Wannon River insights part 6: 2024 saw “average” winter rainfall in the upper catchment, but what happened to flows?

In the last newsletter, Mark presented a table comparing rainfall at Mt William (Duwul) to surrounding towns on the plains – Ararat and Hamilton. The Mt William weather station is the highest in southwest Victoria, at an elevation of 1150 m above sea level, and catches a lot more rain than other areas at lower elevations. This is due to orographic lift whereby air rises over the mountains, cools and the moisture falls out as precipitation. In fact, Mt William is the highest mountain range west of Melbourne, all the way over to the Western Australian Coast, so its capacity to draw rain from the humid winds off the Southern Ocean is significant.

We now know that the 18 months leading up to the end of last year (2024) resulted in severe rainfall deficiencies in southwest Victoria, and this has been attributed as a contributing factor to the recent Grampians bushfires. I thought I’d take a deeper dive and see how, against this wider regional pattern of rainfall deficiency, rainfall across the southern Grampians stacked up for 2024. And, how that translated into flows for the upper Wannon River, leading up to the recent bushfires which burnt almost its entire catchment.

There are a few caveats to my investigation. Firstly, I have used Silo point data to fill in missing data from the BOM weather station records. Point data is constructed using actual weather station data, and interpolation using other nearby weather stations where data is missing. In addition, Mt William weather station has data going back to 2006, so that does limit our window. This time frame also falls after we started to see an overall, regional downward trend in rainfall (since 1996), compared to patterns from the 1950s onwards so what I’m referring to as “average” is all in this context.

Based on the collated data, the 2024 rainfall was close to the 20-year average during the runoff months of June to October. Annual rainfall was below average whereas 2023 annual rainfall was above average. In fact, we are coming off three relatively wetter years (2021, 2022, and 2023). Mirranatwa (20 km to the southwest and at 248 m elevation) and Willaura (36 km to the southeast and at 250 m elevation) had below-average rainfall in 2023 and 2024. in alignment with conditions across the wider region. Something else worth pointing out is the lower rainfall at Willaura compared to the sites on the western side of the range (i.e. Mirranatwa), particularly from June to October. This reflects the rain shadow effect, whereby the prevailing south-westerly airflow, and its moisture, are intercepted by the mountain range.

To understand the relationship between seasonal rainfall and flow, it’s useful to look at cycles in terms of water years – this is the period between March and February and takes into account the wetting up of the catchment following summer, rather than January to December as above. The chart below shows a comparison of rainfall and flow data across water years from 2006-7 to 2024-25. Flow data is available on the Victoria Water Management Information System and I have summarised this into flow volumes for three phases of the water year: i.e. early catchment wetting (March to May), peak runoff period (June to October), and end of flow cycle and catchment drying (November to February). I’ve used data from the upper Wannon River gauging station at Jimmy Creek campground and also from a station where diversions from the upper Wannon tributaries enter Fyans Creek (Wannon diversion pipeline outlet). From these data, we can see that the flows down the Wannon River last year (2024) were the lowest we have seen in the past 20 years.

The last really dry year we have seen is 2015 and I wrote an article last year flagging the dry start to 2024 and comparisons with 2015. If we have a look at the cumulative rainfall patterns again, we can see that 2024 rainfall surpassed 2015 at day 294 (21st October), despite the start of 2024 being drier. At the time that article was written (September) we were seeing a small pulse of flow making its way through the lower sections, at Lynches Crossing and also flows entering Gooseneck and Brady Swamp, and there was some hope for the system receiving a top up leading into summer. But flows waned and the system dried out rapidly during November and December. So while rainfall in the higher catchment area wasn’t noticeably lower, a combination of lower rainfall away from the highest areas and a shorter flow season (less rainfall through June and July) meant that the downstream sections didn’t really get a chance to wet up before evaporation started to overtake precipitation and inflows.

So coming into 2025 we are in a similar hydrological pattern to 2016, when refuge pools in the upper Wannon really dried down. We don’t want a lot of rain until the burnt catchment vegetation has had a chance to regenerate but contributions from the headwaters, draining off the Mitchell Plateau, have an important role to play. Looking at recent satellite imagery we were relieved to see that part of the Mitchell Plateau didn’t burn in the recent fires and this includes the tributaries of the First and Second Wannon Creeks, as shown below. For now, we will just have to wait and see what autumn and winter rainfall brings, and embrace the signs of recovery already happening across the Grampians.