I moved to Bacchus Marsh on a bit of a whim and only got the bug for understanding the bioregion afterward. Certainly, I’ve never lived somewhere so dry. Our average annual rainfall, about 505mm (roughly 20 inches) is a bit more than the average for Australia (which was 486mm for 2000-09: ABS), but that’s not saying much! Many recent years have been drier than that.
The area is in a rainshadow due to the Otway ranges to the south-west, and the uplands directly to the west. Both these catch a lot of the rain from the prevailing south-westerly winds. Weather from directly south and from the south-east is the best bet for good rain generally (in my experience) but is less common. Rain from northerly directions, also less common, often seems to be substantial enough to make it over the range and land here as well.
Conveniently, Melbourne Water and the Bureau of Meteorology keep some records which are available online. Here’s the Bacchus Marsh monthly averages for the period 1880-1962, according to the Bureau of Meteorology (it appears this weather station was discontinued after 1962). Average annual rainfall in this period was 505mm, while the median was 513mm.
This long-term average prior to global warming taking hold is a useful baseline. To compare, Melbourne Water have data available for their two local rainfall monitoring stations. One is at Darley, the northern suburb of Bacchus Marsh; the other is at Parwan Creek , a few kilometres south of Bacchus marsh on the road to Rowsley. Darley is probably a better proxy for the old Bacchus Marsh station, but since the town is spreading out, I think it’s useful to include both stations to get an idea of the regional variability.
The data I’ve got are from December 2005 onward. This encompasses the second half of the millenium drought, and the wet years following 2010-2011. In this period (December 2005-December 2014), the average annual rainfall was 505mm for Darley, and 428mm for Parwan.
Here’s the graph of annual total rainfall for the period:
And to give more “granularity” (as I think the stats nerds call it) here’s monthly rainfall for the same period. Obviously, rain doesn’t fall uniformly in every month, any more than it does in every year, so it’s useful to get some sense of the variability if you’re planning to grow anything.
Like in the long-term (1880-1962) figures, you get a sense of when rain generally falls most: in the spring. And, that it usually falls a bit more in Darley up against the Great Divide, than at Parwan Creek — but not always! We do sometimes get quite localised, but heavy, downpours that cross south of the town.
Going back to the annual averages above, while 2010-11 were obviously very wet years, and 2012 pretty much average, it’s been pretty dry again for the following two years — and going into 2016, we’re facing another El Niño, so it’s likely to be dry again.
Here’s the monthly averages for 2006-14.
The pre-1960 average had 4 months a year below 40mm average rainfall. In the average for the last ten, only two are above 40, although the annual average is still similar.
If you’re expecting a dry year, the wet years included in this average may skew the totals, especially as the heavy rain in the wet years 2010-11 didn’t all fall in the normally wet spring months.
To get a bit of a sense of what to expect in dry years (which may be most of them, with climate change affecting our rainfall) here’s a graph of average monthly rainfall for 2006-14, leaving out the wet years 2010-11.
These rainfall figures are not all the story, if you want to use them for anything. How well rain soaks into the ground (or how much it runs off) depends on slope, soil structure, rain intensity and vegetation cover. How much evaporation occurs also depends on these, as well as temperature, aspect (slope facing north, or south) and wind exposure. All these have great effect on how your garden, or revegetation planting, will fare.
Climate change (and El Niño) can bring more summer rain, less predictable rain, more evaporation from heatwaves, and other effects not common in the baseline period preceding. If we don’t stop runaway climate change it could be catastrophic, but even if we halt it where it is now, the increased heat in the atmosphere and oceans is having a big effect on our weather. We can’t assume past averages will remain relevant to future rainfall.