandates.

The researchers are using unique transdisciplinary approaches to derive solutions and tools from non-traditional disciplines such as accounting and business for potential integration with traditional fields of hydrology as the beginnings of a way forward for improving dam decision-making across scales to support the health and safety of environments and communities during La Nina events.

The El Nino Southern Oscillation (ENSO) does not have as large an influence on weather in southwest Western Australia as compared to Eastern Australia, where it has a much stronger effect.

However, one of the most significant effects of ENSO on WA is its influence on the Leeuwin current, which flows south along the WA coast from Indonesia, bringing warmer waters further south.

In early 2011, we had a very strong La Nina event, which led to a particularly strong Leeuwin current, resulting in a significant marine heatwave event. However, this La Nina event is not predicted to be as strong as the 2010-2011 event, hence we need to wait and see what this might mean for the Leeuwin current.

It is critically important to realize that not all La Nina (or El Nino) events are the same, and hence, the impacts will differ from one La Nina (or El Nino) year to the other, and there are other factors at play. ENSO is just one important part of the puzzle.

On the upside, a La Niña should translate into a lower risk of bushfires and extreme heatwaves over the eastern states this summer.  Unfortunately however we can expect an increased likelihood of flooding rains and tropical cyclones.  
 
La Niña events generally increase summertime rainfall over eastern Australia, which can bring drought relief to farmers, but it can also lead to extremely damaging flood events such as we saw in Brisbane during the summer of 2010/2011.