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Climate adaptation in my backyard - #3: Capturing Rainwater

Updated: Oct 18, 2021

Growing my own food in central Victoria where I live also means needing a fair bit of water. Especially in summer when it is dry and hot.

Where I live in central Victoria is at the drier end of temperate. The mean annual temperatures are between 2.9 and 11.8°C in July and 13.5 and 28.4°C in February, and mean rainfall is 590.8mm year (data measured from 1966-2018). We also have extreme differences in temperature with the lowest minimum temperature on record being -6.3°C and the highest maximum temperature 43.9°C.

Given these extremes, I want to capture water for a few reasons:

· Store water for the hot summer months when water demand is greatest

· Water without restrictions

· Less demand/reliance on town water supply

· To spread out the water I have captured across my landscape to improve soil, water, vegetation and microclimates – leading to landscape cooling.

· To have a 'back up plan' for the reduction in rainfall expected with climate change

But what does capturing water practically mean?

When we started thinking about how much water we could or should capture, my partner had a lot of questions, such as:

· How much water do we actually need? How much will be using in summer compared to the tanks filling up?

· What is the evapotranspiration in our yard?

· What about using rainfall data?

· Can we plan for the shed, the house and new roof area over time?

· Should we integrate it with a green/glass house?

My reply was simple – capture as much as you possibly can with the roof area you have available. We could spend a lot of time analysing the exact needs of the yard. My thinking was that rainfall, evapotranspiration and garden needs are all highly variable and we’d have rough estimates at best. And we haven’t really planned the garden yet either*. Mind you the idea of doing a proper analysis made me think about bringing in a modelling friend…

Dams in the goldfields are a sanctuary for plants and animals (photo: J Drake)

Keeping it simple – basic calculations for capturing water

I want to keep it simple. Let's pretend you have no design or financial constraints, and lets just focus on how much water you can capture.

We started by calculating the roof area of the house (138m2) and shed (45m2). I wanted to consider climate change in my analysis, so I used average monthly rainfall records from Castlemaine since they started recording (1966-2018) and compared that to average monthly rainfall for the last 15 years (2003-2018).

I found that annual rainfall from 1966-2018 was 590.6mm, whereas it was 535.4 in the last 15 years. This is a (very) rough 9% decrease**. Using the average monthly rainfall for these two time periods, I calculated the amount of rain that can come off the house and shed for the cooler months (May - November) and for the warmer months (December - April). I assumed we want to collect 100% of the rainfall in the cooler months, and that is will be our target capacity – 70,508 L using the 15 year data, and 74,170 L if we use all recorded data.

Now we can aim for a tank system in that range, right?

Wrong. Climate change is not that simple

I did some more thinking about climate change and rainfall. I have heard and read all sorts of different predictions about rainfall. At a Connecting Country talk I went to on weeds, an academic said that the Castlemaine-Bendigo area is likely to have 10% less rainfall by 2035 and that the rain will become summer dominant. I thought - what is that compared too?

I decided to use Climate Change in Australia climate analogues to get a better idea of likely rainfall decline. I set the model to Bendigo (Castlemaine was not an option), to the highest emissions scenario (RCP 8.5), maximum consensus, and time period of 2030. The outcome was that there was a 0% change in rainfall expected. That changes to 5% decrease by 2050 and 12% decrease by 2090 using the same model factors.

If I use the same model at highest emissions scenario (RCP 8.5) and then also the hottest and driest prediction, then things get really bad. We start with a 17% decline by 2030 and up to a 25% decline by 2090. This means we will transition to a drier landscape – for comparison the Little Desert in the west of Victoria gets as little as 400mm year, closer to the rainfall of our future.

A track through the Little Desert Park (photo: J Drake)

I had a quick look at the Climate Futures Tool to gain a better understanding of seasonal impacts of climate change on rainfall. When I used the November-April and May-October groupings for RCP8.5 and by 2035, I found that there would be little change (-5% to +5%) in November-April, but it would be either little change or drier (-5 to -15%) in May-October. These predictions are based on 33-66% of the models used agreeing with each other – termed as moderate agreement. So overall, the model really couldn’t tell us much about seasonal variation – more data is needed.

How does climate change effect the volume water I am trying to capture?

It is possible that we will have up to a 17% decrease in rainfall by 2030, with some of the decreases possibly occurring between May to October. What does that mean for my numbers for capturing water?

If we use the full data available (1966-2018), then a 17% decrease is 490.2mm/year. That makes me feel a bit sick. If we assume, for now, that the rainfall seasonality doesn’t change, then we only need 61,561 L of storage. If we assume the maximum seasonality difference of -15% (noting that my dates are slightly different), then we need 63,044L of storage.

So, if we purchase water storage making the assumption that we won’t have any decrease in rainfall (74,170 L) then we have 12,609 L of excess capacity by the worst predictions, or perfect space if it doesn’t change too much.

However, weather will also be more extreme with climate change. Introducing systems such as tanks can moderate these extremes – allowing space to capture rainfall generated in big events and release it slowly to the environment. Tanks as an enclosed space also reduce evaporation loss compared to dams, meaning we have more water for our environment that can be slowly released when we need it the most.

What did we decide?

We would like to go with the maximum capacity if we can. That is 2 x 35,000 L ready made tanks, depending on cost and if they will fit in the space we have available. They will fit the water we want to hold now, and capture more during extreme events so we can use it later.

Getting any tanks you can fit in the space you have available and with the money you can afford*** is going to be a win in a drying climate!

*We actually need to put in tanks first due to a drainage issue.

**I have not done a full data analysis to come up with this number, just a quick percentage using the annual average rainfall data on the BOM website for Castlemaine Prison. We note that median or mode may be more useful in a full analysis of rainfall data.

***Let's be real. Being sustainable means having the upfront cash to make a change. It is not cheap upfront, but can be cost effective in the long term.

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