Clouds shuffle across a sullen sky, staring down at a landscape that is waiting in limbo until the autumn rains. The rainwater tank is getting low, plants not heavily watered have shut down growth as they run out of soil moisture to draw on. Rain will come, but when? And how much?
Previously I’ve had a bit of a stab at charting long-term rainfall trends using Bureau of Meteorology historical data for the town, compared to the recent data from Melbourne Water’s nearby weather stations. With a bit more thought and work I think I have improved the analysis. I’ve also included more data.
The result, in short, is that rainfall has been declining over the 137 years since the records began. This appears to be caused mainly by a large decline in autumn rainfall.
I also looked more carefully at whether the south end of town gets less rainfall than the north (as I had previously assumed). It appears that is true and likely to hold with current weather patterns. Continue reading →
Will dry-climate species spread from their current rainshadow niches, where they are a botanist’s curiosity item, to recolonise the surrounding, warming landscape? Are they waiting in these refuges for the opportunity, or merely relics on their way out?
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.