Note to Yarns from the Farm readers: this is the penultimate instalment of a series on climate change that I’m writing for our local monthly newsletter, the Southern Midlands Regional News (SMRN). Previous articles can be found in the preceding Yarns from the Farm. The image above is a small waterfall on my farm, at the headwaters of Curly Sedge Creek, present when there is enough rain to get significant runoff.
Last issue I promised you an article about extreme weather events related to climate change. I will now confess that I’ve really struggled to write this article. While all of us who live close to the land have a feeling that things are not as they were even a couple of decades ago, Australia’s naturally variable weather patterns can easily obscure the effects of climate change.
For instance, we have the feeling that our dry cycles are coming more often and lasting longer, and that our flooding rains are less frequent but more devastating.
Pragmatically, we humans want to know how our water supply, plant growth and fire risk will be affected by the variability of our climate system, with or without changes stemming from global climate change.
The reality is that all of these variables: water supply, soil moisture and fire risk, are event-driven: they reflect sporadic, often intense events superimposed on our highly variable underlying climate system. And whether a given rainfall event results in recharging our water supplies may depend as much on the state of soil moisture before the rainfall as it does on the details of the recharge event itself.
So, how do we define the variables that would let us discover whether our feelings about long-term changes in our local weather patterns reflect objective reality—whether they are actually true? And just what do we really mean by extreme events?
To make this more tangible, let’s look at the most recent runoff event in the Southern Midlands. Over three days in June we received just over 50 mm, which corresponds to the long-term average monthly rainfall. This event filled farm dams and increased soil moisture markedly. In contrast, we received 74 mm in April, without any significant run-off at all. Did the April rainfall events set up the soil moisture for the June recharge? Probably. How do we account for that in analysing our historical rainfall records?
I’ve been keeping fairly accurate rainfall records on my farm south of Oatlands since 2005—not long enough to do any useful statistical analysis on events that happen every few years. A quick look back over my record, though, shows 8 monthly average rainfall amounts of over 100mm, which we might use as an indicator of flood or recharge events.
Of those 8 high rainfall months, half had 3-day events over 50mm, like our recent recharge event. For one that didn’t—September 2009—my farm journal notes “Awash! Rapids in Curly Sedge Creek!”. Clearly, it was a flooding rainfall event, even though it didn’t reach the 50mm-in-3-days threshold. Flooding was presumably preconditioned by the steadily above-average rainfall of the preceding months, two of which had over 100mm of rainfall, but no 3-day event over 50mm.
So, it’s not a simple matter of doing the usual kinds of statistics if what you really want to know is how often water supply recharge events have occurred in the past, let alone forecasting them for the future. Since I’m now clearly working well above my pay grade, I’ll see if I can engage some scientists I know to help me figure out how to take the next steps.